1 Purpose of This Lab Exercise:

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1 Physics Experimental Physics Temple University, Spring C. J. Martoff, Instructor J. Tatarowicz, TA Physics 4796 Lab Writeup Hunting for Antimatter with NaI Spectroscopy 1 Purpose of This Lab Exercise: Acquaint you with NaI(Tl) spectrometer and a few pieces of nuclear electronics (photomultiplier tube, HV supply, pre-amp power supply, computer multichannel analyzer). Explore the interactions of gamma rays with matter by analyzing the energy loss spectrum they produce in the NaI(Tl). Use measured spectra to search for evidence of the existence of antimatter. Specific Goals (see also AN 34 reference below) Obtain gamma ray spectra from room background with and without iron shield and the following sources with the shield on: grocery store 40 K source, Red Fiestaware plate with nat U glaze, and one of the rock samples provided or another interesting item (Brazil nuts, bananas, fireplace ashes). Calibrate the Multichannel Analyzer (MCA) using the 40 K and 208 Tl peaks which the instructor will identify for you. Use the MCA software calibration function and also perform the calculation by hand and compare the results. Assign as many gamma ray photopeaks as you can, using the references provided (particularly Ref. 1 under NORM). For each peak, give the expected energy, the decay scheme, and the emitting isotope. Identify, and explain the energy and physics origin of the following features in a spectrum: photopeak, Compton edge, backscatter peak, escape peak or.511 MeV gamma ray peak. 1

2 Estimate the activity (in Becquerel) of one of your samples from the NaI counting data. Approximate time needed: Two to four lab sessions. Suggested References: General information on NaI spectroscopy; 1. ORTEC Application Note AN34 Experiment 3. Available on the internet at: Note: Since this is a piece of official manufacturer s literature, made available on the manufacturer s website for the convenience of customers, it is acceptable as a reference in the lab report. Note that this Application Note assumes the use of certain equipment slightly different from that used in our experiment. Procedures in AN-34 therefore must be slightly modified for use with our setup. 2. Melissinos, Experiments in Modern Physics, 2nd Ed. 3. Knoll, Radiation Detection and Measurement. 4. Moore, Building Scientific Apparatus 5. Horowitz and Hill, The Art of Electronics, Appendix A Naturally Occurring Radioactive Material (NORM) 1. B. Minty, AGSO J. Aus. Geol. Geoph. 17, 39 (1997) (on course website) 2. U.S. Environmental Protection Agency MARRSSIM manual, Appendix A. Posted on the course website and available from: (Web-posted material from government institutions is citable, though in some cases, particularly for foreign governments, it may not be reliable.) 3. Glasstone, Sourcebook on Atomic Energy, Ch. 5 (in Paley) 4. Brown and Firestone, Table of Radioactive Isotopes 5. Korean Atomic Energy Research Institute gamma ray peak energy collection at: 2 Preparation: Before starting this lab you must complete the Prelab questions. This will require studying at least some of the References above. The appropriate chapter of Knoll s book and the ORTEC Application Note will be particularly useful. We will use a small NaI(Tl) crystal about 50 mm diameter and 40 mm thick, which lives in an iron shield to absorb background from natural radioactivity in the room. A large 200x250 mm crystal is available for people interested in more elaborate experimentation. Both spectrometers photomultipliers require POSITIVE high voltage of about 1000 V. 2

3 2.1 Cabling With all power off, connect one end of the HV cable to the small NaI spectrometer and the other end to the Power Designs HV supply. Note: the HV cable connections used are coaxial cables of a standard type known as SHV ( safe high voltage ). They are indeed pretty safe, with all the hot HV conductors well recessed into hollow insulators at the ends of the cable and the mating connectors. They connect by carefully pushing the cable onto the mating connector with the connector pins aligned with the slots in the cable connector, then twisting through about 30 degrees to lock the connection. Even so, you should know that the Power Designs HV supply can supply 40 ma at 3KV, which is enough to make you very dead. Don t modify or play around with anything connected to HV (the NaI spectrometer assembly for example). Signal cables (called BNC s, for bulkhead naval connectors) look different from SHV cables- there is no protruding hollow insulator. Do not mix them up or you will ruin cables or equipment. Connect a signal cable from the spectrometer preamplifier output to the input of the spectroscopy amplifier supplied by the instructor. Connect another cable from the spectroscopy amplifier output to oscilloscope channel A (or 1). Be sure to use a T and a 50 Ohm terminator at the oscilloscope to avoid reflections (see Sec , Building Scientific Apparatus, or The Art of Electronics Sec ff). Connect the 9-pin power cable from the connector on the rear of the spectroscopy amplifier in the NIM (Nuclear Instrumentation Modules) bin to the pre-amp power input of the NaI spectrometer base. 2.2 Checking Signals Set oscilloscope as follows: Vertical Mode: Channel A (or 1 on some scopes) Trigger Source: Internal, A only (or 1 on some scopes) Trigger Mode: Normal Volts per Division:.2 V/cm Time per Division:.050 µs/cm Trigger Slope: positive (or rising on some scopes) Trigger Amplitude: near zero Intensity: fully clockwise (maximum). If this produces a glaring spot at the left side of the display, back off a little on the intensity setting. Now it is time to apply HV with the Power Designs supply. First check that the polarity switch on the supply is set to POSITIVE. Set all voltage knobs to zero, then turn AC ON with toggle switch. Wait 10 sec., turn HV ON, then finally set voltage to V with voltage dials. Look for signals from NaI on the oscilloscope while carefully adjusting the trigger amplitude until the green Triggered indicator light glows steadily. This indicates that the scope is triggering, and you should see a waveform on the scope screen. The trigger level adjustment may be quite sensitive. If no trace is visible on the screen, adjust the Vertical Position A knob to 3

4 get the trace on scale. If you can t get a trace or a trigger in five minutes, see the instructor. 2.3 Radioactive Source Check Obtain a 60 Co or 137 Cs radioactive check source from the instructor. If you lose one of these sources you will be condemning the instructor to endless Nuclear Regulatory Commission paperwork, with obvious (negative) consequences for the high regard in which the instructor initially holds you. Alternatively, the instructor may give you a radioactive source purchased at the grocery store (KCl salt substitute, activity about 0.4 nci/g). 137 Cs emits (indirectly) a single gamma ray line of MeV energy. It is a fission product (produced in nuclear reactor fuel as it burns ) with a 30-year half-life. 60 Co is a very interesting source, emitting two gamma rays one after another (a gamma ray cascade ) with energies 1.17 and 1.34 MeV. It has a 5.3 year half-life. 60 Co is also a fission product. Strong sources (several million times stronger than the one we have) are used for radiation therapy of cancer tumors. These lab sources are so low level as to be quite safe. At a distance of 10 cm with no shielding, these sources give you a radiation dose rate about the same as natural background (several times 10 7 Sv/hr). For comparison, flying to Europe at a time of low solar activity gives you an additional radiation dose of at least Sv due to less atmospheric shielding between you and the solar and cosmic radiation. At the time of high solar activity or a big flare, this rate can be larger by a factor of 100 or more. Place the source in contact with the NaI can and notice the effect on the scope display (this may be impossible to see with the KCl source). Adjust the HV to get a band of source-associated pulses about 4 V tall. Do not exceed 1500 V without instructor s consent. Have instructor check your display before proceeding. Sketch the source-associated pulses from the NaI pre-amp in your lab book and record their risetime (10% - 90%), fall time and a rough estimate of the rate. 3 Energy Spectrum Measurement Turn off HV by turning all voltage knobs to zero, HV switch OFF, wait 10 sec, finally turn AC OFF before proceeding with this section. Turn on the data acquisition computer. Remove the cable from the oscilloscope connect it instead to the input (marked IN ) of the multichannel analyzer card in the data acquisition computer. The function of the multichannel analyzer (MCA) is described in the References. It produces a histogram of the peak amplitudes presented to its input. You do not need a T-and-terminator at the MCA input. Insure that the computer recognizes the MCA- see instructor if acquisition refuses to start when you click GO button. 4

5 Acquire a spectrum for 5 minutes. Have the instructor check your results. This short count may need to be repeated with a different HV setting to get the peak in a convenient location in the spectrum. When the instructor gives you the OK, use the software to determine the number of counts in the highest channel of the highest energy peak in the spectrum. Calculate how long it will take to get a spectrum with 1000 counts in this channel. Clear the data from the spectrum. Use the preset time option of the software to start a new count for the time you calculated to get 1000 counts in the high channel. Do this long count and have the instructor check your results. When the instructor gives you the OK, repeat the long count with another radioactive source. Perform all the calculations in AN-34 Experiment 3.1. Use this setup to complete the activities in AN-34 and in the Specific Goals at the beginning of this writeup. All numerical answers must include an analysis of uncertainty and a statement of the total uncertainty in the measured number. 4 How Radioactive Is It? For the activity estimate, center the source at 12.4 cm from the detector, use the MCA software to integrate the number of counts in the photopeak, and use the following formula: ( ) Σ(peak) Σ(BG) 1 Activity = t ǫ(e γ )f Here Σ(peak) is the number of counts in the photopeak channels for the source spectrum after counting time t, Σ(BG) is the number of counts in the same channels for a background spectrum measured or scaled to the same counting time t, f is the fraction of decays for the parent nuclide which emit gamma rays (see Brown and Firestone reference; your peak must be properly assigned to determine f from this reference), and ǫ(e γ ) is the energy dependent photopeak efficiency for our NaI (see Knoll reference for an explanation). Take the photopeak efficiency from the following table (from Monte Carlo calculations made at the University of Guelph), using interpolation if necessary to get to your measured energy: E γ ǫ 511 kev ± kev ± kev ± kev ± kev ± To further explore the efficiency, estimate the geometrical efficiency from ǫ geom = A(det)/4πD 2 where A(det) is the area of the front face of the NaI (1) 5

6 and D is the source-to-detector distance (12.4 cm in this case). The geometrical efficiency is the probability that a gamma ray emitted isotropically from the source will strike the detector. Why is the number you get so much bigger than the photopeak efficiency listed in the table above? (Hint- see Knoll reference). Measure the photopeak fraction (fraction of total counts minus background that appears in background-subtracted photopeak). from your spectra. Compare this to the quotient of ǫ(e γ )/ǫ geom. These two numbers should be about the same- explain why and compare their values. 6

Gamma Ray Spectroscopy with NaI(Tl) and HPGe Detectors

Nuclear Physics #1 Gamma Ray Spectroscopy with NaI(Tl) and HPGe Detectors Introduction: In this experiment you will use both scintillation and semiconductor detectors to study γ- ray energy spectra. The