From Physics 111-Lab Wiki

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

I. Gamma-ray Spectroscopy

II. Pre-Labs must be printed separately. GMA Pre-Lab .PRINT, FILL THIS OUT, Get it Signed by 111-Staff, turn in your Signed Pre-Lab Sheet with your report, to the 111-Lab Staff.

III. Error Analysis Notes

IV. Gamma Ray Layout

V. NaI Crystal Information

VI. DG535 Digital pulser

Reprints and other information can be found on the Physics 111 Library Site

Before The Lab

View the 'Radiation Safety Video' From Inside the 111-Lab, Paste into your Browser [ U:\Advanced Lab Share\Safety Manuals and Videos ] and then click on Rad Safety Video located in the My Computer directory on the 111-Lab Network Share Drive, get Radiation Safety form from GSI, then fill it out & sign the Radiation pink form, and get a Radiation Ring.

Discuss the Physics about this experiment with the faculty or the GSI's in the 111-Lab before starting.

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.

View the Gamma Ray video, discuss the pre-lab questions with an instructor, and get the GMA Pre Lab Questions and Staff Sign Off Sheets.....PRINT, FILL THIS OUT, Turn it in

signed.

Prerequiste Reading Materials

Index to the Prerequisite Reading Materia [1]

Knoll, Radiation Detection and Measurement, John Wiley and Sons, New York, 1979.
1. Ch. 1 §IV pp 16-25 (Information about the sources of EM radiation),
2. Ch. 2 §III pp 62-71 (How gamma rays interact),
3. Ch. 3 §I-VI pp 79-95 (Detectors in general)
4. Ch. 9 §I-§III pp 272-284 (Photomultiplier tubes)
5. Ch. 10 §I-IV(B) pp 306-338 (Scintillators).
6. RCA Photomultiplier Tube Manual

All the above references are in the reprints for this experiment on the [Physics 111-Lab Library Site]. Start with (i), (ii), and (iii), but read the rest, as well as any other part of Knoll that interests you (there is a good chapter on the constituents of background radiation, for example).

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

Safety

When working on the gamma ray apparatus, you must wear a radiation ring. You should also always wear rubber 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.

Source Type	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

Detection

The gamma rays in this experiment are detected by a thallium-doped sodium iodide [NaI(Tl)] crystal. 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 Pulse Height Analyzer.

The Pulse Height Analyzer (PHA) accepts pulses of varying amplitudes between 0 and 8 volts and divides this range into 2048 increments of 8/2048 = 1/256th of a volt per interval. When a pulse representing a detected gamma ray is received, the PHA determines its height rounded to the nearest 1/256th of a volt, and adds 1 to the memory location which corresponds to that height. 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 times that gamma rays of different energies have been detected.

Procedure

Before you turn on the high voltage power supply (bottom rack), check to see that the POLARITY is NEGATIVE, and that the VOLTAGE is set for -2100 volts. Once set, do not change these controls! You will change the high voltage using the controls on the 5KV VOLTAGE DIVIDER located in the rack: set the METER switch for the Photomultiplier Tube (PMT) that you are using, (probably 1), and use the knobs under "1" (to the left) to change the voltage. The LOW/OFF/OFF/HIGH knob determines the range of the voltages: in the LOW position the range is from roughly -1.2 kV to -1.5 kV, and in the HIGH position the range is from roughly 1.5 kV to -2.0 kV. The 1000:1 VOLTAGE DIVIDER just scales down the voltage so that the DMM can read it-a reading of -1.5 V on the DMM, for example, corresponds to -1.5 kV going to the PMT.

Fig. 1: PMT Output, ¹³⁷Cs Source Scope: 50 mV/div; 0.5 $μ$ sec/div
Set the high voltage to -1500 volts, and place the ¹³⁷Cs source 10 to 20 cm away from the detector. Look at the output of the PMT on a fast scope (Tektronix 455). To match impedance, use a 50-ohm terminator on a BNC "tee" going into the scope. Look for a faint signal about 0.5 $μ$ sec 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 the many negative pulses of different amplitudes coming from the PMT.
The CANBERRA AMPLIFIER (amp), located in the equipment rack, amplifies the small pulses from the PMT into the range (0 to 8V) necessary for the Pulse Height Analyzer (PHA). Feed the output of the PMT into the amp (you don't need a terminator here). Set the INPUT switch to NEG (since we're inputting 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 $μ$ sec 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 $μ$ sec/div
The PHA we have for this experiment is the Tracor-Northern 7200. It has a lot of features, and you should look over the manual for instructions. But, to get you started, here is a brief overview:
1. To start, plug the output of the amp into the PHA DIRECT INPUT located at the rear of the unit. Make sure that the INPUT switch is set to DIRECT, and that the COUPLING switch is set to DC and the COINC/ANTI switch is set to ANTI. Turn the PHA on (using the Intensity knob), choose MEMORY GROUP 1/1 so that you are using all 2048 possible data channels, hit the ERASE DATA button to clear the screen, and hit ACQUIRE to begin collecting data.
  
  Figure 3: PHA display. 'Counts vs Energy of ¹³⁷Cs'
2. The double-arrow buttons control the vertical and horizontal scale. If you set these appropriately, you should see a spectrum similar to Figure 3. The other knobs you should experiment with are the LLD and ULD: these set the Lower and Upper Level Discriminators so that the PHA will ignore pulses that are outside the range that you set. This way the system doesn't spend its time with unwanted pulses like low-level noise. To set them properly, do the following. Set the ULD all the way to the right (dial reads 10), so that everything on the high end passes (all large pulses pass through). Set the LLD to 0 (DEAD TIME will read a very high value), and then turn it to the right until you see the DEAD TIME fall to zero, and then add a little extra for drift (instability of electronics). You may have to change this for different sources or configurations. The DEAD TIME indicator tells you what percentage of the time the PHA is processing the signals it has acquired (each pulse requires some time to determine its height and increment the appropriate bin)-keep this below 10%. Too small a dead time and you are not going to get any data; too large a value and you are overloading the system - it can't keep up with the large data flow.
3. Finally, you should know that the cursor (large knob) should not be used. Use the computer to display the channel number and counts of any channel that you want to see.
4. After setting the LLD and ULD, STOP the unit from acquiring, ERASE DATA, 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.
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 5 KV VOLTAGE DIVIDER only, and watch to see where the Cs peak moves. Because the peak channel number is equivalent to some input voltage, 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 -1300 volts, and record as much data up to about -1700 volts 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 negative) PMT voltage levels.
1. 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 Kay Electric Co 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; watch the Dead Time.)
  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= 85000 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 12 μs (i.e. frequency of 85000 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? A handy feature for obtaining the total number of counts in a peak is the Region of Interest (ROI). To use the ROI feature with the 7200 (see Section 9 of the PHA manual), press a number key to choose an ROI, and then move the cursor to the beginning of the peak. Press ATTACH LOW, and wait for the additional information to appear at the top of the screen. Then press ATTACH LOW again to set the lower ROI limit. Move the cursor to the end of the peak, and press ATTACH HIGH twice to set the upper limit. The information at the top of the screen gives you the ROI number (0-9), the beginning and ending channel numbers, and the total (gross) number of counts contained in the region. To disable the ROI, press CLEAR (NO), and to re-enable the ROI, press ENTER (YES).
1. 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 ⁶⁰Co source) should appear near the end of your spectrum, not in the middle. Then obtain a spectrum for each source listed.
2. 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.
3. 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.
4. Use the computer nearest 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 ANALYZE or EXCEL to perform background subtraction and peak-fitting. See the 111 Lab Computer Use packet on how to transfer your spectral data and how to use ANALYZE (the analyze section is in the back)
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.
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?
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 $I_{measured}=I_{absolute}(n(E)\frac{\Delta\Omega}{4\pi})$ 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 = 15 cm.
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 accepted values. Think about doing background subtraction, and be sure to include some discussion of uncertainties.
2. When measuring mass attenuation coefficients it is important to have proper geometry. Your source, collimators, absorber, and detector must all be in a line. The source, which is a small disk, should be placed perpendicularly to 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.