hallium- Doped Sodium Iodide and Selected Properties of, Exams of Engineering

Download hallium- Doped Sodium Iodide and Selected Properties of and more Exams Engineering in PDF only on Docsity! NASA Reference 1 :;:ation ' September 1984 I Engineering and Design properties of ~hallium- Doped Sodium Iodide and Selected Properties of - sod ium-~o~kd Cesium Iodide K. Forrest, C. Haehner, T. Heslin, M. Magida, J. Uber, S. Freiman; G. Hicho, and R. Polvani NASA Reference Publication % I 3 1 19984 National Aeronautics and Space Administration Scientific and Technical knformetion Branch Engineering and Design Properties of Tha oped Sodium Iodide and ected Properties of Sodium-Doped Cesium Iodide K. Forrest, C. Haehner, T. Heslin, M. Magida, and J. Uber Goddard Space Flight Center Greenbelt, Maryland S. Freiman, 6. Hicho, and R. Polvani National Bureau of Standards Washington, D.C. ENGINEERING AND DESIGN PROPERTIES OF THALLIUM-DOPED SODELFFA IODIDE AP4D SELECTED PROPERTIES OF SODIUM-DOPED CESIUM IODIDE K. Forrest, C . Haehner, T. Heslin, M. Magida, J. Uber NASAIGoddard Space Flight Center Greenbelt, Marylarzd 20 771 S. Freiman, G. Hicho, R. Polvani National Bureau o f Standards Washington, D. C. 20234 INTRODUCTION The experiments selected for the Gamma Ray Observatory (GRO) will utilize over 1000 kg of inorganic NaI(T1) and CsI(Na) scintillators. Many of the individual blocks of scintillators will be larger than any that have previously been launched in a space vehicle. The Energetic Gamma Ray Experiment (EGRET) scintillator, for example, is a composite assembly of NaI(T1) approximately 0.76 m square by 20 cm thick, and the Oriented Scintillation Spectrometer Experiment (OSSE) uses a 0.33 m diameter cylinder composed of a 10 cm thick piece of NaI(T1) and a 7.6 cm thick piece of CsI(Na). Proper mechanical systems design for the encapsulation, bonding, and support of these large scintillator blocks is necessary if performance degradation from cracking near support regions or failure because of creep or clouding from hydration are to be avoided. On the other hand, overdesigned support systems that result in large weight penalties are also to be avoided. Although the alkali halides have been studied for years, most of the work in the 1950's and 1960's centered on NaCI, KCI, LiF, and cubic materials such as MgO because these materials were model systems for delineating solid-state theories about defect structures that were the subject of extensive research at the time. Other than some work on CsI done for the High Energy Astronomy Observa- tory in the early 1970's (references 1 and 2), the only engineering properties study of NaI was a report of limited circulation by Stanford University (reference 3) and undoped, single-crystal elastic constant data in Landolt-Bornstein (reference 4). To optimize the mechanical design of the scintillator supports and housings for experiments, the GRO systems team authorized a program for dete~mining accurate and statistically significan"ca1ues for the mechanical and thermal properties of Nal(T1) and selected properties of CsI(Na). The ensuing materials evaluation program was conducted by the Materials Control and Applications Branch of the Goddard Space Flight Center (GSFC) and the Fracture and Deformation Division of the National Bureau of Standards under contract to GSFC. Emphasis of the program was placed on the needs of the EGRET and OSSE programs. This test report was compiled from the results of this test program. In addition, other test results and literature data that were not developed in this pro- gram are cited to make this compilation as comprehensive as possible for those who wish to use these two materials in mechanical designs. Enough test description and analysis of the data is presented to facilitate comparative testing in the future. In the following section, a series of ten test requirements are outlined, beginning with coefficient of linear thermal expansion and ending with ingot variation of strength. This sequence of properties is the topic outline for each succeeding section in this report, beginning with summary of results and ending with the conclusions. TEST REQUIREMENTS The following properties of single-crystal and polycrystalline NaI(T1) were required for meeting the needs of the various GRO experiments: 1. Coefficient of linear thermal expansion 2. Thermal conductivity 3. Thermal-shock resistance 4. Heat capacity 5. Elastic constants a. Density b. Young's modulus c. Shear modulus d. Mechanical damping efficiency (internal friction) 6. Ultimate strengths a. Modulus of rupture b. Shear c. Compressive d. Critical stress intensity factor (KIc) 7. Creep a. Bending b. Compression 8. Hardness 9. Susceptibility to subcritical crack growth 10. Ingot variation of strength In addition, it was determined that compressive strength and compressive creep measurements were necessary for single-crystal CsI(Na). Material for all tests was purchased from the Harshaw Chemical Company, Solon, Ohio, and the Bicron Corporation, Newbury, Ohio. SUMMARY OF RESULTS This section summarizes the test results for single-crystal and polycrystalline NaI(T1) and single- crystal CsI(Na). After each property heading, the symbol subsequently used to refer to the property is listed, and the range of values measured for that property is given without regard to the values referring to single-crystal or polycrystalline material unless otherwise stated. Finally, an evaluated average, plus or minus one standard deviation, is given when applicable along with the number (n) of data points used to arrive at the standard deviation. The evaluated average is arrived at in the section entitled "Test Results and Statistical Analysis" and usually represents the average of the largest group of specimens that cannot be statistically differentiated from each other using a pair-wise t-test of the equality of means at the 90- or 95-percent confidence level. In the short text after each heading, some details on the effects of orientation and position are given for single-crystal and Polyscin* materials, and the effects of ingot variation are cited. An abbreviation key referring to position and orientation designations for Polyscin and single-crystal material appears in Table 4. Coefficient of Linear Thermal Expansion, a, @ Range 45.65 X loe6 K-' to 50.20 X K-' at 253 to 323 K @ Evaluated average (47.21 + 0.44) X lom6 Kml at 297 to 323 K, n = 9 Measurements were made on three oriented single-crystal and four Polyscin NaI(Tl) specimens. Single-crystal material exhibited the same coefficient of linear thermal expansion in the <loo>, <011>, and < I l l > directions. The coefficient of linear thermal expansion of Polyscin is slightly less in the direction perpendicular to the extrusion axis than in the direction parallel to it, and there is no significant difference between similarly positioned material taken from the beginning or end of an extrusion. Literature values are as follows: e Single-crystal, undoped NaI 44.7 X K - I , 293 K (reference 5, page 11 16) Single-crystal, undoped CsI 48.3 X K"', 293 K (reference 5, page 1098) *"Polyscin" is a polycrystalline form of NaI(T1) produced by the Harshaw Chemical Company. C, , = 2.45 X 1 O4 MPa (reference 4, page 8) C12 = 6.7 X 1 O3 MPa (reference 4, page 8) C,, = 6.3 X 1 O3 MPa (reference 4, page 8) Ultimate Strength @ Modulus of rupture (MOR) <loo> cleavage, range = 1.57 to 2.7 1 MPa Polyscin, range = 2.85 to 6.49 MPa @ Shear <loo> cleavage, range = 6.42 to 9.74 MPa Polyscin, range = 5.71 to 8.00 MPa @ Compression <loo> cleavage, range = 1 1.19 to 16.72 MPa Polyscin, range = 9.1 1 to 23.12 MPa @ KIC Polyscin, range = 0:304 to 0.436 MPa m1I2 Evaluated average= 0.382 + 0.042 MPa m1I2, n = 9 Material from two separate logs of Polyscin and nine different NaI(T1) single-crystal ingots was tested. Although Polyscin is homogeneous with respect to elastic moduli, its MOR strength in one extrusion varied significantly (a factor of 1.4 on average) with position in the log, whereas in the second extrusion, MOR strength was homogeneous. Single-crystal strengths are ingot-dependent within the ranges cited. Csl (Na) at 300 K (0.2-percen t yield) @ <loo> single crystal, range = 3.27 to 7.49 MPa @ Polycrystal, average= 2.98 + 0.22MPa, n = 5 Material from three single-crystal ingots and one polycrystal ingot of CsI(Na) was tested. Yield strength varied extensively from ingot to ingot (as much as a factor of 2.3), but strength within an ingot was homogeneous. Literature values are as follows MOR = 6.45 MPa (reference 3, Part 2, page 1 ) Proportional limit = 0.276 to 2.35 MPa (reference 2, page 14) K,, = 0.279 to 1.206 MPa m1I2, single crystals* Creep e Compression Less than 0.005 percent in 2000 hours at 1.38 MPa (Polyscin) Less than 0.01 7 percent in 3000 hours at 1.08 MPa (<loo> single crystal) e Bending (four point) 12.7- by 2.54- by 2.54-cm specimens, 10.6-cm outerspan, 3.386-cm inner span Less than 5.08 X 1 0-5 cm in 4000 hours at 1.93 MPa (Polyscin) Less than 7.62 X lom4 cm in 4000 hours at 1.48 MPa (<loo> single crystal) Compressive and flexural creep in NaI(T1) at 300 K can be expressed linearly using a t1I3 plot, where t is time. An indication of creep rate is measured in terms of the slope of such a linear plot. An increasing slope indicates an increasing creep rate. Significant creep is considered to be the point of maximum rate of change in a plot of slope versus applied stress (Figure 19). Material from one extrusion of Polyscin was tested in flexure and in compression. Material from one single crystal was tested in compression while material from another single crysial was tested in flexure. In general, single-crystal material crept more than Polyscin for the same test. Compressive and flex- ural creep was greatest for single-crystal material stressed in a principal-axis, <loo> direction, com- pared to material stressed in the other symmetry-axis directions, <O11> and <11 l>. For Polyscin, compressive creep i s very small (<0.005 percent in 2000 hours) at stresses below 1.38 MPa. Material from the end of an extrusion, near the surface, creeps less than material from the interior of an extrusion near the beginning by a factor of about 0.72. This is attributed t o a dif- ference in the degree of work-hardening in these two regions. In either case, relatively large creep rates (slope >10 mi~rostrain/hrl/~) do not occur until the proportional limit of the material is exceeded, which is, on the average, 2.43 MPa for interior material and 3.44 MPa for surface material. Maximum compressive creep was measured in a single crystal loaded in a principal-axis <loo> direction. A strain of 0.135 percent was measured in 350 hours at 2.89 MPa. *D. Lewis, Personal Communication, Ceramics Branch, Naval Research Laboratory, Washington, D.C. In flexure, creep in Polyscin begins when the maximum flexural stress exceeds 1.93 MPa. Below this stress level, no observable creep was detected in 4000 hours with a sensitivity of 5.08 X 10""cm. In contrast with compressive creep, flexural creep in Polyscin is homogeneous with respect t o position and orientation in an extrusion. Relatively large creep rates (slope >5.08 X c m / h ~ l / ~ ) do not occur until the proportional limit of the material is exceeded, which is, on the average, 3.03 MPa. Maximum flexural creep was measured in a single crystal loaded in a principal-axis <100> direction. A displacement of 7.62 X 1 0 ' ~ cm was measured after 4000 hours at 1.72 MPa. Csl(Na) at 300 K (Compression) @ Less than 0.858 percent in 979 hours at 2.25 MPa (polycrystal) Less than 0.139 percent in 1126 hours at 2.69 MPa (<loo> single crystal) The dominant feature of creep behavior of the CsI(Na) is the high variability between crystal ingots. Vickers Hardness, H @ Range 7.74 t o 9.5 kg/mm2 (single crystal) @ Evaluated average 8.1 2 + 0.6 kg/mm2 The hardness of single-crystal NaI(T1) is independent of crystallographic direction. Susceptibility to Subcritical Crack Growth Stress-rate tests indicate that Polyscin NaI(T1) is not susceptible to subcritical crack growth in the stress-rate range from 0.1 32 to 63.8 MPa/second. Plastic deformation behavior is obsesved in this stress-rate range. At higher rates, in excess of 1.4 X 1 O4 MPalsecond, strength increased relative t o the values exhibited at the lower stress rates by a factor of 2 to 5 times, and brittle fracture was observed. Ingot Variation of Strength A study of the MOR, <loo> single-crystal cleavage strength of 38 NaI(T1) specimens from eight different ingots indicates that strength is ingot-dependent but homogeneous within an ingot. Aver- age strengths varied from 1.75 to 2.5 8 MPa. A similar study of the MOR strengths of 24 specimens from one extrusion of Polyscin and eight specimens from another extrusion indicated that the second extrusion was homogeneous with respect to strength, whereas the first one was not. Average MOR strength is extrusion-dependent, and varied from 4.03 to 5.6 MPa. The thallium concentration in NaI(T1) was measured for each of the MOR test specimens. No correlation was found to establish a strength dependency on the basis of thallium concentration. Table 1 (Continued) 7.62 X 1.27 dia 7.62 X 1.27 dia 12.7 X 1.90 X 1.90 12.7 X 1.90 X 1.90 12.7 X 1.90 X 1.90 *Table 4 defines symbols that appear in this table. Table 2 GSFC Purchase No. 2 (1982) *Table 4 defines symbols that appear in this table. **Four specimens from each of six different ingots. Table 3 N B S Purchase (1 980) 1.27 X 0.635 dia 1.27 X 0.635 dia 'Table 4 defines symbols that appear in this table. **One specimen from each of three different ingots. For <loo>, <011>, and < I l l > specimens, the mutually perpendicular direction pairs <010>, <OOl>; <0i 1>, <loo>; and <l TO>, <i 12>, respectively, were indicated on the specimen, and for parallelepiped specimens, these directions were perpendicular to prismatic faces. The 12.7- by 2.54-cm faces of the elastic moduli and Q-I specimens (property No. 5) were perpendicular to the <0i l > direction for <01 I> specimens and perpendicular t o the <Ti 2> direction for < I l l > specimens. Specimens were received from the manufacturers in sealed cans. The cans were opened, and the specimens were inspected in an N, gas-purged glove box. The specimens were subsequently inven- toried, stored in a bank of file drawers inside the dry glove box, and purged with the dry N, boiloff from an LN, tank as shown in Figure 1. The dryness of the purged gas was periodically measured with a hygrometer (Great Eastern Model 500) and was consistently found to be below a dew point of 253 K. CsI(Na) specimens were also kept in a dry glove-box facility at the National Bureau of Standards similar to the one shown in Figure 1 ; however, precautions to maintain the low humidity necessary for testing NaI(T1) were not necessary for CsI(Na). These specimens were tested in a laboratory environment (<50-percent relative humidity at 295 K). NaI(TL) specimens were kept dry and were isolated from ambient humidity at all times. They were transported, from dry storage boxes through air locks to the testing facility, in sealed containers filled with anhydrous CaSO, . Table 4 Symbols Key for Tables 1, 2, and 3 Material Type Key Nal (TI) Polyscin Nal (TI) single crystal Harshaw Chemical Company material Bicron Corporation material Initial end of extrusion Final end of extrusion Heart of extrusion Surface of extrusion Top portion of a single-crystal ingot Middle portion of a single-crystal ingot Bottom portion of a single-crystal ingot First dimension parallel to extrusion direction First dimension perpendicular to extrusion direction First dimension parallel to a Miller index <loo> crystallographic direction First dimension parallel to a Miller index <01 I> crystallographic direction First dimension parallel to a Miller index <I 1 I> crystallographic direction Random (orientation unknown) Three methods of humidity reduction were necessary for maintaining moisture levels below a dew- point of 253 K, depending on the size of the enclosure. Small enclosures were continuously purged with dry N, from LN, tanks or compressed-gas bottles. Larger enclosures maintained a dry N, purge coupled with open containers of anhydrous CaSO,. Because of the size of the creep facility, however, i t was necessary to install a drying train to maintain sufficiently low humidity levels. In some cases, polished scrap pieces of NaI(T1) were used as a visual moisture indicator; cloudiness indicates unacceptably high moisture levels. Coefficient of Linear Thermal Expansion Measurements were made according to ASTM E228-71 with a Custom Scientific Instruments Company model CS-128, quartz push-rod dilatometer. A National Bureau of Standards copper standard reference specimen was used to calibrate the instrument. The smallest displacement mea- surable with the apparatus was 7.62 X cm. one was a single crystal. In addition, two 3.81- by 1.27-cm diameter Polyscin rods were tested. These specimens were made from broken ultimate shear specimens. Finally, nine 6.35-cm long by 2.54-cm square bars were made from broken MOR specimens and were tested. Five of these speci- mens were single crystals, and the remaining four were Polyscin. Oil testing was done by using Penetone TPC cutting fluid as a medium. This fluid is a very low- viscosity oil in which NaI(T1) is quite insoluable and is used by manufacturers as a coolant in machining this material. Haake type FS and FK constant temperature hot and cold circulating baths were used for heating and quenching. Capacity of each unit was about 1.5 liters of fluid. The volume of the largest specimen was 0.1 5 liters. Both baths had a <1 K thermal gradient across the 10 centimeters from one side of the bath to the other. Temperatures of the bath fluid were mea- sured to within k0.5 K. Before testing, all specimens were closely inspected for flaws such as edge, face, or corner cracks. Flaw size and location were recorded for later use in fracture analysis. The testing involved heating the specimens for 30 minutes to allow them to thermally equilibrate, then transferring them as rapidly as possible to the cold bath, increasing the temperature difference between the two baths, if necessary, by 2 K increments until fracture first occurred. The difference temperature between the two baths at fracture was assumed to be equal to the thermal gradient at the specimen surface. Measured values of ATc at fracture were compared with predicted values for the various geometries and critical flaw sizes using the relation (reference 10): where E = Young's modulus = 2 X 10'' N/m2 v = Poisson's ratio = 0.3 14 K,, = critical stress intensity factor = 0.3 82 X 1 O6 ~ / m ~ / ~ G = fracture energy = K;, /2E = 5.1 J/m2 a = coefficient of linear thermal expansion = 47 X 1 OS6 K- I N = number of cracks per unit volume = 3 X lo5 m-3 Q = crack length (m) B = a geometric factor (4.67 for rods, 4 for square bars, and 3.25 for square plates, reference 12) k = thermal conductivity of NaI(T1) = 1.7 1 X 1 0-2 W/cm K a = characteristic length of specimen (volume to surface-area ratio) = 0.75, 0.54, and 0.29 cm for plate, bar, and rod specimens, respectively hc = calculated average surface heat-transfer coefficient, assuming forced convection in light oil = 0.019, 0.024, and 0.037 W/cm2 I(: for plate, bar, and rod, respectively As agreement between observed and calculated ATc was good, the foregoing equation was used to calculate an average heat-transfer coefficient in still air, assuming a crack size equal to the average in Polyscin and an average ATc observed for rod specimens tested in air. Air Quenching Two Polyscin rod and two Polyscin bar specimens were tested in air by heating the specimens in a convection oven for 30 minutes and then quenching into still room-temperature air (297 K). Tem- perature in the oven was read with a thermometer. The oven temperature was raised by 5 K incre- ments until fracture occurred on quenching. The critical temperature (ATc) was taken as the difference between oven temperature and room temperature. Thermal gradients in the oven were negligible over the dimensions of the specimen. The effects of specimen size were evaluated by plotting ATc versus surface area to volume ratio for both oil- and air-tested specimens. Heat Capacity The heat capacity of single-crystal and polycrystalline NaI(T1) was measured at 310 K using a Perkin-Elmer differential scanning calorimeter model DSC-1B with a heat capacity kit (P-E part No. 21 9-0136). This instrument uses a comparative method to measure the amount of heat required to raise the temperature of a specimen to a specified temperature at a predetermined rate. Ln the determination, a sapphire standard, the specific heat of which is accurately known and documented, is used as a comparison material. The measurement is based on the electrical power required for maintaining the temperature control of a specimen and a reference pan embedded in a specimen- holder assembly block. The system monitors and controls the average temperature of the specimen- holder assembly block and the difference temperature between the reference holder and the speci- men holder. Thus, by increasing the temperature of the specimen-holder assembly block to a specified value at a predetermined rate and monitoring the differential power needed for maintain- ing the same temperature in the specimen holder as in the reference holder, a measure of the energy supplied to the specimen and its holder is obtained relative to the reference holder and its contents, as a function of time or temperature. In practice, the average temperature of the specimen-holder assembly block is recorded on the abscissa while the differential power needed for maintaining an equivalent temperature between the reference holder and the specimen holder is recorded on the ordinate of the system's chart recorder. This method involves (1) heating the specimen-holder assembly block to a predetermined tempera- ture at a specified rate, using sapphire and an empty reference pan, (2) making a blank determina- tion with both pans empty, and finally, (3) scanning the same range using the specimen and an empty reference pan. In each determination, an ordinate pan shift from one baseline to a new base- line occurs. After correcting the ordinate amplitudes recorded for the specimen and the reference holder in light of the blank determination, the heat capacity of the specimen is calculated as: As Wr C (specimen) = --- X - P X Cp (sapphire) *I- s where As = ordinate amplitude (specimen) Ar = ordinate amplitude (sapphire) Wr = weight (sapphire) Ws = weight (specimen) Precision (standard deviation divided by the mean) of the measurement was less than 2 percent. Table 1 describes the specimens. Elastic Constants Density was determined 'for NaI(T1) compression specimens from first and second shipments by measuring their weight a id dimensions inside the dry glove-box storage facility. The elastic stiffness constants for single crystals, and Young's modulus and shear modulus for Poly- scin specimens, were measured in two ways. The first and most accurate method used a Panametric model 5054 time intervalometer to measure the transmit time of an ultrasonic pulse in the material, using a technique called pulse-echo overlap. In this technique, an acoustic transducer is used to inject a shear or longitudinal wave packet into a specimen. The same transducer is then used to pick up the echo of the wave packet after it is reflected from a surface that is perpendicular to the transmis- sion direction and that is situated at an accurately known distance from, and parallel to, the intro- duction plane. The transducer signal is applied to the vertical axis of an oscilloscope, and the horizontal axis of the oscilloscope is swept sinusoidally at a frequency whose period is the round- trip transit time of the wave packet, thus superposing the injected wave packet with its echo. Because the horizontal drive frequency can be generated and measured accurately, the time of flight of the wave packet can be determined accurately. The time between wave-packet injections was large and the period of vibrations within a wave packet was small compared with the round-trip time of a wave packet. This enabled multiple echoes to die out before new echoes were generated and enabled the injected wave packet and its reflections to be well-defined entities on the oscilloscope rather than interferring with one another. Longitudinal and shear wave transducers had a period of 0.44 ps. The time between wave-packet introductions was 0.01 or 0.001 times the typical round-trip flight times, which were 20 to 40 ys. Accuracy and precision of the measurement was 0.03 percent and was limited by the accuracy and precision of the measurement of specimen dimensions and density. Ultimate compression and MOR bend-test specimens were used for this measurement (Table I ). Similarly, Poisson's ratio is given by (reference 18, page 1 84) : where, in this case, the subscripts on v, E and the direction cosines (hkl) again refer to directions. Subscript 1 refers to the direction of axial strain, subscript 2 refers to the direction of lateral strain, and v12 is the ratio of lateral strain along direction 2 to axial strain along direction 1. In terms of the measured C 5 , the formulas for Eh,, , GI,, and v12 are: For polycrystalline specimens, the shear wave velocity is rotationally invariant, and only one longi- tudinal velocity (V,) and one shear velocity (V,) were measured. The formulas used to determine Young's modulus, the shear modulus, Poisson's ratio, and bulk modulus for these specimens were as follows (reference 19, page 1 8) : R = V,/V, v = (R2 - 2)/2 (R2 - 1) F = [(I - v)/( l + v)(l - 2v)I ' I 2 The other technique for measuring elastic constants and moduli was to determine resonant fre- quencies of vibration of appropriately held specimens, using an acoustic spectrometer. The elastic modulus calculations using measured resonant frequencies, specimen dimensions, and density are an iterative process and were done on a computer. For a rectangular bar of material, Young's modulus (E) and shear modulus (G) are calculated from the flat-flexural fundamental and'torsional fundamental as (reference 18, pages 84 and 90): where E = Young's modulus (dynes/cm2 ) G = shear modulus (dynes/cm2 ) p = density (g/cc) n = order of vibration = 1 f, = flat-flexural fundamental (Hz) F, = torsional fundamental (Hz) v = Poisson's ratio Q = specimen length (cm) h = specimen height (cm) b = specimen breadth (cm) T, and R are finite length correction factors. From these values of E and G, Poisson's ratio is calculated as (reference 19, page 89): Since v appears in the calculation for E, the procedure for calculating Young's modulus is to assume an arbitrary value of v equal to 0.3 in the calculation for E, then determine G and calculate v from the above equation, and iteratively arrive at a value for E. This iterative process is done by computer to less than 0.1-percent change in v. The accuracy and precision of the measurement was limited by how closely the resonant frequency could be read and by positioning the specimen grips. It is esti- mated that an accuracy and precision of about 1 percent was obtained. For single-crystal specimens, E and G are calculated as for Polyscin above. The elastic compliances are then calculated from the directional Young's modulus and shear modulus given by equations 2 and 3. For <loo> specimens, This gives two of the three elastic compliances. Compliance S,, is calculated by using the values of E iteratively derived for the <011> and < I l l > specimens and using the above-determined values for S,, and S,, from a <loo> specimen. For a <01 I> specimen, Figure 2. Closeup of the MOB test fixture used in test 6a (Table 1) . DIRECTION OF PUSH AND DIRECTION OF VELOC!?'! MEASUREMENT <oil > <001> DIRECTION <loo> <loo> (a) <loo> MOR SPECIMEN (b) <011> MOR SPECIMEN <I l l> DIRECTION OF PUSH AND DIRECTION OF VELOCITY MEASUREME <010> <loo> (c ) <I l l> MOR SPECIMEN F i g u r e 3. Diagram of single-crystal MOR specimen orientations. the test fixture had three-point loading with a span of 2.15 cm. In addition, the specimens were smaller (Table 3), being nominally 2.5 by 0.6 by 0.3 cm. Crosshead rates of 5.08 X lo-', 5.08 X 5.08 X 1 0-3, and 10.1 6 X 1 0-4 cmlminute were used. In addition to the modulus of rupture, the 0.2-percent offset proportional limit was determined for loading rates of 5.08 X lop1, 5.08 X 1 0-3, and 10.16 X 1 0-4 cmlminute. Shear-strength tests were made by using a double-shear fixture that is shown schematically in Fig- ure 4a. The fixture could also be used in single shear as shown in Figure 4b. Machine crosshead speeds of 0.0127 cmlmin and 0.0508 cmlmin were used. The specimens were nominally 1.27-cm diameter, 7.62-cm long rods, as indicated in Table 1. Compression testing of NaI(T1) at GSFC was performed in the enclosed Instron facility with a self- aligning loading fixture shown schematically in Figure 5. To prevent localized crushing from occur- ring at the ends, each specimen was placed into 0.158-cm thick end plate. At the beginning of each test, a crosshead speed of 0.00508 cm/minute was used until the 0.2-percent yield point had been reached. As the test continued, the crosshead speed was incrementally increased from 0.00508 to 0.127 cmlminute until cracking began and, finally, load-bearing capability was lost. Initial cracking was observed visually, using a high-intensity lamp. The increase in strain rate was necessary because most specimens exhibited a total strain greater than 20 percent. From load-deflection curves recorded (a) DOUBLE-SHEAR FIXTURE SHEAR AREA (b) SINGLE-SHEAR FIXTURE Figure 4. Full-scale drawing of test fixtures. 2.2 2 .o PURE BENDING I THREE-POINT, S/W = 8 I 1.8 THREE-POINT, S/W = 4 I I , / The coefficients A have the following values: Pure bending . . . . . +1.99 -2.47 +12.97 -23.17 +24.80 Three-poi nt: S / W = 8 . . . . . . . +1.96 -2.75 +I 3.66 -23.98 +25.22 S/W = 4 . . . . . . . +1.93 -3.07 +14.53 -25.1 1 +25.80 Figure 7. K calibrations for bend specimens (reference 22). RH and 300 K with two removable dryerlheater units, one of which is depicted in Figure 9. Each unit contained 4 pounds of anhydrous calcium sulfate. Two fans within each dryer recirculate the atmosphere within the RTV-sealed acrylic enclosure that covers the creep facility. Also within the recirculating unit is a thermostatically controlled cone heater for maintaining the temperature with- in the enclosure at 300 K. Sliding doors seal off the dryers from the enclosure for servicing and for regenerating the dryer material every 72 hours. Two units, one at each end of the enclosure, are capable of dropping the humidity of the N, gas in the enclosure from 25- to 0.5-percent RH in 2 hours. Inside the creep facility, fixtures were suspended from an I-beam frame mounted on a 1.22- by 1.83-m pneumatically suspended isolation table. The fixtures were dead-weight loaded. To accom- modate 18 simultaneous tests, six multiple-specimen fixtures were made, each designed to allow test- ing three specimens with one set of weights, as shown in Figure 10. In each case, the load is applied at the bottom of the fixture and is transferred through the lower specimen to the middle specimen, MULTIPLE TEST FIXTURE Figure 8. Multiple-creep-test facility. through to the upper specimen, and finally to the upper fixture in a chain-like fashion. Creep dis- placements were measured with a Pickering model 7303W-4A0, 6-volt input dc-dc linear variable- differential transformer, which gives a 0 - to 2-volt dc output for a 0 - to 0.508-mm displacement with a sensitivity of 0.508 X 1 0e3 mm. Initially, the middle fixture in each assembly was loaded to twice the nominally expected maximum service stress for the EGRET scintillator. The maximum expected service stresses were 1.29 MPa in flexure and 0.710 MPa in compression. In compression, stresses applied by the top and bottom fix- tures in a loading chain were about 20 percent more and less, respectively, than the stresses applied by the middle fixture in an assembly. In bending, loading of the top and bottom fixtures in a chain differed from the middle fixture by about 30 percent. The loading schedule for the assembly is shown in Table 5. As time progressed and some of the specimens were not observed to creep, the applied load was increased. Creep behavior was analyzed in terms of (time)lI3 equations: E = a + bt'I3 compression 6 = a + bt1I3 flexure Figure 9. Dryer/heater unit for enclosures. 33 TEST SPECIMEN WOBBLE ADJUSTMENT TRANSDUCER (LVDT) WER FRAME MEMBER Figure 1 1. A one-half scale cross-sectional view of the Csl (Na) creep test frame. test frames were built. The displacement transducers were LVDT-type transducers (Schaevitz Engineering model PCA-220-loo), which have a displacement resolution of 0.003 mm and a less than 0.1 percent deviation from linearity. The use of a wobble-plate linkage to couple the specimen and the transducer provides a means of averaging out errors in the signal that result from misalign- ment of the specimen. The CsI(Na) creep data were collected manually and were plotted using Dataplot (reference 9). Dataplot is an NBS-developed statistics, curve-fitting, and graphics program. The data were plotted in terms of a natural logarithm of time equation: where a,, a,, and a, are curve-fitting parameters. To perform extrapolations of the test results, another computer program, Runnonlin, was developed (reference 9). Using Runnonlin, the test results were extrapolated to times outside the test data base with confidence limits based on the test-data scatter. Hardness Vickers microhardness of NaI(T1) was measured using loads of 100, 300, and 500 g. The specimens were cleaned with toluene before indenting to remove any surface hydration, and no attempt was made to control relative humidity below laboratory ambient. The specimens were oriented so that their surface was perpendicular to the indenter and were aligned so that dimensions of the square- sh'aped surface impression were parallel to known crystallographic directions. Vickers hardness (Hv) was calculated using the equation: where P = load in grams d = mean diagonal of indentation in pm Susceptibility to Subcritical Crack Growth In addition to the stress-rate testing of Polyscin performed by NBS and described under modulus- of-rupture ultimate strength testing, GSFC performed three-point instrumented impact tests on single crystals of NaI(T1). An analog oscilloscope trace of the impact event was recorded as a load- time trace and was evaluated to obtain load to failure and time to failure. From these measurements, failure stresses and stress rates were calculated. Most specimens were cleaved from broken MOR bars. In addition, four 76- by 13- by 13-mm specimens were provided gratis from Bicron Corporation specifically for this test. Impact specimens of various dimensions were cleaved from these single- crystal bars by hand using a razor blade. After cleaving, the specimens were wet-ground and polished by hand using a fixture to maintain parallel sides. Pentone Corporation's TPC solvent and General Electric's silicone (SF97'-50) oil were used for grinding and polishing mediums, respectively. Speci- men edges were chamfered. The l o ~ g axis of a specimen was in the <100> direction, and prismatic surfaces were perpendicular to the other two principal axes. The majority of specimens were nomi- nally 6.4 by 6.4 by 64 En?. A total of 22 of these specimens were tested. The impact tester was a modified Custom Scientific Instruments, Incorporated, pendulum-type tester. The machine was modified by changing the impact mode from tension to three-point bending with a 50.8-mm span. A piezoelectric quartz load cell (Kister model 910) was used to pick up the impact event. The oscilloscope was externally triggered by the interruption of a heliumlneon laser beam to a solar cell by a metal flag attached to the pendulum head. The impact tester was enclosed in a sealed acrylic chamber that had four flapped-iris ports for access. Surgical gloves were worn when handling the specimens. Specimens were broken at stress rates ranging from 1.0 X 104 to 4.0 X lo4 MPa/s, compared with quasi-static stress rates of about 0.1956 MPa/s measured with a universal testing machine. Ingot Variation For NaI(Tl), ingot variation was investigated in a controlled manner (i.e., the same specimen sizes, test equipment, personnel, etc.) in terms of elastic moduli, modulus of rupture (MOR), and com- pressive initial cracking strength. This was done by measuring these properties for two Polyscin extrusions and by measuring the MOR of material from eight single-crystal ingots and the elastic constants for material from two of these single-crystal ingots. An uncontrolled measure of ingot variation of strength was obtained by comparing MOR values measured at GSFC with MOR values measured at NBS in stress rate testing to assess susceptibility to subcritical crack growth and with some early Polyscin strength measurements performed by Stanford University. For CsI(Na), ingot variation was investigated by comparing compressive strength (0.2-percent yield) and creep for three single-crystal ingots and for polycrystalline material from a fourth ingot. TEST RESULTS AND STATISTICAL ANALYSIS Specimens were classified and differentiated on the basis of: e Material type-NaI, CsI, polycrystal, or single crystal e Ingot-Four Polyscin ingots, eight single-crystal ingots e Position within an ingot-Initial and final end, surface, or interior of a Polyscin extrusion or top, middle, and bottom of a CsI ingot as designated in Table 4 e Orientation-Specimen dimensions relative to symmetry axes as designated in Table 4 In most cases, a statistical analysis determined whether observed differences in the mean value of a property for various classifications of material were significant. This analysis was done by pair-wise t-testing for significance of differences in mean values, after an F-test for equality of variances was In the temperature range 297 to 323 K, groups 1 and 3 do not differ significantly at the 90-percent confidence level. The grand average for these five material types is a = 47.21 + 0.44, n = 9. Thermal Conductivity (Nal(TI) at 300 K ) Multiple measurements were made on three oriented single-crystal NaI(T1) specimens and four Poly- scin specimens, and the results are summarized in Table 7. Table 7 Summary of Nal(TI) Thermal Conductivity Measurements 'Refer to symbols key in Table 4. There is no significant difference between ISl, FHR, FSl, <loo>, and <O11> specimens at the 90- percent confidence level. A grand average for these specimens is = (1.7 1 k 0.28) X 1 0e2 W/cm K, n = 43. Thermal Shock Resistance (Nal(TI) at 300 K) Critical thermal gradient (ATc), as a function of crack length (Q), was calculated for plate, bar, and rod specimens of NaI(T1) tested in light oil and for the condition of infinite heat transfer coeffi- cient using equation 1 (Figure 12). A minimum in the critical temperature gradient occurs a t a crack length of about 4.5 mm. The median edge-crack size observed in Polyscin is 2.5 mm. The area above a curve is a region of crack instability, and the area below a curve is a region of stability. Critical thermal gradient results for oil quenching are shown in Table 8. Comparison of these results with ATc determined from Figure 12 shows generally good agreement of predicted and observed results when the median flaw size is known. Effects of specimen size were extrapolated by plotting ATc versus surface-area-to-volume ratio for both oil- and airquenched specimens (Figure 13). As agreement between observed and calculated ATc was generally good, equation I was used to cal- culate an average heat transfer coefficient in still air, assuming a crack size equal to the median in Figure 12. ATc versus Q, specimen size, and specimen geometry for NaI(TI) quenched in light oil. Table 8 Results of Oil-Quench Testing of Nal(TI) 1 2 3 4 SURFACE AREA TO VOLUME RATIO (cm-') Figure 13. ATc surface-area-to-volume ratio for Nal(TI1 in air- and oil-quench tests. Polyscin (2.5 mm) and the average ATc observed for rod specimens tested in air (67 IS). A value of h = 1.2 X 1 0-2 W/cm2 I< was calculated. Heat Capacity (Nal(TI) at 300 K) Multiple measurements were made on two single-crystal and two Polyscin specimens, and the results are summarized in Table 9. Pair-wise hypothesis testing for equality of the means indicates that the means are equal at the 90- percent confidence level. A grand average is cp = 0.347 -t 6.61 X J/gK, n = 28. Elastic Constants (Nal(TI) at 300 K ) Results of the density measurements for Polyscin NaI(T1) are summarized in Table 10. An F-test for equality of variances fails at the 95-percent confidence level. A t'-test for equality of means indicates that the means are equal at the 95-percent confidence level, giving: Table 12 Elastic Stiffness Constants Mechanical damping efficiency (internal friction) was measured by using forced- and free-vibration measurements. Forced-vibration measurements of Q-' were used to assess material effects on Qml. These measurements reflect Q-' measured at the maximum amplitude attained by the specimen, and these results are shown in Table 13. Free- and forced-vibration measurements for single-crystal material and for Polyscin were averaged to assess the frequency dependence of Q-l, using free- vibration estimates of Q-l of maximum amplitude. These results are shown in Table 14. Finally, strain amplitude dependence of Q-' was observed for many specimens at the flexural, torsional, and longitudinal fundamental frequencies, manifest as a nonlinear decay curve of log amplitude versus time in the free-vibration method. Internal friction was highest at maximum amplitude and dropped to a low limiting value with time as the vibrational amplitude dropped to zero. Ultimate Strengths Na I (TI) a t 300 K Tables 15 and 16 summarize single-crystal bending strengths for NaI(T1) and Tables 17 and 18 summarize Polyscin bending strengths. All the specimens tested were of the large variety (1 2.7 by 2.54 by 2.54 cm). Although smaller Polyscin bend specimens (2.5 by 0.6 by 0.3 cm) were also tested, the results for them are summarized in the section on "Susceptibility to Subcritical Crack Growth." Figures 14 and 15 with Tables 19 and 20 give Weibull analyses for Polyscin and single- crystal material, respectively. Thallium ion concentration measured by atomic absorption using pieces of the fracture surfaces ranged from 416 to 1423 ppm. No correlation of strength to thallium ion concentration was ob- served. Tables 21 and 22 summarize single-crystal compressive strength results for NaI(Tl), and Tables 23 and 24 summaize the results for Polyscin. Table 13 Material Effects on B;Lrce, of Na l (TI) Resonance Specimens at Fundamental Resonance Frequencies *Refer to symbols key in Table 4. Table 14 Internal Friction of Nal(TI) a t Various Frequencies Table 15 Nai (TI) Siiigle-Crystal Critical Resolved Tensile Stress iii Bending 'Refer to symbols key in Table 4. "Applied stress rate of 0.1 956 MPaIs. Table 16 Results of Pair-Wise t-Testing of Groups 8 Through 13 from Table 15 t signifies arithmetic means are equal at 90-percent confidence level. X signifies arithmetic means are not equal at 90-percent confidence level. Table 19 Pol yscin Wei bul l Analysis of Large Specimensx Table 20 Single-Crystal Weibull Analysis of Large Specimens* '12.7 by 2.54 by 2.54 cm. t~ = n / ( N + 1 ). Table 21 Nal (TI 1 Single-Crystal Cornpressi\~e Strength *Refer t o symbols key i n Table 4. Table 22 Results of Pair-Wise t-Testing of Groups 1-6 from Table 21 t signifies arithmetic means are equal a t 95-percent confidence level. X signifies arithmetic means are n o t equal a t 95percent confidence level. Table 23 Nal (TI) Polyscin Compressive Strength Data *Refer t o symbols key i n Table 4. Table 24 Pair-Wise t-Test Results of Groups 1-4 t signifies arithmetic means are equal at 95-percent confidence level. X signifies arithmetic means are not equal at 95-percent confidence level. Tables 25 and 26 summarize single-crystal and Polyscin shear strength results, respectively. The NaI(T1) criticalstress intensity factor was measured by using two methods: the double canti- lever beam method and the notched-beam method. Tables 27 and 28 summarize the test results for the double-cantilever-beam method and the notched-beam method, respectively. Using the data from Table 28, Figure 1 6 is a log plot of the stress intensity equation, where Y = a geometry constant of = the stress at failure a, = flaw size at failure Csl (Na) at 300 K Tables 29 and 30 summarize 0.2-percent compression yield strength results for CsI(Na). In addition to the single-crystal results, five polycrystalline specimens from ingot 1 had an average yield strength of 2.98 + 0.22 MPa. Creep Na I (TI) at 300 K Table 3 1 and Figures 17 and 18 summarize flexural creep results for Polyscin and single-crystal NaI(Tl) materials. The equation that was fitted to the creep data was: Table 29 Csl (Na) Single-Crystal 0.2-Percent Compressive Yield Strength (MPa) Ave +S 3.49 + 0.33 Ave +S 5.1 1 rt 0.14 *Refer to symbols key in Table 4. Table 30 Increase in Average Compressive 0.2-Percent Y ie ld Strength o f Csl(Na) w i t h T ime *Five polycrystalline specimens. Table 31 Summary of Nal(TI) Flexural Creep Results 'Refer to symbols key in Table 4. 'second loading after initial creep at lower stresses. where 6 = midpoint deflection (em) t = time (hours) a = intercept constant in a t1I3 plot of 6 b = slope constant in a t1I3 plot of 6 Tables 32 and 33 summarize compressive creep results for Polyscin and single-crystal NaI(T1) ma- terial, respectively. The equation that was fitted to the creep data was: 3 .O 2 .o Symbol Location Slope x lo6 (cm/hrl l3 1 Figure 17. Flexural creep of Polyscin Nal(TI). 6.0 - SYMBOL ORIENTATION Q <loo> 5.0 - A <Ill> <Oll> rn I POLYSClN CURVE SLOPE ~"1' (cm/hr'l3) Figure 18. Flexural creep of Na l (TI) single crystals. 10 20 30 SLOPE (rnicr~strain/hr"~) Figure 19. Compressive creep summary for Polyscin. Table 34 Summary of Constants and Residual Standard Deviations for Csl (Na) Creep Curves Applied Ingot Stress No. (MPa) 1 2.54 Pol y-X 1 4.23 Pol y-X 2 2.69 100 3 5.99 100 3 11 .oo 100 4 2.07 100 4 4.21 100 4 9.72 100 4 4.49 4 18 4 2.4 1 111 a, = a dimensionless fitting parameter (microstrain) a, = a fittikg parameter with the dimensi~ns of time (hours) Hardness (rcda!(TI) at 300 K) Table 35 summarizes the hardness values for single-crystal NaI(T1). Table 35 Vickers Hardness Values for Single-Crystal Nal (TI) I Load. The mean value for Vickers hardness is 8.12 + 0.600 kg/mm2 . Susceptibility to Subcritical Crack Growth (r\lal(TI) at 300 K) Tables 36 through 39 summarize the results of the stress-rate testing for Polyscin NaI(T1). Figure 20 and Table 38 give a Weibull analysis of groups 3 and 4 from Table 36. Ingot Variation In extrusion 1, Polyscin material from the beginning of an extrusion is consistently weaker than material from the end of an extrusion, whereas in the second extrusion, this distinction is not so clear cut. Analysis of the results of the single-crystal NaI(T1) MOR indicates that each ingot exhibits a characteristic strength with a small standard deviation. On the other hand, Polyscin groups exhibit higher average strength and a rather large standard deviation relative to <loo> cleavage strengths. This difference can be explained in terms of the rather small Weibull modulus for Polyscin (-4) relative to the Weibull modulus measured for single-crystal material (- 17). Materials with a large Weibull modulus typically exhibit a characteristic strength because of the rather small range in the magnitude of worst flaws. Such materials typically exhibit very little size effect on strength. In con- trast, materials with a low Weibull modulus exhibit a large amount of scatter in strength values be- cause of the large range in magnitude of worst flaws. Such materials typically exhibit a significant size effect on strength since the adequate sampling of a flaw population with a large range in the magnitude of worst flaws becomes difficult with small specimens. Table 36 Na I (TI) Polyscin Stress-Rate Bend-Test Resu Its *These specimens were broken in three-point bending with a gauge area of 2.15 by 0.6 cm. Specimens reported in ingot variation testing (Table 17) were broken in four-point bending with an inner and outer span of 3.39 and 10.15 cm, respectively. Because for brittle materials, strength is a function of specimen size and type of loading, it i s necessary to make the measurements comparable by using a formula derived from Weibull statistics: where - al = average strength in three-point loading E2 = average strength in four-point loading m = Weibull modulus for Polyscin S1 = area of a 2.15- by 0.6tm tensile face of the small three-point bend specimens S2 = area of a 10.16- by 2.54-cm tensile face of a hypothetical large three-point bend specimen The first factor in the above equation converts four-point loading to an equivalent three-point loading. The second factor converts a large specimen tensile surface to a smaller one, and the third factor converts a square cross-section specimen to an equivalent rec- tangular cross-section specimen (references 26 and 27). Table 37 Pair-Wise t-Testing Results for Average Flexural Strengths Listed in Groups 1-4 from Table 36 t signifies arithmetic mean strengths are equal at 95-percent confidence level. X signifies arithmetic mean strengths are not equal at 95-percent confidence level. Figure 20. Weibull plot of Polyscin MOR data (small specimens). 2. Strength of single-crystal material is ingot-dependent in both NaI(T1) and CsI(Na), with CsI(Na) showing the greatest variation. Strength of material within a single-crystal ingot appears to be homogeneous. 3. Strength of Polyscin NaI(T1) is extrusion-dependent and dependent on position within an extrusion. 4. Strength of Polyscin NaI(T1) exhibits a significant size effect as predicted by Weibull analysis. 5. The 0.2-percent yield strength of CsI(Na) appears to become greater with time. 6. Creep in both NaI(T1) and CsI(Na) is ingot-dependent. 7. Although evidence shows that NaI(T1) does not exhibit subcritical crack growth at stress rates of less than 65.8 MPa/s in less than 3-percent RH room temperature environment, there is some mechanism that more than triples its average strength between stress rates of 65.8 and 1.0 X lo4 MPals. Possible explanations for this behavior are a susceptibility to subcritical crack growth at very high stress rates and/or a strength-enhancing brittle trans- ition redon between stress rates of 65.8 and 1.0 X 1 O4 MPals. 8. No correlation was found to establish a strength dependency on the basis of thallium con- centration. RECOMMENDED TESTING FOR DESIGNERS Polycrystalline forms of NaI(T1) and CsI(Na) can be produced by using various forming techniques that utilize temperature and pressure to produce a grain structure in material that is originally a single crystal. Modulus of rupture and compressive creep are considered to be the most diagnostic strength-related measurements for these materials. Creep testing should use a wobble-plate linkage to couple the specimen and the transducers as a means of averaging out errors due to specimens mis- alignment. These tests, coupled with a K,, measurement and determination of elastic moduli using the pulse-echo overlap technique, should provide a good indication of how these newer materials compare with extruded NaI(T1) and single-crystal NaI(TI) and CsI(Na). ACKNOWLEDGMENTS This study was initiated by Mr. William Cruickshank who, before his death, contributed greatly to its direction and scope. The authors wish to thank Dr. Robert Hartmann, Mr. Earl Angulo, and Dr. Carl Fichtel of the Goddard Space Flight Center for their assistance in determining materials to be tested and for their continued support. Appreciation is also expressed to Dr. Guy Eubanks and Mr. Walter Viehmann for their guidance and critical analysis. Special thanks are given to Messrs. Ronald Hunkeler, Edward Sanford, and Carl Walch for their technical support in making measure- ments. Helpful suggestions from Drs. Barrie Hughes and Robert Hofstad'ter of Stanford University, Dr. James ICurfess of the United States Naval Research Laboratory, and.personne1 from both the Harshaw Chemical Company and Bicron Corporation are greatfully acknowledged. Engineering and Design Properties of Thallium-Doped Sodiurn Iodide and Selected Properties of Sodium-Doped Cesium Iodide Performing Organization Code Goddard Space Flight Center Greenbelt, Maryland 20771 ~ o n s o ~ n g Agency Name and Address National Aeronautics and Space Administration Washington, D.C. 20546 I 11. Contract or Grant No. I 13. Type of Report and Period Covered Reference Publication 1 14. Sponsoring Agency Code I 15. Supplementary Notes K. F o r r e s t , C . H a e h n e r , T . H e s l i n , M. M a g i d a , a n d J . U b e r : G o d d a r d S p a c e F l i g h t C e n t e r , G r e e n b e l t , M a r y l a n d . S . F r e i m a n , G. H i c h o , a n d R. P o l v a n i : N a t i o n a l B u r e a u o f S t a n d a r d s , Mechanical and thermal properties, not available in the literature but necessary to structural design, using thallium-doped sodium iodide and sodium-doped cesium iodide were determined to be coefficient of linear thermal expansion, thermal conductivity, thermal-shock resistance, heat capacity, elastic constants, ultimate strengths, creep, hardness, susceptibility to subcritical crack growth, and ingot variation of strength. These properties were measured for single and polycrystalline materials at room temperature. Alkali halides, Mechanical properties, STAR Category 3 1 Scintillators, Gamma-ray detectors Unclassified-Unlimited 1 Unclassified I Unclassified 1 76 1 A05 I "For sale by the National Technical In format ion Service, Spr~ngfield. 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