A low-background ?-ray spectrometer facility with composite detecting modules

Abstract

In order to refine the low-background facilities based on large-area semiconductor detectors [1], we developed a new type of detecting module [2, 3] composed of two (main and guard) p -i -n -Si(Li) detectors, produced on a common large-diameter single crystal of silicon. A schematic diagram of this module is shown in Figs. 1a and 1b. The distinctive feature of its diffusiondrift phase is that the lithium ions, instead of penetrating through the thickness of the crystal, drift to a strictly determined depth dictated by the conditions of the physical problem being solved.It should be noted that, in this case, the requirements for the thickness of the diffusion n + regions (entrance windows) are particularly stringent. On the one hand, they must ensure a sufficiently high detection efficiency for β rays (i.e., be thin), and on the other, they should provide high quality electrical and mechanical contact. The high electrical conductivity of the residual p region used as a common negative contact for both the main and guard detectors is ensured by the low resistance of the raw silicon, which imparts the required contact properties to this region.Two p -i -n -Si(Li) detectors were made in a common silicon single crystal with an area of 25 cm 2 and a thickness of the active region w i = 1.5 mm. At U rev ≈ 20-30 V, the detectors had dark current I ≈ 1-2 µ A, capacitance ë ≈ 180-200 nF, and noise energy E noise = 25-30 keV. Figure 2 presents β -ray spectra of 207 Bi with peaks corresponding to energies = 976 keV and = 1050 keV. The energy resolution and the pulse height of detector D 1 are slightly lower than those of detector D 2 . This is evidently caused by the thicker diffusion n + region (the "dead" layer of the entrance window) of detector D 1 . This difference in thickness of n + regions is attributable to the nonsimultaneous sputtering of Li onto the end surfaces of the crystal during the diffusion phase.It should be mentioned that the separation of D 1 and D 2 into the main and guard detectors is a matter of convention. The identity of their physical and geometric properties allows them to be considered as identical units, both capable of detecting nuclear radiation and cosmic rays and measuring their spectra over wide spatial angles.Abstract -The features of twin pin Si(Li) detectors produced on a common large-diameter silicon single crystal are described. The intrinsic noise of the facility is suppressed by reducing the relative proportion of the structural materials (casings, insulating compounds, metal contacts, etc.). (b) (a) 5 3 2 4 i p -Si i D 1 D 2 + -+ 3 4 2 1 1 Fig. 1. (a) Cross section of a semiconductor detector: ( 1 ) diffusion region; ( 2 ) compensated region; ( 3 ) p -Si layer; ( 4 ) ùäãÅ-10Å insulating epoxide compound; ( 5 ) organic glass case; and ( D 1 , D 2 ) detectors. (b) Overall view of the head stage of the low-background facility containing two Si(Li) detectors with a working surface of 25 cm 2 :( 1 ) cassette for a β -radioactive sample;( 2 ) semiconductor detector;( 3 ) common organic-g...