Gamma Ray Coincidence Detection with Sodium Iodide Scintillators: Setup and Data Analysis, Lab Reports of Art

Download Gamma Ray Coincidence Detection with Sodium Iodide Scintillators: Setup and Data Analysis and more Lab Reports Art in PDF only on Docsity! 1 COMPTON SCATTERING EXPERIMENTAL OBJECTIVES: To verify the kinematics of Compton scattering. In addition one should verify that the relative probability of scattering into a given solid angle is given by the Klein-Nishina formula. Note that you will have to make energy dependent corrections to your data, make measurements of the solid angles of the source and both detectors, and measure count rates, taking into account detector efficiencies. Please realize that you have a finite amount of time to complete this experiment. The most time consuming part of the experiment is the setup. Plan to spend a lot of time in the lab the first couple of weeks, so that by the end of the second week you have already taken at least a sample data run. REFERENCES: Read Mellissinos 6.3 or any Modern Physics text such as Tipler for the Physics background. Read Mellissinos 5.4 and any of the many detector catalogs for a description of scintillation detectors, and the Lecroy catalog on time coincidence techniques. Use Mellissinos and/or the MIT experimental procedures as guidelines for general procedures and data analysis. Our setup differs from Mellissinos' by replacement of the inert aluminum scatterer by a Sodium Iodide (NaI) scintillator connected to its own photomultiplier tube. This allows us to define an event as the coincident arrival of pulses from two detectors, thus reducing background noise by orders of magnitude. In addition we get as a bonus the ability to measure the energy spectrum of the recoiling electron. We’ll refer to this detector as the electron energy analyzer (EEA). The 2 scattered gamma ray (the 662 keV photon from a Cesium (Cs) 137 source) is detected by a second NaI(Tl) scintillator connected to another photo-multiplier tube, henceforth referred to as the photon energy analyzer (PEA). NaI is used because it is an efficient gamma ray detector. The sum of the energy in the electron and the photon analyzer/detectors add up to the original 662 keV for all scattering angles. This experiment lends itself to a computerized data acquisition system. THE APPARATUS: The apparatus, shown in the figure, is comprised of the two photomultiplier tubes (PMTs) and scintillation detectors, pre-amps, voltage amps, shaping (spectroscopy) amps, NIM (Nuclear Instrument Module) discriminators, a coincidence unit (AND gate), and a TTL Gate Generator (used for gating the pulse height analyzers). 5 for proper techniques and operation of the electronic modules before doing the experiment. The other critical aspect of the experiment is calibration and gain adjustment of the photomultiplier tubes (PMT's). The height of the pulses from the PMT's and hence the gamma energies (and electron energies) must be adjusted to fall within the display range of the Pulse height Analyzer. The height of the charge pulse (gain) is a function of PMT voltage and the amount of amplification of the pulse signal. Insufficient gain will cause the pulse height spectrum to be shifted too far to the left; excessive gain will cause the spectrum to appear shifted too far to the right. Refer to the schematic diagram for the system. Because of the particular voltage and impedance requirements at each step in the signal path, three different types of amplifiers must be used - each performing a specific type of function. The pre-amps within the PMT bases are charge sensitive amplifiers that adapt the impedance of the PMT to that of the Spectroscopy amplifiers. The Philips X10 amps boost the signal voltage from the PMT anodes to an appropriate range for the NIM logic modules. You’ll note there are two different signal outputs on each of the PMT bases. One output is from the pre-amp, and the other is taken directly from the anode of the PMT. The direct signal has a negative polarity which is suitable (after amplification) for the NIM logic modules. The output from the pre-amps has positive polarity and is suitable for the Spectroscopy amplifiers. The logic circuits are used to generate the GATE, and the Spectroscopy amp signals are sent to the PHA for energy analysis. Use the oscilloscope at each step of the electronic setup, as you must visually determine that your signals have the correct amplitude and timing. The gain, or amount of amplification is controlled first by the amount of high voltage applied to the PMTs, and secondly by the gain settings of the spectroscopy amplifiers. Excessive PMT voltage will overdrive the pre-amps and amplifiers causing pulse shape distortion and poor energy resolution. This distortions appears on the oscilloscope as a flattening to the peaks of the larger pulses. Make sure the high voltage is set low enough that none of the pulses in the range of interest become distorted. Ultimately, you must adjust the high voltage such that the amplitude of the pulses at the input to the discriminators fall between the -30 mv and –1V discriminator limits. Then set the gain of the spectroscopy amps to the point where both the highest and lowest energies to be measured in the experiment will appear on your PHA spectrum. 6 It is suggested that you initially place the millicurie Na22 sources directly between the plastic scintillator and the NaI. Na22 is a positron (e+) emitter. The positron stops in the source and annihilates with an electron (e-), it's antiparticle. This releases two 511 keV gamma rays in opposite directions simultaneously (the rest mass of the e+, e- is 511 keV). It is these two coincident gamma rays which allow you to observe a substantial coincidence rate. Check to make sure that the pulses from both of the detectors arrive at the coincidence unit (via the discriminators) within a few nanoseconds of one another. Adjust the propagation delay time using the delay box and/or various lengths of cable. Once your system timing is properly set, and you are able to produce spectra, you will need to make an energy calibration. It is suggested that you calibrate each detector in the singles mode, which means one detector at a time – rather than in coincidence. This is easily done by disabling one of the inputs on the NIM coincidence module. You should use the microcurie Na22 source as well as a microcurie Cs 137 source (not the millicurie one since the NaI detector will be saturated if it is directly in line with this source). Also calibrate at a lower energy such as 122 keV with Co 57. You should adjust the spectroscopy amp gain so that all energies of interest (for example between 100 KeV and 662 KeV) corresponding to the different scattering angles fall between channels 1 and 2048 of the PHA. You will need to know the energy of each gamma ray used in the calibration. Use the Chart of the Nuclides to find this information. Record the channel number that corresponds to the photopeak for each gamma energy used. Fit a line to the data to determine the calibration factor in energy per channel. You may prefer to use the calibration utility incorporated into the computer program. Note that if you have a reasonable coincidence rate but are not acquiring counts on the screen, the pulses from the detector may be either too large or too small to display within the range of the pulse height analyzer. If this is the case and the analyzer is receiving a proper gate pulse, then adjusting the high voltage or the spectroscopy amp gain may bring your spectrum into range. (Hint: if the pulse height spectrum extends all the way to channel 2048, you may be missing something off to the right of the spectrum.) Adjust the gain so that the spectrum fills as much of the screen as possible without going overrange. This will allow you to resolve the lower energies during the experiment. At the bottom (low energy edge) of the spectrum, make sure the discriminator threshold is not set above the pulse height that corresponds to the minimum energy you wish to record in your experiment.