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Queen’s University Department of Physics

PHYS 352

M.C. Chen and R. Knobel

GAMMA RAY SPECTROSCOPY Introduction There are two aspects to radiation detection: counting and spectroscopy. A counter detects the presence of radiation and can be sensitive to individual ionization events. Measuring the rate of events in a counter gives information about the intensity of the radiation. In spectroscopy, it is the energy of the radiation quanta that is of interest. Many radiation detectors function simultaneously as counters and spectrometers. The energy of the gamma rays being emitted by a given sample corresponds to the energy level difference in the nuclear transition that was responsible for the emission of those gamma rays. If a sufficiently distinctive measurement of this energy can be made, it could indicate the presence of a specific isotope in the sample. Applications of gamma spectroscopy include isotope geochemistry, environmental measurements and neutron activation analysis (a process in which a sample is activated in a neutron flux and the gamma rays emitted by the newly-activated isotopes give information on the elemental composition of the sample). In general, one might consider gamma spectroscopy for any situation when one wishes to determine the elemental (or isotopic) composition of a natural or man-made substance, ranging from monitoring of nuclear materials to the dating of old bottles of wine! How do gamma-ray detectors work? A gamma ray must first interact in the detector before it can be detected. A gamma ray that passes through a detector without interacting is not detected because it leaves no trace of its passage – that’s because the photon is a chargeless particle. How often gamma rays pass through a detector without interacting is one factor that determines a detector’s efficiency. Gamma rays interact in matter in three ways: photoelectric absorption, Compton scattering, and pair production. In photoelectric absorption, an atom absorbs the gamma-ray photon and an energetic electron is ejected, leaving behind an ion. Compton scattering is the limit of the previous process when the binding energy of the electron becomes negligible compared with the energy that’s transferred from the photon to the recoiling electron – it can be modeled as scattering off of quasi-free electrons. A photon with energy greater than 1.022 MeV has enough energy to convert into an electron-positron pair. Pair production only occurs within the vicinity of the nuclear Coulomb field. In all three cases, the photons that interacted produce energetic electrons. It is the subsequent energy loss of these energetic (charged) electrons that produces more

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ionization along the recoiling electron’s short path. Gamma ray detection thus comes down to detecting this ionization produced by recoiling electrons in the detector. Roughly speaking, there are three ways to detect ionization. A gas-filled detector, such as a Geiger counter, collects and measures the ionization by drifting electrons and ions in a gas, using large electric fields. With the development of semiconductor technology came the ability to fashion detectors in which charge (in this case electron-hole pairs) can be transported within the semiconductor solid, obviating the need for gas. The third category is scintillation. In some materials, ionization excites optically active states in the material (molecular energy levels or electronic bands in a crystal). Light is given off when these states de-excite – this process is called scintillation. The amount of scintillation light is usually proportional to the amount of the ionization; hence, measuring the amount of scintillation light serves to measure the total ionization produced (or energy deposited) by the gamma-ray interaction. In this lab, you will look at two types of gamma-ray detectors that are commonly used: sodium iodide (NaI) scintillator and a high-purity germanium (semiconductor) detector. The focus of this lab is on energy resolution, very important for spectroscopy. Resolution What determines the energy resolution of a gamma-ray detector? Let’s start by considering that to produce ionization there is some average energy per ionization that is required. If the total energy deposited by a gamma ray is E = 1 MeV (the energy of the recoiling Compton-scattered electron, for example), and ε = 2.98 eV is the average energy required to make an electron-hole pair in a Ge detector, then on average there would be 336,000 electron-hole pairs to be collected. If a detector could collect 100,000 of these electron-hole pairs, this would be a statistical sampling, or measurement, of the total amount of ionization produced. Poisson statistics should describe the distribution of the amount of charge collected. Thus, this measurement of the ionization has a standard deviation of = 316, which gives rise to an uncertainty in the number of electron-hole pairs detected. If 100,000 electron-hole pairs corresponds to 1 MeV, then an uncertainty of 316 corresponds to ΔE = 3.16 keV, and the relative resolution is: ΔE/E = 0.3%. The statistical nature of the measurement process is often the dominant factor that determines the energy resolution of a detector. Note that there are two “counting” processes: the counting of electron-hole pairs that occurs internally (microscopically) and this determines the energy. There is also the counting of the number of radiation interactions in the detector, a measure of the intensity of the radiation incident on the detector – don’t confuse these! Compare the above example to scintillation. In NaI it takes 25 eV of ionization energy, on average, to produce a scintillation photon. For the same 1 MeV of energy deposited there would only be about 40,000 scintillation photons to be collected. Usually you are doing pretty well if you can collect 1/10 of these emitted photons. Thus, a scintillation measurement of 1 MeV would have a standard deviation = 4000, or 1.6%, as a

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rough estimate. For spectroscopy applications you can expect that a Ge detector, with its better energy resolution, would be superior. Nevertheless, NaI scintillators are still used in some circumstances because of their cost effectiveness per size and their efficiency. Experiment There are two setups for this lab. One group will work with the Ge detector while the other works with the NaI detector, then you will swap. Photos of the equipment are in Fig. 1.

Fig.1 Ge detector (left) and NaI scintillator (right).

1) Ask the TA or lab instructor about the equipment. Make sure you understand how each component works and what is connected to what. Understand the differences between the setups for the NaI detector and the Ge detector. In your report, draw schematic diagrams.

2) Start the data acquisition software and calibrate the energy spectra that are being collected. Sources that emit gamma rays with known energies are available. Use 2-3 peaks to calibrate the detector. Briefly describe the calibration in your report.

3) Using thorium oxide, 137Cs and 60Co radioactive sources, measure the energy resolution (both absolute and relative) of various peaks as a function of energy. It will be easier to accomplish this with the Ge detector, due to its high resolution; nevertheless, try to do this with the NaI detector also. Be sure to include the 2.6 MeV gamma ray from the thorium source, and try to measure another 4 peaks with the thorium source, spanning across all energies.

4) Ge detector only: record the spectrum from a 22Na source, a positron emitter.

Measure the energy resolution of the 511 keV gamma line. In your report, you will answer the question: why does this “line” appear broader (appears to have worse energy resolution) than you would predict from plotting your other data (like in Question 6)?

5) Measure the efficiency of the detector (both Ge and NaI) and compare them in

your write-up. To measure the efficiency, place a source in a known geometry with respect to the Ge detector and determine the count rate (number of detected interactions per second) for events in the full-energy peak. Then, place the same

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source in the same, known geometry for the NaI detector and determine the count rate in the same fashion. Calculate the efficiency of each detector. How do Ge and NaI compare?

6) Plot the energy resolution of the Ge detector as a function of gamma-ray energy (this plot should include the 511 keV data point). Plot the energy resolution of the NaI detector as a function of gamma-ray energy.