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Identifying Unknown Isotopes Using the SAM-935 Surveillance and Measurement System

Identifying Unknown Isotopes Using the SAM-935 Surveillance and Measurement System. V11 Firmware Revision . Overview. Why I.D. Unknown Radioisotope Instrument Operating Principle Instrument Set-up Startup/Calibration Monitoring and “Capture” Sampling Interpreting Results

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Identifying Unknown Isotopes Using the SAM-935 Surveillance and Measurement System

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  1. Identifying Unknown Isotopes Using the SAM-935 Surveillance and Measurement System V11 Firmware Revision

  2. Overview • Why I.D. Unknown Radioisotope • Instrument Operating Principle • Instrument Set-up • Startup/Calibration • Monitoring and “Capture” Sampling • Interpreting Results • Hands-on Scenario

  3. Overview • SAM 935 is a complex instrument that has many capabilities, including some that overlap our other radiation detection equipment • Our primary use of the 935, however, will be to determine the isotope(s) of an unknown source • ADM-300, Innovision 450P, TLDs, etc better for our other needs – so we will not focus on 935 capabilities outside of identifying unknown

  4. Why I.D. Unknown Radioisotope • Understand potential contamination concern • I-131 ½ life = 7 days . . . gone in months • Ra-226 ½ life = 1600 years . . . here for awhile • Understand potential source of material • I-131 probably medical dose • Cs-137 probably industrial • Pu-239 probably needs FBI involvement . . . • Basic info needed for disposition/disposal

  5. Instrument Operating Principle Many radioactive materials give off electromagnetic radiation in the form of gamma or x-rays

  6. Gamma/x-ray energy measured in electron volts (eV) , KeV, MeV • Each radioactive material produces gamma/x-rays with specific energy level(s) • A material may have several types of decay and each type may produce a different, but specific energy level gamma/x-ray 477 KeV Gamma Be-7 1.37 MeV Gamma 197 KeV Gamma O-19

  7. When Radioactive material decays, it can create radioactive “daughters” that also have gamma/x-rays with specific energy(s) 722 KeV Gamma Zr-95 765 KeV Gamma Nb-95 Mo-95

  8. If we can detect and measure the gamma/x-rays, we can compare to library and determine the isotope 765 KeV Gamma (from Zr-95) 722 KeV Gamma (from daughter Nb-95) Zr-95

  9. But background full of gamma/x-ray from other sources 100 KeV x-ray 722 KeV Gamma 255 KeV Gamma 50 KeV Gamma 3 MeV Gamma 765 KeV Gamma 722 KeV Gamma Zr-95

  10. Our SAM-935 uses an external sodium iodide “2 x 2” detector to measure the rate/intensity of gamma/x-rays (counts/min) and also the energy level of the gamma/x-rays • Plots the spectrum of energies vs. the intensity called a “spectrograph” # hits at each energy level Energy level of the individual gamma/x-rays hitting the probe

  11. Trying to find “peaks” that correspond to energy levels of gamma/x-rays associated with a particular isotope (or decay chain “family”) • Compares peaks to a library of radioactive materials and their peaks Peaks

  12. Instrument Set-up Probe “Window” On/Off Switch External “2 x 2” Probe Power Cord LCD Display Screen SAM-935 Our version has no internal detectors (other model 935s might)

  13. Instrument Set-up • “2 X 2” External probe has a sodium iodide detector near the front end • Probe body is resin that absorbs low energy gamma/x-ray • Front face of probe is thin plastic “window” • Want the “window” facing the material you are trying to identify “2 x 2” detector inside probe Probe “Window” is face of tube

  14. Instrument Set-up • If your SAM has an External Sodium Iodide probe, lining up the red dots on the cable/plug while power is off. • Plug in power cord (or operate off battery up to 8hrs). • Turn the 935 on - first screen should be: • Allow 935 to warm up (typically 30 seconds) Verify battery “good” If battery “low”, operate with power cord and recharge asap Contrast Bklight OK Wait

  15. Calibration + Background • When first turned on, the 935 will self-test and auto-calibrate. This is a good time to take a new background (start of every new survey location) • Press F3 to start calibration or F2 to cancel and go straight to Background. Press F2 to start collection 15-Mar-04 22:37:51 Auto calibration is required. Perform calibration now? Util Adjust F2 F3 F2

  16. Dose Rate Mode • Most days, 935 will go directly into Dose Rate mode: • Each time the instrument power is cycled or the temperature shifts +/- 5 degrees, the instrument will automatically calibrate. ( for example, going from inside to outside, etc)

  17. Dose Rate Mode • 935 starts in this mode, showing the “full” dose rate of everything it sees from background sources • First step -- see what your background “full” level is without input from any sources • Keep probe away from sources (including your check sources) and see background range (typically 1-10 uR/hr) Trigger list being used Dose rate of everything 935 is detecting Total dose recorded this “alarm” cycle Srch On Monitor Calib Capture

  18. Surveillance/Monitoring • Move the 2 x 2 probe window as close as possible to the source you want to identify (remember inverse square law) • When 935 finds peaks above background it associates with a particular isotope in the trigger list, it lists the isotope and the estimated dose rate that isotope contributes to “full” • 935 will also list “unknown” if it sees peaks it does not associated with isotope in library

  19. Surveillance/Monitoring • Isotopes w/steady reading are possibly a good detection • Transient/flickering means not a good match • Surveillance/monitoring only tentative i.d. -- follow with a “ID” sample Dose rate of everything 935 is detecting “IND” = typical industrial source Trigger list being used Dose rate of specific isotopes Typ Source Abv: IND = Industrial NRM = Natural MED = Medical SNM = Weapon

  20. Capture with ID • The “capture” function collects data over a longer period of time to get better spectrum for library comparison • Default capture time is 1 minute (vs. 1 sec for surv/mtr) • Place probe window near source and press F3 to start an ID from surveillance/monitor mode F3

  21. Identify • The 935 will collect data over the acquisition period, showing a real-time spectrograph • When finished, “ID” will end in an “MCA report” • Use arrow buttons to scroll down report MCA REPORT15-Mar-04 22:37:51 MCA REPORT Report scroll bar DATE: 15-Mar-2004 22:30 EN CAL DATE: 15-Mar-2004 07:03 BKG DATE: 15-Mar-2004 07:04 ELAPSED LT 59 USE F2 OR F3 TO PRINT REPORT Use arrow buttons to scroll down/up report Return P-Full P-Short

  22. MCA Report • Scroll down report to table with energies where gamma/x-ray counts were found significantly above background • Isotopes are listed next to an energy line the 935 associated with an isotope definition in the library • The uncertainty (“UNC”) should be lower than 11% for a “good” identification and lower uncertainty is better MCA REPORT15-Mar-04 22:37:51 Not a good i.d. Uncertainty too high ENERGY NET UNC NUCLIDE (keV) (CPM) (%%) 16.5 551.28 +- 9.32 125.8 293.15 +- 43.3 Co57 430.2 2309.2 +- 5.35 659.7 33763 +- 0.58 Cs137 Report scroll bar Good i.d. Low uncertainty USE F2 OR F3 TO PRINT REPORT Use arrow buttons to scroll down/up report Return P-Full P-Short

  23. MCA Report • Scroll down further to see isotopes “found” • “Cor” correlation is how well the 935 thinks it matched the energies it found to the library energies for the isotope • Correlation may be high even for bad i.d. (uncertainty high) • Shows how many energies are identified in library for isotope and lists the ones found MCA REPORT15-Mar-04 22:37:51 This report saying both Co57 and Cs137 were found, but we know Co57 uncertainty was too high Correlation for each isotope “found” 1/1 Lib Lines for Co57; Cor = 0.80 LINE PEAK NET CPM 123.7 126.9 293 1/1 Lib Lines for Cs137; Cor = 0.80 LINE PEAK NET CPM 661.7 659.8 33762 USE F2 OR F3 TO PRINT REPORT Return P-Full P-Short

  24. MCA Report • Scroll down further to see all the isotopes on your trigger list that were “not found” MCA REPORT15-Mar-04 22:37:51 NUCLIDES NOT PRESENT 1/2 Lib Lines for Cd109; Cor = 0.50 1/6 Lib Lines for Eu152; Cor = 0.05 0/3 Lib Lines for Ra226; Cor = 0.00 USE F2 OR F3 TO PRINT REPORT Return P-Full P-Short

  25. MCA Report • Scroll down further to see the energy levels where significant gamma/x-ray counts were seen, but not associated with an isotope on your trigger list • So where the heck did those unassociated gamma/x-rays come from? Bremstrallung? Compton? MCA REPORT15-Mar-04 22:37:51 LINES NOT ASSOC WITH NUCLIDES Energy Net CPM Eff Corrected 16.5 551.3 551275.6 C 199.3 3020.5 80284.9 C 430.1 2309.2 134990.8 C USE F2 OR F3 TO PRINT REPORT Return P-Full P-Short

  26. Check Background vs Peaks • From MCA report, press F1 “Return” once, “UTIL” button twice, and then F1 ”Wide” to view the capture spectrum • Move vertical cursor by using arrow buttons • EN = the energy line being looked at , CNTS = the counts at that energy Vertical cursor on energy line being looked at MCA DISPLAY15-Mar-04 Alarm 17 22:37:51 CH=22 EN=48keV CNTS=1225 Normal Log Strip

  27. Check Background vs Peaks • Verify the peaks in your spectrum are at least 20 times higher than the “background hump” (using the “counts”) • If peaks are not 20 times higher than background, the identification may not be solid – get health physicist support to look at spectrum in detail • Exit MCA Display by pressing “Util” twice, then F1 “Exit” to return to surveillance/monitoring Good Poor Peak Peak Bkg Bkg

  28. Interpreting MCA Report • False positive: just because MCA report lists an isotope as found with low uncertainty, is it really there? • Possible misidentification: • Co-57 . . . often mis-identified because its primary energy falls in typical “background” area • If the “real” isotope is not enabled in library, isotopes with similar energy line(s) may get mis-identified • Shielded sources - Compton scattering can create gamma with “random” lower energy levels, increasing the “background hump”

  29. Interpreting MCA Report • False negative: just because MCA report does not list an isotope as found, is it confirmed not present? • The “unknown” isotope may not be “enabled” in library? • Did you run “capture” long enough: if unknown is “screaming”, probably seeing enough gamma/x-ray, but if not very “hot” may take longer to get enough counts – adjust capture time • Isotope may have gamma/x-ray energies outside 935 range (13 Kev – 3 MeV) or not emit gamma/x-ray • Shielding may absorb the low energy lines of an isotope needed to match library definition • If there is strong activity/strong peaks with no identification, a health physicist can interpret spectrum

  30. Interpreting MCA Report • So when can you be confident? • Good calibration and background • Activity at least 10 times “Full” background • MCA report identification with uncertainty less than 11% and does not have a ? next to the isotope. • “Peaks” at least 20 times greater than “background hump” • Still good to send spectrum for health physics confirmation

  31. Adjusting for Longer ID • If 1 minute capture does not clearly identify unknown, may need to adjust to a longer capture to get even better data set • Re-run capture and see if longer time has identified isotope

  32. Changing Between Trigger Lists • The 935 can only look for 20 isotopes at one time in surveillance/monitor mode, so we build several trigger lists • To change to an alternate trigger list (A, B, or C) • Press Util button, scroll to trigger list of choice and press enter • Press F1 “Return” 3 times to return to Dose Rate Screen Note trigger list being used Srch On Monitor Calib Capture

  33. Sending MCA Report/Spectrum • This slide waiting for software and RS-232 cable • Explain how to save a capture MCA report/spectrum and send by email to AFIOH HP support. • Questions?Truly unknown radiation source? Call DOE RAP team in your region

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