Calibration of gamma spectrometer PDF

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Summary

Manual on how to set up a gamma ray spectrometer, how it works, what to do, and how to calibrate it to get accurate readings when taking data....

Description

LABORATORY INSTRUCTION FOR PHYSICS 277 GAMMA RAYS SPECTROMETER 1. Introduction to gamma spectroscopy system The components of a standard gamma spectroscopy system are shown in Figure 1 and include a scintillation crystal, a photomultiplier (PMT) tube, a high voltage supply, an amplifier, and a multi-channel analyzer. Optional data output devices include CRT monitor, a plotter, and/or a PC. Using a set like that one can detect gamma radiation, as well as establish its frequency (or energy per quantum) composition. SHV cable

USB cable

Linear Amplifier

Scintillation Detector

Multichannel Analyzer (Easy-MCA)

BNC cable

PMT Tube

High Voltage Power Supply

Laptop (Maestro software)

BNC cable

ORTEC ACE Mate

Fig.1a. Components of a gamma spectroscopy system. Schematic representation.

The Model P-2000 Scintillation Detector is an example of one of the most common gamma ray detectors in use. The scintillator is sodium iodide crystal containing about 0.1% of thallium iodide impurity, which activates the scintillation process. Sodium iodide material is chosen for its high density (3.67 g/cm3) and high effective atomic number (Z=53 for iodine), which both increase probability of gamma interaction. When interaction takes place, certain fraction of the energy left in crystal by the gamma ray is transformed into a flash of visible light, called "scintillation". With NaI(Tl) scintillator this conversion is specially efficient. The visible light emission takes place at wavelengths around 410 nm, matching the maximum response of the photodetector (PMT), and resulting in its high voltage output. Pasco set uses a 1.5x1 inch cylindrical chunk of a NaI crystal for the scintillator. The purpose of having large size crystal is to maximize the probability of absorbing gamma quanta. The scintillator and the PMT reside in an aluminum can, which allows gamma radiation in while screening out the unwanted ambient light.

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The PMT tube detects the light from the crystal and produces a voltage spike of magnitude proportional to the intensity of the observed light. Since the amount of light in the scintillation flash is proportional to the energy deposited in the crystal by gamma ray, the PMT output voltage can be considered a direct measure of the energy of the absorbed gamma ray. The electronic amplifier built into the Ortec ACE Mate unit provides both appropriate pulse shaping and amplification. The amplifier differentiates and amplifies the PMT pulse and produces a fast-rising positive voltage pulse that has approximately Gaussian shape and lasts about 2µs. That pulse, like the original PMT output, is proportional to the energy deposited in the crystal by the gamma ray. Pulses from the amplifier are directed to the input of the analog-to-digital converter of the multichannel analyzer (MCA). MCA's memory accumulates and stores pulse height information in a digital form. This inforamtion is then collected by the MAESTRO software to display a spectrum, typically of pulse count (how many events) versus the pulse amplitude (Energy) of each detected radiation event. Since the gamma quanta produced by a radioactive sample possess identical energies if the detector response was ideal, the PMT voltage spikes would, in principle, have the same amplitude, and all pulses from the amplifier would be collected in a single MCA memory location (channel) for the same gamma ray. In reality, even for monoenergetic gammas, MCA would register somewhat diverse pulse amplitudes due to random fluctuations in energy transformation ratios in the detector and to alesser extent, amplification. Moreover - several physical processes (to be investigated during the next lab) may lead to a partial absorption of gamma energy in the crystal of the detector and the appearance of wide variety of pulses with drastically lower amplitudes, in addition to the typically nice and symmetric "total absorption peak". Sources with several original gamma energies have, correspondingly, more complicated spectra. The purpose of the lab is to become familiar with the operation of the gamma spectrometer, determine its energy resolution, and use it for identification of an unknown gamma radiation source. 2. Experiments a. Energy resolution of the NaI(Tl) detector Introduction Resolution is a measure of a detector's ability to resolve (differentiate between) gamma rays of different but similar energies. The usual way to express the resolution is by giving the Full Width of the total absorption peak at Half of its Maximum (FWHM). For a NaI(Tl) scintillator detector, it is conventional to measure FWHM using a monoenergetic source of gamma rays from 137Cs and to express resolution as a ratio of peak width (in

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channels) to the peak centroid channel number. The result is then recorded in percent. Typical resolution for detectors used in this lab is between 6 and 8% which is pretty good even for real research detectors. The smaller the resolution, the better is the detector, since smaller energy differences between incident gammas can be detected and identified.

Equipment P-2000 Scintillation Probe ORTEC ACE Mate signal amplifier and detector bias supply Analog-to-digital converter and Multi-channel Analyzer (ORTEC Easy-MCA) Laptop BNC, SHV, and USB cables Radioactive Sources

Procedure -

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Set up the gamma ray spectroscopy system as shown in Fig.1. Position a 137Cs source in front of the detector. Begin collecting gamma ray events (counts). Adjust the amplifier gain until you can see the 137Cs spectrum highest peak ("total absorption peak", or "photopeak") at the lower half of your spectrum. Acquire a spectrum until the peak is very well formed and clearly visible. Record the spectrum (Save) in ASCII format (Ask your instructor). Fit the 137Cs Photopeak with a gaussian curve (e.g. using SciDAVis) and estimate the photopeak amplitude (in channels on the x-axis – proportional to energy). Determine the Full Width at Half Maximum (FWHM) from your fit or as shown from your spectrum and compute the resolution in percent. (Don’t forget to calculate the error!) Comment: How good is this detector’s resolution?

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Fig.2 Typical 137Cs photopeak and resolution determination. The numbers shown for the resolution could vary between detectors and are provided only as an example. Yours will be (most probably) different.

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Role of the amplifier

Introduction The amplitude of the voltage pulses reaching the MCA depends on the amount of energy the gamma rays leave inside the scintillator, and on the gain setting of the linear amplifier. In this experiment we demonstrate that the amplifier gain influences the energy calibration (gamma energy - peak position relation), but practically has no effect on the spectrometer resolution. Equipment Gamma spectrometer and gamma source used for part "a". Procedure

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Keep the 137Cs source in front of the detector. Acquire a spectrum. Check if the photopeak is still at the lower half of the spectrum. What happens when you double the gain? Has the centroid channel for the photopeak moved to double the channel number on your spectrum? Can you explain why? Does the peak look wider? Calculate again the resolution. Has it changed? Reduce the gain by a factor of 2 with respect to the starting situation. Acquire a third spectrum. What happens to the peak centroid position, and resolution? Summarize your observations in your own words. Calibrating the energy scale

Introduction One can consider the multi-channel analyzer in our experimental set-up as a sorting machine for voltage pulses originating at the PMT. Its role is to register the voltage pulses at different memory channels, according to pulse amplitude (or if you think about it, proportional to the absorbed gamma energy). The exact relation between gamma quantum energy and channel number in which the voltage pulse gets registered must be determined by experiment, by investigating sources of "known" gamma energies. Once this calibration procedure is finished, the gamma spectrometer can be used for the determination of gamma energies produced by unknown sources, and for their identification.

Equipment Standard set of equipment, set of gamma sources. Procedure -

You will be given a set of gamma sources. To find the gamma energy of any source you can use the following link: http://nucleardata.nuclear.lu.se/toi/ Adjust the gain of your spectrometer so that you can see the most intense photopeaks of the given gamma sources. Record the gamma energies (from the online database) and the corresponding photopeak positions (from the monitor screen) in an excel spreadsheet. Plot the energy calibration curve for the spectrometer, showing gamma energy on vertical axis and the centroid channel number on the horizontal. Your points should fall along a straight line for this detector. This is the "energy calibration curve". Determine the line slope. How many different energy points are necessary to get a reliable calibration? Use the least squares formulas to calculate in your spreadsheet the error in the slope and in the intercept of the fit.

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Acquire a spectrum for an unknown gamma source supplied by the instructor. Keep the spectrometer running, until all major photopeaks contain enough counts so their centroid channels can be established with confidence. Use the spectrometer calibration curve to determine gamma energies represented in the unknown source spectrum. Consult the above online resources to identify each gamma-emitting nuclide in your sample. Estimate the energy error in channels of your measurement of the peak energy and use it as a guide when searching for the reference data.

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