Gamma-ray Spectroscopy

Gamma Ray Description

1. Note that there is NO eating or drinking in the 111-Lab anywhere, except in room 286 LeConte on the bench with the BLUE tape around it. Thank You the Staff.

The purpose of this experiment is to study some properties of the gamma ray. First this lab will walk you through some tests to show you how the equipment is affected by your equipment settings. You will then use the gamma rays from some known sources to calibrate your detector, and verify the inverse square law. Finally, you will make some measurements that will allow you to calculate the mass attenuation coefficients for several materials at several energies.

The experiment offers a good acquaintance with a number of important devices, including a pulse height analyzer, scintillator and photomultiplier tube (PMT). It is well suited for data analysis with a computer.

The gamma rays enter a NaI(Tl) scintillator crystal, which converts them into many lower energy photons. These photons travel through the crystal to the photocathode of a photmultiplier tube, where they are converted into electrons by means of the photoelectric effect. The photo-electrons are sent through a series of electrodes where the number of electrons is multiplied. At the anode, a pulse of current is produced.

The number of lower energy photons produced in the scintillator is very nearly proportional to the energy of the incident gamma ray, and the number of electrons produced is proportional to the number of photons incident on the photocathode. Therefore, the pulse height is proportional to the energy of the incident gamma ray. A pulse height analyzer is used to display the spectrum of pulses from the photomultiplier tube.

1. Pre-requisites: None
2. Days Alloted for the Experiment: 6
3. Consecutive days: No

All pages in this lab. Note To print Full Lab Write-up click on each link below and print separately

I. Gamma-ray Spectroscopy

This lab will be graded 20% on theory, 30% on technique, and 50% on analysis. For more information, see the Advanced Lab Syllabus.

Before the Lab

Complete the following before your experiment's scheduled start date:

1. View the Gamma Ray video.
2. View the 'Radiation Safety Video' After watching the video in the 111-Lab, get a pink Radiation Safety form from a 111-Lab staff person. Fill it out & sign the form for getting a Radiation Ring.
3. Now complete the Radiation Safety Training Radiation_Safety After completion of Training turn in all forms to Don Orlando.
4. Complete the GMA Pre Lab and Evaluation sheets. Print and fill it out. The Pre-Lab must be printed separately. Discuss the experiment and pre-lab questions with any faculty member or GSI and get it signed off by that faculty member or GSI. Turn in the signed pre-lab sheet with your lab report.
5. View the Introduction to Error Analysis video and Error Analysis Notes.
6. Also view Light Sources and Detectors video.

Suggested Reading: 1. Knoll, Radiation Detection and Measurement, John Wiley and Sons, New York (1979):

a. Ch. 1 §IV pp 16-25 (Information about the sources of EM radiation),

b. Ch. 2 §III pp 62-71 (How gamma rays interact),

c. Ch. 3 §I-VI pp 79-95 (Detectors in general)

d. Ch. 9 §I-§III pp 272-284 (Photomultiplier tubes)

e. Ch. 10 §I-IV(B) pp 306-338 (Scintillators).

2. RCA 6810A Photomultiplier Tube PMT 6810

3. Harshaw Scintillation Phosphors; Harshaw Scintillators and NaI Crystal Information

More References

You should keep a laboratory notebook. The notebook should contain a detailed record of everything that was done and how/why it was done, as well as all of the data and analysis, also with plenty of how/why entries. This will aid you when you write your report.

Introduction

The measurement of energy levels of atomic, molecular, and nuclear systems constitutes a large part of experimental physics. This experiment examines gamma rays, which come from transitions between nuclear energy levels, with emphasis on their interaction with matter. This experiment is a little different from most in the 111 Laboratory in that a lot of what you do will be oriented towards learning about the equipment and its capabilities, rather than striving to achieve some experimental result.

The goals of this lab are to learn about gamma ray spectroscopy using a doped sodium iodide scintillator, a photomultiplier tube and a pulse height analyzer. In particular, you will measure the spectra of several radioactive sources, verify the inverse square law for radiation, determine the absolute intensity of the cesium source, and determine the mass attenuation coefficients of several materials at several energies.

Apparatus

1. PMT with base and Fluke 415B High Voltage Supply
2. Pre-Amplifier
3. DG535 Digital Pulser
4. High Voltage divider Box (1000:1)
5. National Instruments NI-5124 Digitizer Card (Used for digital Soft Scope and Pulse Height Analyzer)
6. Oscilloscope

Safety

Remember the Lead Bricks are heavy, at least 30 to 50 pounds each. If it is knocked off the bench and falls onto someones foot it will smash it to pieces.

You must wear a radiation ring when you are working with the gamma-ray apparatus,. You should also always wear vinyl gloves when handling the radioactive sources, but remove and discard them when adjusting the equipment or you will defeat the purpose. The sources are located in a lead container, and this should be kept closed. Keep the sources in their plastic bags. When you use the sources, put the clamps on the bags, not on the source itself inside the bag. Any rupture of the source package will cause leakage of radioactive materials - very low-level radiation and not a serious health hazard, yet it will require discarding the source and a decontamination of the bench area or wherever the source has been.

Do not stack the lead bricks on the lab bench - it would be very dangerous to do so. They are quite heavy and the kinetic energy they would gain from a one-meter fall is more than that enough to shatter a human toe (we know this from direct experience). In addition, you will find experimentally that some of the gamma rays can Compton scatter off these bricks and contribute to the scattering portion of your spectra.

The radioactive sources are in plastic bottles with the radioactive material embedded in epoxy. The original activity and date are on the label of the bottle. The sources emit radiation in all directions. To use a source, hang the bottle by grasping its top with a clamp so that the source is at the same height as the center of the detector.

The following are the radioactive sources used in this experiment.

Source Energy (MeV) Half-life
22Na 0.511, 1.28 2.6 years
137Cs 0.6616 30 years
60Co 1.17, 1.33 5.2 years
54Mn 0.84 312 days Source weak

Detection

The gamma rays in this experiment are detected with a thallium-doped sodium iodide [NaI(Tl)] crystal (1.5" in diameter and 1" in height) mounted on a PMT (Ortec 905-4 Model; 3M3/3X with tube base Ortec # 266). The maximum voltage supplied to the PMT is +800 Volt DC. This crystal emits photons in the visible range when struck by gamma rays, and is hence called a "scintillating" crystal. These visible photons are detected and amplified by the photomultiplier tube, which outputs a pulse of electrical current with an amplitude proportional to the energy of the incident gamma ray. This pulse is further amplified with an external amplifier and then fed into a NI-5124 Pulse Height Analyzer.

The Pulse Height Analyzer (PHA) is a LabView program, "PHA-5124.exe", using the National Instruments NI-5124 Digitizer Card. National Instruments donated this card to us for the Physics 111-Lab. You first check the signals out of the pre-amplifier with the Soft Scope program using the NI-5124 Card. Pulses of varying amplitudes between 0 and 8 volts are sampled into 1024 increments (or what you select in program control Tab). When a pulse representing a detected gamma ray is received, the PHA-5124 determines its amplitude and puts it into the correct channel (bin). Because the amplitude expressed as a voltage is proportional to the energy of the incoming gamma ray (a fact you should explain in your report), the accumulated counts form a distribution called an energy spectrum on the screen - the number of gamma rays that have been detected in each energy bin.

Procedure

1. Before you turn on the Fluke high-voltage power supply (at the top of the rack), check to see that the POLARITY is POSITIVE and all black dials should be set to zero. Turn on the power supply by toggle the switches on the front panel, the 'power' one first and then the other one when the 'stand-by' light comes on. When the red lights are on, set the VOLTAGE to +780 volts. Once set, do not change these controls! The 1000:1 VOLTAGE DIVIDER just scales down the voltage so that the DMM can read it -- a reading of about +0.76 V on the DMM, for example, corresponds to +780 volts going to the PMT. To have stable operation, it is better to leave the high-voltage power supply and the voltage divider running during the period that you sign up for the experiment.
Fig. 1: PMT Output, 137Cs Source Scope: 50 mV/div; 0.5 μsec/div
Set the high voltage to +780 volts. Place the 137Cs source 25 to 30 cm away from the detector with lead colliminator in front of the source. Make sure that there is no other lead brick or stacks of aluminium plates near the source (why?). Look at the output of the PMT on a fast scope (Tektronix type) then look at it with the "Soft Scope program using the Digitizer NI-5124. To match impedance, use a 50-ohm terminator on a BNC "tee" going into the scope. Look for a faint signal about 0.5 μsec wide and -50 mV high (see Fig. 1). You'll have to play with the triggering to get this, and you'll probably have to turn up the trace intensity and shield the screen from glare. Each trace of this signal is the electron pulse coming from the PMT that represents a gamma ray striking the detector. The signal is blurred because of many negative pulses with different amplitudes coming from the PMT.
2. The CANBERRA AMPLIFIER (amp), located in the equipment rack, amplifies the small pulses from the PMT to the range (0 to 4V) necessary for the PHA-5124 program. Feed the output of the PMT into the amp (you don't need a terminator here). Set the INPUT switch to NEG (since the PMT output are negative pulses), and the MODE switch to UNI (since the pulses are unipolar - that is, they are entirely negative as opposed to a signal that has positive as well as negative parts). Look at the output of the amp (without a terminator); you should see a positive pulse about 1 μsec wide followed by a negative pulse that is of no interest (see Figure 2). Watch the effects of varying the gain controls. Set the gain to produce a pulse with an amplitude of around 4 volts.
Fig. 2: Amplifier Output Scope: 2 V/div; 1 μsec/div

Pulse Height Analyzer (PHA Digitizer)

1. The PHA-5124 is explained here. For this experiment we use the National Instruments Digitizer Card NI-5124.
1. To start, plug the output of the amp into the Digitizer channel 0 located at the rear of the computer. Using a 60Co source you should see a spectrum similar to Figure 3.
Figure 3: PHA display. Counts vs Energy of 60Co (Click on Picture to enlarge)
2. The other adjustments you should experiment with are the LLD on the Control Tab in the program: this sets the Lower Level Discriminator so that the PHA will ignore pulses with amplitudes below the setting. This way the system doesn't spend its time with unwanted pulses like low-level noise. To set it properly look at the Soft Scope program display. Setting the LLD too low will allow the program to trigger on noise, and overload the program. Set the LLD at a reasonable level that is above the noise level, but below the level of the physical processes that you are interested in.
3. Use a computer with Matlab or some other program to display the channel number and counts of any channel that you want to see.
4. After setting the LLD, STOP the unit from acquiring, and ACQUIRE again. You should see one peak start to form in the middle of the display, along with a "mess" forming to its left. The peak corresponds to the 0.662 MeV 137Cs gamma rays; what are the peaks to its left? Press STOP to stop the accumulation of counts when the peak is reasonably well formed. See Figure 3.
2. Remember The Maximum Voltage is +800 VDC. Measure the relative gain of the photomultiplier tube as a function of high voltage using the photopeak of the Cs source. The purpose of this exercise is to gain familiarity with the gain of a photomultiplier. It is not an examination of the non-linearity of the gain curve. To do this, vary the high voltage to the PMT using the knobs on the 3 KV High Voltage power supply only, and watch to see where the Cs peak moves. Because the peak channel number is equivalent to some pulse height, by recording the peak channel number at each voltage we can determine the relative gain as a function of the PMT high voltage. Start at around +400 volts, and stop at about +800 volts (change the high voltage by 10 volts at a time) to determine the relation between gain and high voltage. Do not exceed 800V. Plot the results. Explain the plot, particularly the behavior at the higher (more positive) voltage settings.
3. To examine the capabilities and linearity of the amplifier, use a Pulse Generator (PG) to simulate the detected pulses that you saw coming from the PMT in step 1. To make the PG's pulses as small as those from the PMT, connect the PG to the dB attenuator (a small box with a row of toggle switches that selects the attenuation). Use a BNC tee to send the attenuated output of the PG to the input of the amplifier as well as to the scope - you need again a BNC tee and 50 ohm terminator on the scope-input to match impedance. Set the PG such that the pulse width is approximately the same as that of the pulses from the PMT. Now choose an amp gain setting, and vary the amplitude of your input pulses and record the corresponding peak channels. Do this for at least three gain settings and determine whether the amp is linear for each. (Be careful not to set the PG Repetition Rate so high that overwhelms the PHA.)
1. To get you started with the DG535 Pulse Generator try the following settings.Using the Output button select: channel = AB, AB= Var., Load = HighZ, amplitude= 0.5V, offset= -0.5V. Using the Delay button set (This sets the pulse width): B=A+0.000001 s, A=T+0.000000 s, channels C and D are not used. Using the Rate button set (this sets the pulse frequency): Rate= 8500 Hz, Trigger= INT. The output from the -AB (marked with |_| symbol) channel should now be a 1μs-wide square negative pulse at a rate of about 1.2 μs (i.e. frequency of 8500 Hz). Don't forget to use the 50 ohm terminator to view the pulse on the oscilloscope.
2. Also look at the effects of varying the repetition rate of the PG: do the counts under the peak increase as you expect? As you increase the repetition rate, you may begin to see a phenomenon called pileup. This is when a pulse comes along before the electronics have finished processing the previous pulse. The lesson to learn here is that if you place the source too close to the NaI detector, your spectrum will be skewed due to pile up.
4. Choose a high voltage somewhere near the middle of the range and an amp gain setting in the linear region such that you are utilizing the full scale of the PHA; that is, the highest energy gamma ray (which comes not from the 137Cs but from the 22Na source) should appear near the end of your spectrum, not in the middle. Then obtain a spectrum for each source listed above (2-5 minute runs each).
1. Determine carefully the peak channels and the full width at half-maximum (FWHM) of the peaks in the spectra, and then calculate the resolution at the energy of each peak. Compare your measurement with the theoretical line width for each gamma ray. Remember that you need to estimate the uncertainty in each of your determination.
2. Also compute the 180° back-scatter and Compton edge energies of the gamma ray(s) for each source, and compare to the observed spectra. See the book by Knoll for a description of what the plot should look like.
3. Use the computer connected to the apparatus to obtain the data and plots of all of your spectra. There is a built-in timer in the equipment. Once your spectra are available on the computer, you may use Matlab to perform background subtraction and peak-fitting.
4. Once you have determined the best combination of High Voltage and Gain Settings, keep these settings fixed. Eventually, you should calibrate your voltage axis using the known energy values. If you vary your settings, your energy calibration can change drastically.
5. Take another 137Cs spectrum with a large aluminum block behind the source. Explain the difference between this spectrum and the one seen in part 6.
6. Verify the inverse square law for radiation using one of the radioactive sources. *Remember* putting the source closer than 25 cm to the detector will skew your spectrum. For a given detector with a fixed size and gain, how does the rate of data collection vary with distance from the source?
7. Compute the absolute intensity of the 137Cs source using the published values of the NaI efficiency. The NaI crystal must not be near or enclosed by lead shielding for this measurement. Subtract the background. To reduce background, support the source with a low-mass stand. Use $I_{measured}=I_{absolute}(n(E)\frac{\Delta\Omega}{4\pi})$ where n(E) is the intrinsic efficiency and the quantity in brackets (the product of n and the solid angle) is given in the NaI Crystal Information for a distance of d = 30 cm.
1. Use the 22Na and 137Cs sources and several sheets/blocks of different thickness to measure the mass attenuation coefficient, in cm2/gm, of Al, Cu, and Pb [see reference (ii)]. Compare the results to the accepted values. Think about doing background subtraction, and be sure to include some discussion of uncertainties.
2. When measuring the mass attenuation coefficients it is important to have proper geometry. Your source, collimators, absorber, and detector must all be in a line. The source should be placed on this line so that the greatest intensity of gamma-rays will go toward the detector, not toward the ceiling, table, PHA, etc.
3. There should be two lead bricks with holes in them at the apparatus. Use these bricks as collimators (see figure below). The first collimator passes only photons that strike the absorber nearly perpendicular to its face. The second collimator is needed to absorb the gamma rays that have undergone small angle scattering from the absorber. If the angle is small enough these gamma rays might enter the detector and be counted as unaffected gamma rays. It is possible for gamma rays that are scattered by the collimators to reach the detector, but these are of such low energies that they do not affect your results significantly.

References

2. Segre, Emilio. "Appendix A: Scattering from a Fixed Center of Force." Nuclei and Particles, Second Edition, WA Benjamin Inc. London (1977).

3. Yoshizawa, Yasukazu. "Beta and Gamma Ray Spectroscopy of C 137",Nuclear Physics 5, 1958, pp. 122-140. #QC770.N8

5. Equipment Manuals; GMA Equipment Manuals

6. DG535 Digital pulser DG535 Manual

7. R. D. Evans, The Atomic Nucleus. McGraw Hill (1972).

Other reprints and reference materials can be found on the Physics 111 Library Site