Semiconductor detectors with a large working volume are needed to solve certain paroblems in nuclear spectroscopy. The development of such detectors became possible with advances in growing large-diameter silicon single crystals with the required properties, satisfying the requirements for obtaining detectors based on them. One important requirement for obtaining detectors with a large working volume is that its resistance must be high. This is achieved by using the lithium ion drift process in the volume of the semiconductor detector.However, even though the mobility of lithium ions in silicon is anomalously high, a long time is required for diffisuin-drift compensation of lithium in a large volume (W ≥ 2 mm, S ≈ 20 -80 cm 2 ). In addition, the usual method of compensation, which includes drift of lithium ions from a distributed source, produced from one end of the crystal by preliminary diffusion of lithium, does not give a uniform drift front beause of the nonuniformity of the temperature field in a large-volume crystal. The diffusion region can also become very diffuse during prolonged drift, which will result in very thick entry and exit windows and will decrease the effectiveness of the spectromeric characteristics of the detector.A new method for producing p-i-n structures was developed to decrease substantially the time required for compensation of a large volume of silicon by lithium ions and to eliminate at the same time the negative consequences of holding the crystal at a high temperature and under a high voltage. The method consists of the following. Drift of lithium ions from two ends of prepared samples is conducted to a depth sufficient for the required compensation of the initial acceptor impurity in silicon. Figure 1a shows the typical diffusion profile obtained for the distribution of the lithium concentration by the proposed method in p-Si with ρ = 10 Ω·cm and N 0 ≈ 2·10 17 cm -3 . The negligible deviation of the lithium penetration depth at the two opposite end surfaces of the crystal is due to the fact that lithium did not enter simultaneously at both ends.Before the drift voltage is applied, the crystal is shaped in the form shown schematically in Fig. 1b. After the standard required technical operations have been performed, the crystals are placed into a setup where lithium ions drift into the volume of the crystal, as shown in Fig. 2a. The drift regime was chosen taking account of the low resistance of the initial silicon [1]. It is easy to show that in the proposed method the compensation time for a prescribed volume of a p-Si crystal is decreased by a factor of 4. Indeed, according to the empirical formula for the dependence of the depth of the compensation region for the case of unilateral drift where µ is the mobility of lithium ions and U and t 1 are the drift voltage and drift time, respectively.For bilateral drift, the path traversed by the lithium ions is two times shorter, i.e., Hence for the compensation time for bilateral drift and t 2 = 1/4t 1 . t t 1 2 2 / = W U t i / . 2 2 2...

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...