Peaking Procedure Part 1: Cs-137

The following peaking procedure is used in-house at Ludlum Measurements, Inc. for peaking LMI Models 44-10, 44-11, 44-20, 203, and 243 detectors on the 662-keV ¹³⁷Cs peak with a Model 2200 Scaler/Ratemeter. Other scintillation detectors are peaked in the same manner, but have different values for resolution.

The subject detectors utilize a NaI(Tl) crystal scintillator that is 2 inches thick, and the parameters given herein apply to any 2-inch thick crystal scintillator that is peaked under factory conditions. Please note that actual field conditions may necessitate the use of different parameters (such as a wider window). Accordingly, this procedure is offered as an example or guideline only.

1. Switch the M2200 WINDOW to the "ON" position. Set the window at 0.40.
2. Set the threshold dial at 6.42.
3. Peak the detector for ¹³⁷Cs (662 keV) as follows:
   1. Set the high voltage (HV) dial at 1.50. This ensures that you begin the peak routine below the actual peak.
   2. Set the 3-position switch (RATE, HV, BAT) to RATE.
   3. Set the range multiplier to x1.
   4. Begin increasing the high voltage by slowly rotating the HV dial clockwise. Rotating the dial too quickly can easily cause the peak to be missed.
   5. When the meter deflection is at full scale, switch the range multiplier to x10. Press the RESET button and continue to increase the HV until full-scale deflection is again achieved. Switch the range multiplier to x100, press RESET and continue to increase the HV. (NOTE: Range multiplier settings are suggestions. Actual settings will vary with source size.)
   6. When you have located the peak, an  increase or a decrease in voltage should drop the count rate. If it does not, check steps 1 and 2 to ensure proper switch settings and repeat the entire process. Once the count rate drops with an increase or a decrease in HV, proceed to step 4.
4. To confirm the probe is peaked for the 662-keV peak, set the window at 1.0 and the threshold at 6.12. Record the count rate, then switch the window "OFF". Count rate should stay approximately the same if the probe is peaked. If it is not peaked, the count rate will change dramatically.
5. Once you have confirmed the peak, switch the window "ON", re-adjust the threshold dial to 6.42 and the window to 0.40. Take a 6-second count (1/10 minute). Record the count rate and the peak HV.
6. Rotate threshold dial counterclockwise below 6.42 until the count rate is approximately half of the peak count rate determined in step 5. Record the threshold dial setting.
7. Rotate the threshold dial clockwise above 6.42 until the count rate is once again approximately half of the peak count rate determined in step 5. Record the threshold dial setting.
8. Using the example formula below, determine the full-width-at-half-maximum (FWHM), or resolution, of the peaked probe.
   Example

   Threshold dial setting at 6.82 = 5,000 cpm
   Threshold dial setting at 6.42 = 10,000 cpm
   Threshold dial setting at 6.02 = 5,000 cpm

   Energy Peak: Threshold dial setting at peak (6.42) plus ¹⁄₂ window dial setting (0.20) = 6.62 x 100 keV/turn = 662 keV.

   \[\mathsf{FWHM=\frac{6.82 - 6.02}{6.62}\times{100} = 12.08\%}\]

9. NOTE: The following criteria apply to the referenced detectors — that is, those with 2-inch thick NaI(Tl) crystal scintillators — for which resolution is an important factor. The value of the FWHM should be less than 13% to be considered an acceptable probe. The value of a new probe should be approximately 8 - 10%. Any probe with a FWHM value greater than 13% should be considered marginal and sent in for service.
10. A plateau is usually performed after the successful completion of the FWHM test to establish the operating curve of the photomultiplier tube. NOTE: The plateau HV should be higher than the peak voltage if the peaking procedure was performed correctly.

Peaking with isotopes other than ¹³⁷Cs