From Physics 111-Lab Wiki

Contents 1 Development 2 Gamma Ray Description 3 Gamma Ray Spectroscopy Photos 4 Before the Lab 5 Introduction 6 Apparatus 6.1 Safety 6.2 Radioactive sources 6.3 Detection 7 Procedure 8 Pulse Height Analyzer (PHA Digitizer) 9 Gamma Layout 10 References

Development

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Media: DetectorCalibration.pdf
Media: GMA_todo.pdf

Gamma Ray Description

1. Note that there is NO eating or drinking in the 111-Lab anywhere, except in room 282 LeConte on the bench with the BLUE tape around it. Thank You the Staff.

The purpose of this experiment is to measure the energies of gamma rays (photons) emitted from a variety of radioactive sources. The gamma rays enter a NaI 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.

The experiment offers a good acquaintance with a number of important devices, including a pulse height analyzer, scintillator and photomultiplier tube. It is well suited for data analysis with a computer . You will calibrate the system using the known energies of the gamma rays emitted by the radioactive sources, examine the characteristics of the system, observe the Compton effects that occur within the detector, verify the inverse square law of radiation, and determine the absorption coefficients of several materials.

1. Pre-requisites: None
2. Days Alloted for the Experiment: 6
3. Extra Study: 6/10
4. Difficulty of Lab Work: 6/10
5. Analysis: 5/10
6. Combined Overall: 6/10

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 30% on theory, 30% on technique, and 40% on analysis. For more information, see the Advanced Lab Syllabus.

Comments: E-mail Don Orlando

Gamma Ray Spectroscopy Photos

Gamma Ray Rack Click here to see larger picture

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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' on a computer in the 111-Lab. Go to the shared 111-Lab drive [ U:\Advanced Lab Share\Safety Manuals and Videos ] and then click on Radiation Safety Video located in the My Computer directory on the 111-Lab Network Share Drive. 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 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

4. RCA Photomultiplier Tube Manual

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 than 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.

Your goals for this lab are to learn about gamma ray spectroscopy using a 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 a minimum of 30 to 50 pounds each. If it is knocked off the bench and falls onto someones foot it will smash it to pieces.

When working on the gamma ray apparatus, you must wear a radiation ring. You should also always wear vinyl gloves when handling the radiation 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 massive and the kinetic energy they would gain from a one meter fall is more than that required to shatter a human toe (we know this from direct experience). Besides, experimentally you will find that your gamma rays scatter off these bricks, contributing to the Compton scattering portion of your spectra.

Radioactive sources

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

There are four radioactive sources used in this experiment.

Detection

The gamma rays in this experiment are detected by a thallium-doped sodium iodide [NaI(Tl)] crystal mounted to the PMT (Ortec 905-4 Model; 3M3/3X with tube base Ortec # 266). Maximum Detector voltage 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 a photomultiplier tube, which outputs a pulse of electrical current whose amplitude is proportional to the incident photon energy. This pulse is amplified by 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 has 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 and divides this range into 1024 increments (or what you select in program control Tab) of a volt per interval. When a pulse representing a detected gamma ray is received, the PHA-5124 determines its height and bins it. Because the voltage heights are proportional to the energy of the incoming gamma rays (a fact you should explain in your report), the accumulated counts form 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 that the VOLTAGE is set for + 600 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 +0.6 V on the DMM, for example, corresponds to +600 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 +600 volts. Place the ¹³⁷Cs source 10 to 20 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. This signal is the electron pulses coming from the PMT that represents the gamma ray striking the detector. The trace 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, and end up with the gain set to produce a pulse 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 computer back Digitizer channel 0 located at the rear of the computer. Using a ⁶⁰Co source you should see the following
      

      

      Figure 3: PHA display. Counts vs Energy of ⁶⁰Co (Click on Picture to enlarge)
   2. You should see a spectrum similar to Figure 3. 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 that are 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 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 (change the high voltage by 10 volts at a time). 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 record as much data up to about + 800 volts (change the high voltage by 10 volts at a time) as you think necessary to determine the relation between gain and high voltage. Plot the results. Explain the plot, particularly the behavior at the higher (more positive) PMT voltage levels.
3. To examine the capabilities and gain 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 impedances. 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 as to overwhelm 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 the 50 ohm terminator 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?
4. Choose a high voltage somewhere near the middle of the range, and choose the amp gain settings so that you are in a linear region and are utilizing the full scale of the PHA-your largest energy line (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 (20 minute runs each).
   1. Determine carefully where the peak channels are, the full width at half-maximum (FWHM) of your peaks, and the resolution. Compare these to theoretical line widths. Remember that you need an estimate of the uncertainty in your determinations.
   2. Also compute the 180° back-scatter and Compton edge energies for the different sources, and compare to the observed spectra. See ref. 5 for a description of what the plot should look like.
   3. Use the computer connected to the apparatus to obtain complete data and plotted copies of all of your spectra. There is a built-in timer in the equipment. Once your spectra are on the computer, you may use or 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 energy axis using the known energy values. If you vary your settings, your energy calibration can change drastically.
5. Take another ¹³⁷Cs spectrum, this time 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. For a given detector with a fixed size and gain, how does the signal vary with distance from the source?
7. Compute the absolute intensity of the ¹³⁷Cs source using the 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 the background, support the source with a low-mass stand. Use 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 = 15 cm.
   
8. 1. Measure the mass attenuation coefficient \[see reference (ii)\] in cm²/gm for Al, Cu, and Pb for several different thicknesses, using ²²Na and ¹³⁷Cs sources. 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.

Gamma Layout

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References

1. Web Elements Period Table of Elements

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

4. Canberra Counter/Timer 1772 Instruction Manual

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