Peaking Procedure Part 2: Various Isotopes

In Part 1, we discussed the peaking procedure used in-house at LMI for peaking LMI Models 44-10, 44-11, 44-20, 203, and 243 detectors on the 662-keV ¹³⁷Cs photopeak with a Model 2200 Scaler-Ratemeter. Other scintillation detectors are peaked in the same manner, but have different values for resolution. In Part 2, we will cover using the LMI Model 2200 in peaking for other isotopes. The specific detector model is not germane to this discussion, as resolution of the detector will not be addressed.

As mentioned in Part 1, actual field conditions may necessitate the use of different parameters (such as a wider window). The following are examples of such conditions that affect the choice of window width:

1. If isotopes other than the target isotope are known to be present, use of a narrow window may be appropriate in order to isolate the target isotope's photopeak. This is especially the case for isotopes that have a photopeak energy close to that of the target isotope's photopeak energy. Note, however, that the use of a narrow window will decrease the number of counts registered, which may cause the user to miss detection of small quantities of the target isotope.
2. If the target isotope is isolated from other isotopes, a wider window may be used. This is also the case when other isotopes are present but are known to have sufficiently different photopeak energies from the target isotope. Please bear in mind that these and other suggestions written herein are offered as guidelines only. Your particular situation may warrant a different procedure.

To avoid repeating the specific operating procedure for peaking, we will assume that the reader is familiar with these steps. What may prove to be valuable, however, are the setting used at LMI for the Model 2200 with the THRESHOLD dial calibrated to different keV/turn ratios. Below are examples of THRESHOLD and WINDOW settings for common isotopes of interest.

1. 100 keV/turn:
   For ¹³⁷Cs (normal factory setting), the THRESHOLD dial is calibrated to 100 keV/turn and set at 6.42. The WINDOW dial is set at 0.40. (Photopeak energy at 662 keV*.)
   For ^99mTc, the THRESHOLD dial should be calibrated to 100 keV/turn and set at 1.20. The WINDOW dial is set at 0.40. (Photopeak energy at 140 keV*.)
   For ¹³¹I, the THRESHOLD dial should be calibrated to 100 keV/turn and set at 3.44. The WINDOW dial is set at 0.40. (Photopeak energy at 364 keV*.)
2. 10 keV/turn:
   For ²⁴¹Am, the THRESHOLD dial is calibrated to 10 keV/turn and set at 5.00. The WINDOW dial is set at 2.00.
   For ¹²⁵I, the THRESHOLD dial should be calibrated to 10 keV/turn and set at 1.70. The WINDOW dial is set at 2.00. (Photopeak energy at 27 keV*. Note that this window setting may not include the full contribution from the 31 and 35 keV photopeaks.)
3. 200 keV/turn:
   For ⁶⁰Co, the THRESHOLD dial is calibrated to 200 keV/turn and set at 6.55. The WINDOW dial is set at 0.20. (Photopeak energy at 1.33 MeV*.)
   For ⁶⁰Co, the THRESHOLD dial is calibrated to 200 keV/turn and set at 5.75. The WINDOW dial is set at 0.20. (Photopeak energy at 1.17 MeV*.)

NOTE: A peak check is recommended when switching from one isotope to another. Minor voltage adjustments may be necessary.

*REFERENCE: Shareware Radiation Decay - Emission Tables and Spectra by Charles Hacker, Version 2: March 1997. Initial program written by Grove Engineering, based upon RSIC data.