The development of a 6 to 7 MeV photon field for instrument calibration

Abstract A photon source has been developed at the National Bureau of Standards to measure the response of radiological survey instruments to high-energy photons. The 19 F(p, αγ) 16 O reaction has been used to produce a 6 to 7 MeV photon field with a fairly uniform photon flux density of approximately 3 × 10 3 cm −2 s −1 at one meter from the source. The photon flux density is obtained from measurements with a 3 × 3 inch 2 Nal detector whose absolute response has been determined by a Monte Carlo calculation. The spectral characteristics of the high-energy photons have been determined from measurements with a large volume high purity germanium detector. The absorbed dose rate to water was measured with LiF thermoluminescent dosimeters (TLDs) at several depths in a 30 × 30 × 30 cm 3 Lucite phantom. It is planned to compare absorbed dose determinations from the TLD measurements with those computed from spectral measurements. The response of six commercial radiological survey instruments has been measured behind various thicknesses of plastic absorber. The results indicate that approximately 2.5 cm of plastic in front of these instruments is sufficient to discriminate against the associated high-energy electron contamination.