Thoughts on VCD-145 Detector Calibration
In 1980, Don Smith requested that the EG&G Detector Group in North Las Vegas provide a summary of calibrated sensitivities for the VCD-145 detector. The &sired information was provided in a memorandum from Sam Egdorf (Reference 1). A memo from Brent Davis issued a week later described the effect on VCD-145 detector sensitivity that resulted from changing the thickness of the stainless steel entrance window (Reference 2). This memo is intended first to effectively archive those two references, and second to record thoughts about the significance of their contents. Reference 1 lists a total of 118 calibrated values for 80 different VCD-145 detectors, from 1977 to 1980. With only four exceptions, all of the serial numbers from V004 to V087 were included. The earlier calibrations were for detectors with 1-mil entrance windows, and the later ones were for detectors with 2-mil entrance windows. Three of the earlier units were calibrated at both thicknesses by temporarily placing an extra 1-mil sheet of stainless steel across the window. Altogether six different collimator diameters were used, from 60 mm to 95 mm. Some units were calibrated for more than one collimator diameter, and 14 were at some point designated as backup detectors for a second event. Reference 2 describes the effect of window thickness on calibrated sensitivity. Quoting that reference: ''To demonstrate that the sensitivity decrease is solely a function of the window thickness, a standard VCD-145 detector with a 0.001-inch thick window was calibrated with the {sup 60}Co source. Then without changing detector or geometry, a 0.001 -inch thick stainless steel foil (same material as that of the window) was placed directly in front of the detector window, effectively making a 0.002-inch thick entrance window. The detector was again calibrated. This technique was repeated until the detector had an entrance window equivalent to 0.010-inches thick.'' There are multiple reasons to suspect that the accuracy of the sensitivity reported for a vacuum Compton detector is uncertain by one-half percent or so. Differences in collimated diameter, variations in window thickness, and unknown errors in the calibration procedures all seem to have some small uncertainty. If one looks at the situation from another angle, this can be considered good news--because there are no indications of uncertainties at the one percent level. Having studied the data from References 1 and 2 in depth, it is my opinion that an experimenter can be confident that the calibrated sensitivity reported for a vacuum Compton detector is accurate to one percent or better. This may not be the smallest error in a reaction history measurement, and it may not be negligible, but realistically it is not often a concern. The uncertainty in knowledge of energy transmission through several mean free paths of attenuating material will almost certainly be several times larger than the uncertainty in detector sensitivity. It is believed that there is a trend for the VCD-145 sensitivity to decrease as the collimator diameter is reduced. Since this likely aggravates the problem of the negative precursor, it is recommended that when detector time response is important, the detector should only be used with a collimator that is close to the largest allowed.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 15011600
- Report Number(s):
- UCRL-TR-209935; TRN: US200507%%476
- Resource Relation:
- Other Information: PBD: 10 Feb 2005
- Country of Publication:
- United States
- Language:
- English
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