Konishi, Hideaki

Koori, Norihiko

Matsumoto, Yuzuru

Kyoto University Research Information Repository

TitleA Rectangular Si (Li) Detector Adopted to Position SensitiveCounter Telescope (Commemoration Issue Dedicated toProfessor Takuji Yanabu on the Occasion of his Retirement)Author(s)Koori, Norihiko; Konishi, Hideaki; Matsumoto, YuzuruCitationBulletin of the Institute for Chemical Research, KyotoUniversity (1982), 60(2): 165-171Issue Date1982-08-31URL http://hdl.handle.net/2433/76981RightType Departmental Bulletin PaperTextversionpublisherKyoto University  Bull. Inst. Chem. Res., Kyoto Univ., Vol. 60, No. 2, 1982         A Rectangular  Si  (Li) Detector Adopted to            Position Sensitive Counter Telescope             Norihiko KooR!, Hideaki KONISHItand Yuzuru MATSUMOTO                               Received March 15, 1982     A simplified method is described to prepare Si(Li) detectors with a large rectangular sensitive  area which are adoptable to a position sensitive counter telescope for the study of fast-neutron  induced reactions.                 KEY WORDS: Large semiconductor detector/ Counter telescope/                       Instrumentation for neutron-induced reactions/                         I. INTRODUCTION    Silicon detectors with a large sensitive area have been developed for detection of primary cosmic rays') and for monitoring environmental a-rays.2) These have a circular sensitive area of 20-.40 cmz. Thick silicon detectors are essential for applications except detection of low energy a-particles. Thick silicon detectors with a large rectangular sensitive area have not been reported because of no need of rather particular specifica-tion for detectors.    A counter telescope with large solid angle is indispensable to measure charged particles from reactions induced by fast-neutrons, because the intensity is generally low. The authors developed a position sensitive counter telescope (PSCT) with both large solid angle (i. e. wide angular acceptance) and good angular resolution introducing 2 position sensitive proportional counters into the counter telescope.3) It was needed as a energy detector for this PSCT a thick silicon detector that has a large rectangular sen-sitive area (larger than 15 x 60 mm2) and a thickness enough to stop 14 MeV protons (thicker than 1. 5 mm) .    The lithium drifted (Si(Li)) silicon detector assures easily of the required thickness. One can say that methods of preparation of ordinary size Si(Li) detectors have been established in the years of 1960's.4,5) However, a problem has not been solved how one can obtain the rectangular sensitive area of required large dimension.    A method of preparation of the Si(Li) detector with large rectangular sensitive area will be described with obtained performances in this article.                     II. METHOD OF PREPARATION 1. Starting Material of Silicon Wafer                  *,* A : Department of Nuclear Engineering, Kyushu University, Fukuoka,  812. t Present address : Nittetsu Plant Engineering, Kita-Kyushu, 805.                            ( 165 )                       N.  KOORI, H. KONISHI, and Y. MATSUMOTO    Silicon crystal rods of larger diameter than 70 mm are not uniform in the resistivity and carrier life time in the peripheric region of the rods. In this work, a commercially available p-type silicon crystal rod (30 mm x70 mm/),) is chosen and sliced along the central axis to obtain rectangular wafers with uniform resistivity and carrier life time. The properties of the crystal are as follows: Dislocation free crystal pulled up in a vaccum floating-zone. The crystal axis is <1, 1; 1>. The resistivity p1000 fl•cm, the life time of minority carriers r700 fe sec, and the density of oxygen content 1016 atoms/cm' (0. 2 ppm). 2. Evaporation and Diffusion of Lithium    After lapping (finally with Al203 powders of 2000 mesh), washing (with C2HC13, CH3OH and distilled water under ultra sonic agitation, in turn) and etching for 10 min (with standard 3:1:1 etchant of HNO3, HF and CH3COOH), the wafer is heated upto 300°C with a heater and a mask (18 x 61 mm2) for evaporation in a vaccum evaporator. Metallic lithium is evaporated through the mask onto the wafer, and then the temperature is kept for 16 min at 300°C to make a n+ Li diffused layer of about O. 5 mm thick. The wafer is cooled down rapidly without thermal shock. The Li contamination on the surfaces except the evaporated region should be removed by lapping, washing and etching procedures. 3. Lithium Drift Operation    The wafer i. e. n+-p diode is mounted on an automatic drifting apparatus, that is schematically explained in Fig. 1. On the p-side of the diode, the alloy of Ga-In is painted over an area of about 16 x 60 mm2 for electric contact with the devices. Initially, the reverse bias of 350 V is applied at the diode temperature of about 120°C. The                                                               recoder               relay unitcomparator-:ref                                                      in in                                  A B    fan   V r•driftdiode heater currenttemp.                      ninr                    0             ff77thermo -      power  Vre=couple   supply71                    0 I ,                                                  7/7/ I I VAC  Cr'e                                 AC100 V ,bias  DCSSD  o  DC 0 - 400 V                            supply -                                                           heater fan                Fig. 1. Blockdiagram of automatic lithium drifting apparatus.                           ( 166 )                            Rectangular Si(Li) Detector                               (a)       start -                        1day                            2 days                                                                       ( b)           3days_100 0                                                              start                                                                                  o. -   V                                                    C                                                            -                                                              1 day                                                       o.c9     10 7                                                  j...jays                                      100 7                                                                     3 days              II                                                                              •  10100500 10100500              bias ( V )bias (V )           Fig. 2. Variation of the diode characteristics during the Li drift operation. reverse current (drift current) is kept at 8 mA by controlling the diode temperature with a fan or a heater plate. The diode characteristics (reverse current vs. bias and capacitance vs. bias) are measured every cloy at room temperature in order to estimate the thickness of Li compensated intrinsic layer. A variation of the characteristics is shown in Fig. 2. A measure of the capacitance corresponding to the intrinsic layer is calculated by the ormula: C(pF) =1. 05- A(crn2)/147(cm) . The capacitance is about 77 pF for the case of A= 1. 8 x 6. 1 cm' and W=0. 15 cm.    The Li drift operation is terminated when the intrinsic layer reaches to the initially p-side of the diode. Then, the electron injection from the electric contact increases the reverse current, for which the automatic drifting apparatus decreases the diode temper-ature. Time spent until the termination is about 80 hours. It seems that the mobility of Li ions do not change so much by the crystal axis, which is different from the case of ordinary circular Si(Li) detectors. The spread of the intrinsic layer on the p-side surface is observed by staining in hydrofluoric acid for 20 min after lapping of the p-side surface. Generally, the spread is not uniform at this stage. The drift operation should be repeated at the diode temperature of 100°C and the reverse bias of 160-200 V for about 40 h., after lapping, washing and etching procedures. Now, the electric contact is taken at only peripheric part of the p-side surface to spread the intrinsic layer over the surface. After the second drift operation, the spread of the intrinsic layer stained by hydrofluoric acid is obtained about 17 x 60 mm2, which is required for the application, as shown in Fig. 3. 4. Formation of Surface Barrier Junction    The wafer is immersed in hydrofluoric acid for 3 min and conserved in a desiccator                           ( 167 )                       N.  KOORI, H. KONISHI, and Y. MATSUMOTO         0110Mill011.11.101.110.11111.1.1.1110.1111111111011110111.10.100.01.61k                       - z                                          7                Fig. 3. Stained intially p-side surface after cooled drift operation. for 2 days to make oxidation layer on the surface, after lapping, washing and etching procedures. Gold is evaporated on the revealed intrinsic layer to form a surface barrier junction, and aluminum is evaporated on the n* Li-layer of the other side for the ohmic contact. The evaporated arcas arc 15;<60 mm2. It is important to assure the evaporated Au area is included in the intrinsic area. This surface barrier Si(Li) detector is mounted in a detector case.    The diode characteristics after the mounting in the case are shown in Fig. 4.    I II I                             Start (a )    100 7                                                             ( b )            1 day 200_                                                               - u.   -2 days                              100Fstart                      3 days' IS,50_                                           3 days - e>104 days   :4 daEia;-   -6 days -  10100500 10100500             bias ( V )bias ( V )         Fig. 4. Variation of the diode characteristics during the cleanup drift operation.                            ( 168 )                            Rectangular Si(Li) Detector Conservation  of  the detector biased at a low bias in a test chamber improves the characteristics very much as shown in the figure. This improvement is resulted by the effects that the over-compensated Li ions are drifted away to establish higher resistivity in the intrinsic layer and that the periphery of the intrinsic layer is adjusted to the Au evaporated area.                                 III. •                           PERFORMANCES   The surface barrier Si(Li) detector has a rectangular sensitive area of 15 x60 mm2 and thickness of 1. 8 mm. The detector was operated at a reverse bias voltage of 100 V, where the reverse current was several pA and its capacitance was about 60 pF. Particles incident to the Au evaporated side of the detector where the surface barrier was implanted. The uniformity of counting efficiency along the longer axis was measured by using 210Po a-particles through a 1 mm slit. Typical energy spectrum of the 5. 3 MeV' a-particles is shown in Fig. 5 (a). The FM-W. of the peak correspond to about 100 keV, which includes the attenuation of a -particles in the source itself. The energy resolution is good enough to use in the experiment of neutron induced reactions. Fig. 5 (b) shows the uniformity of about 2. 1% in the standard deviation, that means no insensitive part in the Si(Li) detector. Fig. 6 shows a energy spectrum of protons and a-particles from the 28Si(n, p) 28A1 and 28Si(n, a) 25Mg reactions, respectively, with 14. 1            200Po -a• (a)                                                                                            •                                  105 •                c 100                               ker:  o•                                                 .•    0•               0 160 180 200 ch                                         ( b) '        _ _. • .    1000--                  to        3•              0               0           010 20 30 40 50                            position ( mm )           Fig. 5. Typical spectrum of 210Po a- particles (a), and counting efficiency                   over the sensitive area (b).                            ( 169)                      N.  KOORI, H. KONISHI, and Y. MATSUMOTO   ei I I I I I I I I I                     r 3000 —•     IAI  I \_        til1IA I 128Si( n, a)25Mgo_        v. 1         C 1 1 II3 2 1 2000 — 11I \ •4           I1j5          1 028Si ( n,p)28Al—10 li.c111I1 .v• 41•3.2 ,:-Nc' 11..\        1                                       f, --1II Iil1_  .      ,.,1\iITmoo —               11..1I11 ,   I           1.11.7:11IL-                   .1 1i1                          I .iIi                     ,.1-                                                                                                                                                                       .  0 ..I.,III,                                                                      ,;•......,.........,  0    200300400                                   channel number     Fig. 6. Pulse height spectrum from the rectangular Si(Li) detector irradiated by 14 MeV            neutrons. The peaks correspond to the excited levels of 28A1 and 25Mg. MeV incident neutrons. This spectrum can be used for the energy calibration of the pulse height from the detector during the experiment. The FWHM of the peak corresponding to the ground state of 25Mg is about 230 keV, which includes the spread of the incident neutron energy and other contributions from noise sources. The reverse current increased gradually by the irradiation of 14 MeV neutrons during the experi-ment. Appreciable deterioration was observed in about one month experiment, where the total irradiation was estimated to be about 1011 neutrons. The characteristics could be recovered by the re-evaporation of Au and Al after lapping, washing and etching procedures. When the radiation damage in the bulk of intrinsic layer is accumulated, the recovery by the re-evaporation is not effective procedure. The preparation should be tried from the drift operation at low temperature.                      IV. CONCLUDING REMARKS    This surface barrier Si(Li) detector with a rectangular sensitive area of 15x60 mm2 was successfully adopted to the position sensitive counter telescope that had both a large solid angle of 40.0 msr and a good angular resolution of 5°, and covered a angular range of 35°.    It is essential for the preparation of large rectangular Si(Li) detectors that a good starting material of silicon crystal should be sliced along the central axis to obtain rectangular wafers. The lithium drift operation is achieved similarly to the case with                           ( 170 )                           Rectangular Si(Li) Detector normally sliced circular wafers. The spread of the intrinsic layer on the initially p-type surface must be larger than the required sensitive area.                       ACKNOWLEDGEMENTS   The authors are very indepted to Professor I. Kumabe for his continuous encourage-ment and valuable discussions.                           REFERENCES  (  1  ) M. Yoshimori and S. Uchiyama, Nud. Instru. & Meth., 130, 143 (1975).  (2) H. Shiraishi, Genshiryoku koogyo, 27, no. 9, 52 (1981).  (3) N. Koori, T. Goto, H. Konishi, and I. Kumabe, Nuc. Instru. & Meth. to be published.  ( 4 ) F. S. Goulding, Nud. Instru. & Meth., 43, 1 (1966).  ( 5) S. Shirato, M. Tanaka, N. Koori, I. Ogawa, M. Tsukuda, and E. Tajima, S JC-T-68-4 (1968),       University of Tokyo.  (6) ) Shin-etsu Chemical Industry Co., Tokyo.                            ( 171 )

A Rectangular Si (Li) Detector Adopted to Position Sensitive Counter Telescope (Commemoration Issue Dedicated to Professor Takuji Yanabu on the Occasion of his Retirement)

http://repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433/76981/1/chd060_2_165.pdf

A Rectangular Si (Li) Detector Adopted to Position Sensitive Counter Telescope (Commemoration Issue Dedicated to Professor Takuji Yanabu on the Occasion of his Retirement)

Abstract

Similar works

Full text

Available Versions

Kyoto University Research Information Repository