EFFECTIVE AREA FACTOR FOR DETECTOR SN 483 DOE/ERSP TECH NOTE NO. 11.0 DATED: August 18, 1978 AUTHOR: R. J. Jaffe, EG&G Detector No. 483 is an intrinsie germanium planar detector, model IG 1916, produced by Princeton Gamma Tech (PGT). It has been in use by Desert Research Institute at the Nevada Test Site doing in situ monitoring, and was repaired and calibrated by PGT 1 August 1978. It arrived at Enewetak on 11 August 1978, was calibrated and used by the Enewetak counting laboratory and then installed in IMP Ion 16 August 1978. A standard effective area determination was conducted. This consists of duplicate determinations of count rate at four distances between 100 and 250 em from certified 241Am source (119.4 HCi + 2%). The source remains in its plastic container and is fastened to a sample holder tray using one thickness of cloth fiber tape. The attenuation factor (7) for the plastic container top and tapeis estimated at 0.027. Experimental measurements (5 pairs of runs over two days) give 1.037 as the ratio for uncovered/covered source gamma flux. The equation used to calculate Ay with this factor included is: Ao = 2.738 x d2 x Counts x 1078 Counts-see~!/J-sec-1 -em=2 where Counts = Net Counts in 241Am peak for a counting time of 5 minutes. The effective area of detector 483 is 16.6 em2. The previous measurement of detector 483 at Las Vegas was 17.2 em*, A similar difference averaging about 3.5% has been observed in Ao measurements at Las Vegas compared to measurements at Ursula for other detectors as well, and is currently under study. The effective area based on comparison of Enewetak counting laboratory data (normalized to detector 393) is 17.2 em2, The IMP calibration equation is based on a detector effective area of 19 em2, The effective area correction factor for detector 483 is 1.15. B-11-1