1/6/2005 4:09 PM Physics Project Lab

Lab B4: Gamma-Ray Spectroscopy, or the Creation and Annihilation of Antimatter

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Prerequisite: Lab B1: Ionizing Radiation Safety

Learning Goals

• Understand the sodium-iodide scintillation detector.
• Learn how a pulse-height analyzer works.
• Learn the different ways in which gamma rays interact with matter.
• Understand the energy-level and decay schemes for cobalt-60.

I. References

· Absorption of g-rays in matter: R.A. Dunlap, Experimental Physics (Oxford, 1988), pg. 285

· See Dunlap for the theory of the sodium iodide detector (Ch. 10) and the theory of alpha, beta, and gamma decay, and the photoelectric effect, the Compton effect, and pair production (Ch. 11).

· Standard sodium-iodide spectra - look at the Scintillation Spectrometry Gamma-Ray Spectrum Catalogue in the lab.

· Data on ⁶⁰Co and other isotopes: see chart on the wall, and Table of Radioactive Isotopes (a large book), by Browne and Firestone.

· For operation of the MCA, see its manual, Series II Personal Computer Analyzer, Operation and Instruction Manual.

II. Radiation Safety

Before doing this experiment, you should do the radiation-safety experiment, to familiarize you with the survey meter and the personnel dosimeter.

The source used for most of the measurements in this experiment is a cobalt-60 gamma-ray source, encapsulated in a disk of orange plastic. Radiation levels of up to 10 or 15 mR/hr may be observed near the source. We will also use a beta source, Tl-204 (green plastic tabs), and an alpha-source, Am-241. Because the range of alpha particles is so small, the alpha source is less well encapsulated than the others, so be especially careful not to scratch the active surface.

Use the green survey meter (Technical Associates model PUG 1AB, with P-11 probe) to monitor radiation levels in the lab, and wear one of the XETEX model 415A personal dosimeters while in the lab. Record the dosimeter reading at the start of the lab period and at the end. Usual laboratory safety practices should be observed (e.g., no eating in the lab, wash your hands afterwards.)

III. Theory

Gamma rays interact with matter in various ways, all producing energetic electrons. These electrons undergo collisions with other electrons. In a gas or liquid, this eventually produces a large of positive and negative ions. In a solid, electrons are excited from the valence band to the conduction band.

In scintillation detectors these ions or excited electrons produce light. In a good detector, like NaI(Tl), the light output is proportional to the number of excited electrons produced, which is proportional to the energy deposited. A photomultiplier detects the light by the photoelectric effect on the cathode of a multi-stage photomultiplier tube. The photo-electrons are accelerated between stages of the tubes and produce extra electrons on collision with the tube dynodes. The collision electrons are then accelerated and multiplied in the following stages of the tube. The total gain is very large so that the output of a PM is millivolts per incident photon. The gain of the PM is varied by varying the accelerating voltage.

Gamma rays interact with matter principally through three processes - the photoelectric effect, the Compton effect, and pair production. The first two processes are interactions with electrons, and have cross sections proportional to the electron density. The third is an interaction with the nucleus, and has a cross section proportional to Z². Pair production dominates at energies above ≈1 MeV, and is more important for heavy (i.e., high-Z) elements. For a graph of the relevant cross sections, see figure 11.7 in Dunlap. The Feynman diagrams for these processes are shown in figure 1.

The photoelectric effect. Incident photons can be absorbed by electrons only in the field of a nucleus; the nucleus absorbs substantial momentum, but only a negligible amount of energy. The resulting electron produces a cascade of ionizing particles, by brehmstrahlung and direct collision with other electrons. About one ion pair per 30 MeV of energy is produced.

The Compton effect. This process is just photon-electron elastic scattering, and does not require the participation of a nucleus. Note that the electron does not receive all of the energy of the gamma ray. The scattered gamma ray has a wavelength l’ given by

.

Here l is the wavelength of the incident photon, j is the photon scattering angle, and l_C = 2.427 x 10^-2 Å is the Compton wavelength of the electron, as shown in figure 2.

It is fairly common for the scattered photon to escape from a NaI crystal without
interacting. This produces a spectrum of pulses smaller than those corresponding to complete absorption of the gamma ray, as shown in figure 3b.

Pair production. Here a g turns into an e⁺e^- pair, with momentum, but little energy, transferred to a nucleus. The positron comes to rest and annihilates in the crystal via the reaction

.

Both g's have exactly 511 kev of energy. One or both may escape from the crystal, giving rise to spurious "escape peaks." (See figure 3c.)

III. Experimental Procedure

A. Using the Tracerlab Detector. Connect the preamp power (from the rear of a Canberra 816 amplifier) and the high voltage (about 1200V, POSITIVE high voltage); see the equipment list at the end of this writeup for more information. Now look at the output on an oscilloscope. Place a cobalt-60 source on the detector. Can you see the pulses? Sketch a typical pulse in your lab book.

Now put the pulses through a Canberra 816 amplifier, to shape and amplify them for the MCA. Set the gain so that the pulses do not go over eight volts, the saturation level of the MCA. Sketch a typical pulse in your lab book. What does the amplifier do?

Can you see "lines" (i.e. a predominance of pulses of a particular amplitude)?

B. Using the MCA. A gamma-ray line from a nuclear transition, when absorbed in the detector, will deposit a fixed amount of energy. The height of the pulse is proportional to this energy. However, these pulses are mixed in with pulses of other heights from other processes. The instrument we will use is called a Multi-Channel Analyzer (MCA), or sometimes a Pulse-Height Analyzer (PHA). It identifies pulses in the input signal, determines their height, and enters them in a histogram of pulse height. It also displays and prints out the data, provides calibration and peak-finding routines, and performs disk I/O. The instructor will probably start the program for you. The output of the amplifier goes into a BNC connector on the back of the computer. F1 starts and stops data acquisition, and CTRL-F2 erases the data. The program gives good on-line help. A setup configuration is described in Appendix A. For more information, see the PCA-II manual.

C. Gamma-ray Spectra. To make a histogram of the cobalt-60 spectrum, start up the MCA and collect a spectrum. Be sure to record HV, amplifier, and MCA settings in your lab book. Print out the spectrum and put that in your book, too.

Calibration. Look at the cobalt-60 spectrum. It should have a Compton shoulder leading up to a 1.17 MeV peak, followed by an equally intense 1.33 MeV peak and then dropping to a much reduced "sum peak" (use the log output display to see all the features at the same time). A simple way of calibrating the instrument is to adjust the amplifier fine gain until the 1.17-MeV peak falls on bin 117, the 1.33-Mev peak on bin 133, etc. Record the channel number (using the cursor) of the central value for all three peaks as well as the half-height values. Note that if the gain or high voltage is changed then a new calibration is required.

What is the resolution of the detector? That is, about how accurate is the energy determined from one pulse? Nuclear physicists usually measure this in terms of the full width at half maximum (FWHM) of the peak, in energy units. Determine the width of the three lines in this xpectrum. Does the resolution depend on energy?

Cesium-137. With the system calibrated on Co-60, measure the spectrum of ¹³⁷Cs. Then study the square on the Chart of the Nuclides representing this radioactive nuclide. How does it decay? What gamma lines do you expect to see? What other features do you see in the spectrum? Can you explain all of them?

This spectrum should show a number of interesting things. Below the peak corresponding to total absorbtion of the gamma ray, there should be a "Compton tail." Compton scattering with subsequent escape of the scattered gamma ray should produce a continuous spectrum of pulse heights. The maximum electron energy corresponds to a gamma ray scattering angle of j = 180° (backscattering). Calculate this maximum energy, using the formula given earlier.

¹³⁷Cs produces beta rays as well as gamma rays. Do you see evidence for them in your spectrum? Try recording a spectrum in one segment of memory (1 of 8, for example). Then put a thin piece of lead between the source and the detector, so that betas would be absorbed, but not gammas. Take another spectrum in another segment of memory, for the same length of time. Compare the two. What do you conclude?

Finally, compare your gamma-ray spectrum with one from the Scintillation Spectrometry Gamma-Ray Spectrum Catalogue.

Sodium-22. Now try the same thing for ²²Na. Try using the Chart of the Nuclides to predict what you should see. Also consult the appropriate page in Table of Isotopes, available in Xerox form in a binder in the lab. What is the dominant decay mode? What is the meaning of the peak at about 0.5 MeV? Do you understand the source of this peak? Note that sodium-22 is a beta-plus emitter. How well can you measure the mass of the electron?

Based on these measurements, make the best argument that you can for the existence of antimatter. What properties of antimatter have you demonstrated?

Neutron capture. Some interesting things go on near the large neutron howitzer. Thermal neutrons are captured in the paraffin inside the howitzer, and produce a high-energy gamma ray, via the reaction

followed by

.

This gamma ray is very penetrating (exponential absorption length in lead of about 2 cm), and is energetic enough to produce pairs in your detector.

Get your detector as close as possible to the neutron howitzer and take a spectrum. You will see various sorts of background, including that from the high-energy gammas. See what you can learn about anti-matter.

IV. OPTIONAL EXPERIMENTS

The spectrum from the detector depends on the detector geometry. Raise the sample 10 centimeters above the detector. Does the spectrum change? Can you explain the change?

Measure some unknowns. There is a sample of mercury that was exposed to 800 GeV protons. What elements were created in the interactions? There is a very interesting orange milk pitcher in the lab. Can you identify the active isotope in the glaze?

The width of the peaks that you observe is determined by counting statistics on the photons. The width should be proportional to 10, where E is the energy of the gamma ray. Can you see any evidence for this dependence in your data? Can you explain why the width varies with energy as it does?

Appendix A. Setup for the Nucleus PCA-II Computer-based Multi-Channel Analyzer

1. The PCA-II software is installed on halley2. Click on the icon to start it.

2. Use the menu to choose the following options. mode PHA, gain 1024, display 1024, memory 1/8, vertical scale log.

3. Take the signal from the Tracerlab detector with +1200 V high voltage, and put the signal through a Canberra 816 amplifier, with gain = 4 x 4; this should give a sensitivity of about 3 keV/channel

Appendix B: Setup for the Stand-Alone Multi-Channel Analyzers.

Here we give recommended settings for all the parameters, knobs, and switches for taking a first spectrum. There are separate sections for the Canberra MCA and the Northern Scientific MCA.

The Canberra 8100e MCA. Photomultiplier state number 78111, +1000 volts, into ``Amp In'' Preset Count = off Preset time = large, live Coarse Gain = 1000 Fine gain = 0 Function = PHA I/O = ,plot LLD = 0.5 ULD = 10.0 Data = add I/O Cycle = manual Conversion gain = 1024 baseline = 4.4 memory control transfer = 1/1 6 white switches -- all down vertical range = log To take data, push ``collect.'' To see data, push ``collect'' again.

Setup, Northern Scientific MCA Turn memory unit on first, then MCA chassis. For positive pulses, go through a Canberra Model 816 amplifier, gain 4 x 9, unipolar, positive polarity, and enter at "high level." Co v. gain 512 zero level 0 1/1 memory Tracerlab PM, ``med. HV,'' signal to ``Amp In'' Display Selector = data Amplifier Gain = 2.0 Horizontal = x1 Read Mode = Display LLD = 1.0 ULD = 10.0 Multiplier = 1 Memory = 1/1 Baseline Restorer = passive Coupling = AC ADC input = Amp Zero level = 0.0 Conversion Gain = 512 preset = inf Display = Log Program -- all switches up Digital zero offset -- all off Gate Switch = anticoincidence To take data, push OFF, then push START. To see the data, push OFF, then READOUT.

V. Equipment

A. Tracerlab NaI scintillation counter. This is the counting well on wheels, State Number 45292. This counter used to run off a battery pack, Northern Scientific model #308, which supplied both high voltage and preamplifier power. Now it uses preamp power from a Canberra 816 amplifier, and high voltage from a Canberra model 3002 power supply. It uses positive high voltage, HV £ 1200 V.

B. Canberra NaI scintillation counter. This counter is in counting well State # 78111. This counter works fine too. It is in a smaller, blue-glazed shielded holder. The equipment used is: • Canberra Model 802-1 NaI detector (aka Bicron model 1.5M1/2), with Canberra model 802-9 base. • Power supply (POSITIVE high voltage) to go to about 1000 volts. One option is the Canberra model 3002 power supply, 0 to 3000 V. The other option is the supply labeled Phys Tech Group, 500 to 2500V; the connector on the rear is a special one, and t here is a corresponding cable and adaptor box to get you to the type of connector on the counter. • Canberra model 816 amplifier; the amplifier is not necessarily needed, but the preamp built into the counter base gets its power from the rear of the 816 amplifier module.