Experiences with large-area frisch grid chambers in low-level alpha spectrometry.

The properties of parallel-palte gridded ionization chambers with areas of 300 cm2, developed by us for alpha spectrometry of samples with low specific alpha activity are reported. Several practical hints for optimum operating conditions are presented. The chambers can be operated routinely at atmospheric pressure for several days, without the need for purification of the gas filling (P10). The minimum detectable activity at 5 MeV is 0.01 pCi, based on 4.65 standard deviations of background and 1000 min counting time. At the GSF Research Center ionization chambers of this type are used for the analysis of natural alpha emitters, as well as of transuranium nuclides in environmental samples by: (a) direct alpha spectrometry without any previous treatment of the sample, (b) semi-direct spectrometry after removal of organic matter by low-temperature ashing and (c) spectrometry after chemical separation. Some typical examples of application are given. Furthermore the range of application of the chambers in comparison to semiconductor detectors in the field of low-level alpha spectrometry is discussed