Prior to the switch-over, we were using a four-chip TLD badge, all TLD-700, with CR-39 for neutrons. Now, we are using a TLD badge that has 3 TLD-700 and 1 TLD-600 chips with CR-39. We have been operating on the basis that the neutron spectrum through thick shielding is adequately approximated by an Am-Be spectrum. This assumption had been supported by technical notes, verbal communications, and references from other similar facilities, but had not been independently confirmed for Jefferson Lab. As a result, we have always had our neutron dosimetry and instruments calibrated for the Am-Be spectrum. We also did this because our main source of exposure was from our Am-Be calibration source rather than from the accelerator itself.
Our group radiation physicist, Pavel Degtiarenko, calculated several neutron spectra based on typical target configurations, shield geometry, energies, distances, etc., using DINREG/GEANT. We looked at both skyshine and exit spectra. The results show that the spectrum from skyshine (which we believe to be most representative of dose to employees and to the site boundary) is remarkably close to the spectrum from Am-Be. It is also very close to the unmoderated Cf-252 source which is used in our DOELAP (Department of Energy Laboratory Accreditation Program) irradiations.
Skyshine spectrum
Exiting spectrum
Am-Be spectrum
Cf-252 spectrum
Our dosimetry provider performed several irradiations of both the TLD/CR-39 badge to determine a calibration factor for processing the badges, comparing the Am-Be irradiations to the Cf-252 irradiations. The CR-39 responded virtually identically in both fields; the TLD badge responded slightly differently.
The next step in the process is to develop a method for determining dose retrospectively to a person believed to have been exposed in an unexpected location, e.g., from exiting flux rather than skyshine. We will be doing this in the next couple months and hopefully will be publishing the results in some fashion.
We have submitted a new DOELAP application to take into account our new vendor and the new dosimetry.
If you are interested in looking at our preliminary conclusions, which include calculated spectra, you can download the Postscript file from the following location: <ftp://ftp.cebaf.gov/pub/radcon/neutron.ps> (714 KB).
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