Nuclear Instruments and Methods in Physics Research A282 (1989) 315-316 North-Holland, Amsterdam
SEMICONDUCTOR DETECTORS FOR THE CALIBRATION OF FISSILE RADIONUCLIDE TARGETS S.I . LASHAEV, S.M. SOLOVIEV and P.S. SOLOSHENKOV V.G. Kh1opin Radium Institute, Leningrad, USSR
The description of the set of semiconductor detectors with large areas and complicated shapes used in the Radium Institute Leningrad for the calibration of a-activities from fissile radionuclide targets, including targets with low activity and with complex configurations, is presented .
1. Introduction To determine the number of nuclei on targets of fissile radionuclides, the method of measuring the a-activity by means of standard semiconductor detectors, registrating a-particles under a fixed solid angle, is most commonly used. However, in a number of cases it is worthwhile to consider detectors different from the standard ones in size and shape. In connection with this the special detectors for the calibration of low active targets and targets with a nonplanar shape have been developed and constructed in the target group of the Radium Institute . The surface-barrier detectors were made from n-type silicon with a resistivity of 1-2 kS2 cm . In order to increase the operation voltage with the purpose of decreasing the inner capacitance and to improve the energy resolution of the detectors, the oxide layer was created on the silicon surface by means of special etching regimes including successive low temperature treatments, before the evaporation of the rectifying contact was performed. Also a protection of the edge junction by means of silicon-organic compounds was carried out. To increase the mechanical stability of the entrance window, a double-layer rectifying contact was used. Some details of the preparation technology are presented in ref. . 2. Experimental In the following subsections detectors that are presently used for the calibration of targets are discussed. 2.1 . Detector with an area of 50 cm 2
The use of detectors with large areas permits to increase the accuracy of the calibration . In this case the
error in determining the efficiency of the a-particle registration decreases, while the a-particles are emitted from targets in an arrangement with a fixed and comparably large solid angle (e.g., 2/3 of 2m ). Decreasing is also an influence of inhomogeneities of the active layer on the registration efficiency. The detectors prepared with a sensitive region and a diameter of 80 mm reached an energy resolution of 100 keV (for a-particles with an energy of 5.5 MeV) and the reverse current is - 3 WA at a voltage of 200-300 V. On the basis of this detection device a counter with a registration efficiency of (4.759 t 0.004) X 10-2 and (1 .336 t 0.0015) X 10-1, respectively, measured at different distances between the target plane and the detector diaphragm with a diameter of (76.30 f 0.01) mm, has been constructed. The counter was applied for the calibration of 235U targets obtained from the Argonne National Laboratory, Argonne, USA, and the National Bureau of Standards, USA. 2.2 . Detector with the compensating screen
A detector, also with a diameter of 80 mm, with a central hole is the basis of the instrument with a weak dependence of the registration efficiency on the location of the source. For this purpose the screen in the shape of a rotational ellipsoid with a steplike surface is located before the detector; the hole is necessary for its fixing. Such a setup was used long ago  for the registration of a-particles by layers of zinc sulfide in connection with a photomultipher. The application of the semiconductor detector simplifies the construction, improves the energy resolution, and reduces the background of the arrangement. The detector has a resolution of -150 keV, and the reverse current is 1 pA at a voltage of 100 V. The registration efficiency of the new counter is 0.196 t 0.001 and remains constant in the limits of 0.5% IX . FISSIONING TARGETS
S.I. Lashaev et al. / Semiconductor detectors for calibration
at a shift of the point source of 15 mm from the centre, and in the limits of 1% at changes of the distance from the detector of 0 .5 mm . One of these counters was used in particular for measurements of the number of fissile nuclei put on the substrate in the shape of a part of a cylinder . 2 .3 . 21T a-detector The detector registrates all a-particles emitted from the active layer. The sensitive detector surface with an area of 7 .5 cm2 has the hollow shape of the spherical segment with a height of 3 .5 mm, with a basis diameter of 28 mm, and a sphere radius of 30 mm. The measurements can be carried out with targets of diameters of up to 24 mm . The detector has an energy resolution of 40 keV, and it permits to correct a-spectra of the scattering from the substrate and to determine the contents of the radionuclides present as impurities. The 21T a-detector is used for the calibration of low active targets, targets with inhomogeneous layers, with layers of thicknesses up to 2 mg/cm2 , and also for simultaneous measurements of a set of small, identical targets. 2 .4. Cylindrical detector It is made in the shape of a cylinder with a diameter of 5 mm with an axial hole for the ohmic contact and fixing it on the measuring stick . The length of the cylinder is 14 mm, the sensitive region is 10 mm . The technology of its preparation is described in ref .  . The detector has an energy resolution of 40 keV and is used for measurements of distributions of the surface densities of uranium layers, located on the inner surface of elongated tubes . 2 .5 . Canal detector For cylindrical targets with an active layer on the outer side, an efficiency close to 21T for registring activi-
ties can be obtained if one puts the target inside the canal detector . The length of the sensitive region has to exceed the length of the active layer . Such a detector having a sensitive canal with a diameter of 5 mm and a length of 46 mm has been constructed  and was used for the calibration of needle targets with a diameter of 1 mm. The resolution of the detector was 35 keV and it had a comparatively small reverse current of - 0.15 pA at a voltage of 300 V. For the calibration of targets with larger diameters similar detectors with a diameter of the canal of 7 .5 mm have been prepared .
3. Concluding remark As is seen from the mentioned measuring devices, the set of constructed semiconductor detectors permits to solve rather different problems of the measurements of a-activities of various targets .
References [11 S .N . Gamajunov, V.G . Grlii, S .I . Lashaev, V.D . Maisjukov, and I.E . Mirskii, Nowosti IAI 4 (1986) 19 .  V.S. Valayavkin, A.S. Voronin, S.I . Lashaev, S .M . Svloviev, P .S . Svloshenkov, and P.I. Fedotov, Pricl . Yad. Spectr. 10 (1981) 102 .  H.P. Robinson, Proc . Symp . on Metrology of Radionuclides, Vienna (l959) .  A .S. Voronin, S.I . Lashaev, S .M. Svloviev, and P.S. Soloshenkov, Preprint KIYI 81-10 (1981) p . 20.  S.I. Lashaev, S .M . Svloviev, and P .S. Soloshenkov, Pricl . Yad. Spectr. 9 (1979) 52 .  S .I . Lashaev, S .M . Svloviev, and P .S . Soloshenkov, Thes . Dokl. XXXI Sov . po Yad. Spectr . i Strrct . Atom Yadra, Samarkand (1981) p . 630 .