A focal-plane counter equipped with a large silicon detector

Nuclear Instruments and Methods in Physics Research A287 (1990) 499-505 North-Holland

499

A FOCAL-PLANE COUNTER EQUIPPED WITH A LARGE SILICON DETECTOR S. KATO Department of Physics, Yamagata University, Yamagata 990, Japan S. KUBONO, T. MIYACHI, S. OHKAWA, Y. FUCHI, M.H. TANAKA and M. YASUE Institute for Nuclear Study, University of Tokyo, Tokyo 188, Japan Received 22 May 1989 and in revised form 18 September 1989 A focal-plane counter for a magnetic spectrometer was fabricated using a large, home-made silicon detector as an energy counter for particle; identification . The resolution of the particle identification was tested by measuring rarely-happening 8He particles in an environment of a large background of other particles . High-resolution energy signals from the silicon detector were essential for the background reduction. The silicon detector covered an energy range as wide as 5.5%. 1. Introduction

2. Composition of the counter system

A multi-nucleon transfer reaction is very powerful for mass measurement of unknown nuclides . The larger the number of transferred nucleons, the more mass-unknown nuclei one can produce . Because the cross sections of such reactions become smaller as one increases the number of transferred nucleons, higher resolution of particle identification is needed . A magnetic spectrometer is a powerful tool to identify nuclear species of charged particles . It selects particles of a given magnetic rigidity without changing their energies . Signals of the energy and the energy loss from its focal-plane counter are also useful for the identification of the particle species. Because an energy counter must back up a position counter, one often cannot help limiting the sensitive length of the position counter to that of a commercially available silicon detector (typically 5 cm) [1-5], or using a scintillation counter with poor energy resolution [6-10] . This problem is resolved if one can use a silicon detector large enough to back up a position counter of moderate length . We fabricated a focal-plane counter equipped with a large, home-made silicon detector for the energy measurement . The system was successfully tested by detecting rarely-happening 8 He particles from the (cc, 8 He) reaction on 197Au and "` Zr. The reaction on the latter target was difficult to measure because the counter was hit by a large number of alpha particles. In this report, we first describe the typical case (197Au target) in detail and the difficult one (9°Zr target) briefly as a comparison.

The counter system consisted of two position counters, an energy-loss counter and an energy counter. Its cross-sectional view is shown in fig. 1 . The position counters were resistive-wire proportional counters, which had a drift space and proportional chambers. The position information was derived by charge division of signals from the two ends. The counters were modified from the commonly used one [11]. We measured the position twice in order to obtain information not only on positions of the particles, but also on their angles of incidence to the counter . The latter was useful for reducing background counts . The sums of the signals from both ends of the two position counters were also available as two additional pieces of energy-loss information. The energy-loss counter was also a proportional counter. The energy counter was a large, home-made silicon detector with a length of 11.5 cm, a height of 2.5 cm, and a thickness of 2 mm. Its fabrication procedure is described in the next section . The effective lengths of the position counters were limited by the entrance cover of the silicon detector to 11 cm. We put the position counters not along the focal plane of the spectrometer, but perpendicularly to the mean trajectory of the particles to cover an energy range as wide as possible. By the normal-angle incidence of particles to the counter, the degradation of the position resolution resulting from the oblique incidence was avoided . The spreading of the energy-loss signals due to the incident-angle dependence of effective thickness of the counter was minimized . The position where

0168-9002/90/$03 .50 (D Elsevier Science Publishers B.V . (North-Holland)

S. Kato et al. / A focal-plane counter

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the particle intersected the focal plane was derived from the two positions along the two position counters .

p-type epoxy resin

Au electrode

3. Fabrication of the silicon detector The silicon detector must be thick enough to stop 65-MeV alpha particles and must be long enough to cover the necessary energy range. For such purpose, we fabricated a large, lithium-drifted [12], and totallydepleted silicon detector [131 from a crystal for LSI devices . By the recent development in the single-crystal growth technique by Shin'etsu Handoutai Co. [14], large and highly-purified silicon crystals have been available. The company made a 6 in. diameter rod of dislocationfree, p-type crystal by a floating-zone method. A crystal seed of (100) face was used instead of ordinary (111) one because the crystallization. into a large rod is easy. It supplied us with 6-mm-thick wafers cut from the rod. The resistivity was then about 500 SZ cm. We had to enhance the resistivity for the detector use .

ISINIS

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.

.. . .

Fig. 1. A cross-sectional view of the counter. The indicated elements are (A) conductive wire of the energy loss counter, (B) resistive wire of the position counter, (C) potential-shaping wires, (D) grid wires, (E) potential-shaping plate, (F) plate for drift, and (G) silicon detector . The length of the counter wires was 21 cm and that of silicon detector was 11.5 em . The height of the window was 3.5 cm . The foil was made of mylar. Its thickness was 0.025 mm . The counter gas, a mixture of argon (959) and carbon dioxide (5%), was used at 1 atm. The resistive wire was made of nichrome whose diameter and resistance were 17 .5 itm and 5.5 k2/m . The conductive wires were made of gold-coated tungsten . Their diameter was 12 .5 R m. The grid and the potential-shaping wires were made of alloy of copper and beryillium with 50 Rm diameter . High voltage of - 1000 V, + 1130 V and + 1280 V were imposed or. the drift plate, resistive wires and conductive wires.

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AI electrode Si crysta! wafer

Fig. 2. A cross-sectional view of the silicon detector. The size of the sensitive volurne was 115 mm x 25 mm x 2 mm.

We evaporated lithium onto a silicon wafer which had been heated to 400°C in a vacuum evaporator at a pressure of 10 -4 Torr. Then, the temperature was kept for 15 min to urge the thermal diffusion. After a rapid cooling down, we imposed a bias of about 1 kV/cm for two weeks at 100°C in order to make the lithium drift into the crystal. The N+ and P+ layers were removed so as to unify the lithium-drifted depth. The silicon wafer was shaped into a cuboid (115 mm long, 25 mm high, and 2 mm thick). By these processes, most of the p-type impurities were compensated by the drifted lithium . The resistivity of the wafer became about 100 k SZ cm. After cleaning the surface, we mounted the wafer on an epoxy-resin frame with epoxy glue. Gold and aluminum were evaporated to form 200 A and 1000 A layers on each surface as a surface-barrier and an ohmic-contact electrodes . Fig. 2 shows a cross-sectional view of the detector. Current and capacitance as functions of bias voltage at room temperature are shown in fig. 3. The detector behaved as a diode and its capacitance became about 250 pF above 300 V. Alpha particles from an 241Am source were detected at the center of the detector . After subtracting a contribution from electronic circuits, the intrinsic resolution of the detector was about 50 keV at room temperature . We measured a position dependence of the collection of charges, which were generated by a collimated beam

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S. Kato et al. / A focal-plane counter

501 97Au + a REACTION

of 1150-nm infrared light. The non-uniformity of the sensitivity was less than ± 5%, except near the frame.

E = 65 MeV,

e = 8'

4. Signals from the counter system We bombarded self-supporting foils of '97Au and 9°Zr by a 65-MeV alpha beam from the Sector-Focusing cyclotron of Institute for Nuclear Study . The accumt:lated charges of the beam were 68 and 73 mC for these targets . The target thicknesses were 1.0 and 0.3 mg/cm2. Reaction products were analyzed at 8° with a solid angle of 5 msr by the quadrupole-dipole-dipole (QDD) magnetic spectrometer [151. Because the reaction products were analyzed by the magnetic spectrometer, the energy signal was nearly proportional to Q 21A, where Q and A are its charge and mass numbers. Similarly, the energy-loss signal was proportional to (AZ/Q ) 2, where Z is its atomic number . Vertical position on the focal plane was deduced from the time difference between the fast energy signal and the slow energy-loss signal. The time of flight (TOF) was obtained from the interval between the rf signal from the cyclotron oscillator and the fast signal from the silicon detector. The four signals from both sides of the two position counters, the energy-loss signal, the energy signal, the vertical-position signal, and the TOF signal were analyzed by analog-to-digital converters. Counts of eight-fold coincidence were recorded on magnetic tapes event by event. 5. Particle identification by only the energy and the energy loss The magnetic rigidity of R He2+ particles from the R He)'93Au reaction corresponding to the ground t93Au was larger than that of the incident alpha state of beam. Therefore, the number of background particles was small on the focal plane of the magnetic spectrometer . Fig. 4 shows a contour plot of the energy (abscissa) and the energy loss (ordinate) for the reaction products. Because the particles were analyzed by a magnetic spectrometer, counts must be located not continuously, but discretely along the hyperbolae of AE E a AZ 2 . They must satisfy the condition E ac Q21A simultaneously . Although the Q2 /A values for 'He 2+ and 6 Li 2 + are the same, the horizontal locations of these particles were different because the energy change through the entrance foil and the counter gas was not negligible. This effect had to be treated carefully because the energy resolution of the silicon detector was better than the deviations. '97Au(«,

0

W Z W 0

W m Z W Z Z U

CHANNEL NUMBER OF ENERGY

Fig. 4 A contour plot of the energy (abscissa) and the energy loss (ordinate) for the 97Au+ a reaction. Although the full channel numbers are shown by 1024 x 1024 channels, they were reduced to 64 x 64 blocked channels. The contour levels are in logarithmic steps . The solid contours show 1, 10, 100, 1000 and 10000 counts per block (16X16 :hannels) . The dotted contours indicate 2 and 5 times of them. The peak of R He 2+ is expected to appear to the right of the 4 He + peak.

The ratio of the energy-loss signals of 4 He + and 6He2+ was not equal to 16 :9 . The vertical locations also deviated from the expected position . The deviation did not matter because the resolution of the energy-loss signals was not so good. The peak of 4 He + served as a guide to search for the peak of R He 2 + because the energy losses of these two particles are the same. There is a peak of RHe2+ near the expected position . Although its energy-loss signals were not well separated from the tail of 'He 2+ peak, we set an energy-loss window so as to pass the 4 He + particles . Fig. 5 shows a position spectrum after the particle selection by only the energy and the energy loss. Because the particle selection was not complete, there are some spurious peaks of other particles in the figure. 6. Improvement of energy resolution by the position information Fig . 6 shows an energy spectrum after the particle selection by the energy loss. The energy resolution is not good enough for the identification of particles . Because the energy range covered by the presem counter system was not negligible, the energy of each nuclear species was not constant but dependent on the position along

S. Kato et al. / A focal-plane counter

502 197Au + aRF-ACTION 25' . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

197Au +

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E=65 Mev, 0=8*

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Fig. 5. A spectrum of the position along the first position counter after the particle selection by the energy and the energy loss . Taere are some spurious peaks. For the peak identification, see fig. 12.

the counter. Fig. 7 shows a two-dimensional spectrum of the energy (abscissa) and the position along the first position counter (ordinate) . There are some loci which are not perpendicular. The lower-energy tails of the 'He 2 + and 4 He + peaks appeared at the ends of the silicon detector . The tail counts per channel of the energy spectrum were less than 1/1000 of 'He 2+ peak . Because the silicon detector was enough sensitive at the ends, the tail did not matter as long as the particle selection is not affected . By the information on the position, we modified the energy so that these loci became vertical . Fig. 8 shows a spectrum of the modified energy in which the counts from the ends of the silicon detector were temporarily 197 Au + a REACTION

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Fig. 7. A scatter plot of the energy (abscissa) and the position along the first position counter (ordinate). The loci of 8He 2+ and 6 He 2+ particles are discrete because the low-excited states of the residual nuclei are seen . eliminated . The resolution of the energy spectrum was improved to 200 keV (1/220 of the total energy). We became able to set quite a narrow energy gate for the particle identification . 7. Background reduction by the incident angle and the TOF Fig. 9 shows a scatter plot of the two positions without any particle selections . The counts must be distributed on the diagonal line if the characteristics of

E =65 Mev, e= 8'

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Au + a REACTION

197

E=65 Mev, 0=8*

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Fig. 6. An energy spectrum obtained by the silicon detector . The particle selection was made only by the energy loss and the incident angle.

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Fig. 8. The energy spectrum modified by the position and restricted to the useful region .

S. Kato et al. / A focalplane counter 197

Au + a REACTION

503 197 + Au a REACTION

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Fig. 9. A scatter plot of the two positions for the i97Au+ a reaction. Positions along the first and the second position counters are expressed by the abscissa and the ordinate. For the sake of clarity, only the first 1000 counts are plotted. A group of improper particles appeared at the upper-right corner. Their angle of incidence was not normal and dependent on the position . the two position counters were identical. The distance from the line indicates the incident angle to the counter. There is a group of improper incident angle at the upper-right corner. These particles must be scattered near the right (higher-energy) end of the counter. The removal of these improper counts brought the elimination of some improper groups at the lower-left corner of fig. 4.

197Au(a,8He)193Au

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896

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Fig. 11 . A scatter plot of the modified energy (abscissa) and the energy loss (ordinate) after the background reduction by the incident angle and the TOR Information on the TOF was also useful for the reduction of the background. Fig. 10 shows a TOF spectrum after the selection by the incident angle, the energy and the energy loss . An abnormally narrow peak is seen at the lower channels . These counts were excluded . Fig. 11 shows a scatter plot of the modified energy and the energy loss after the background reduction by the incident angle and the TOF. The restrictions by the energy and the energy loss which had been once determined were temporarily released here for a comparison with the previous spectrum of fig. 4. The isolation of 8 He 2+ peak was much improved by the background 197

E =65 MeV, 0=8*

Au(a,8He) 193Au

E=65 MeV, e=8*

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Fig. 10 . A TOF spectrum. Since the vertical scale is k. garithmic, the counts near 50 channel are large.

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Fig. 12 . A spectrum of the position along the focal plane for the 197Au((c, s He)19-'Au reaction .

S. Kato et al. / A focalplane counter

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reduction . If necessary, we can easily delete the lowerenergy tail of the 6He2+ peak by rejecting the counts from the ends of the silicon detector . The high resolution of the energy counter is very powerful for setting a narrow gate to obtain a background-free spectrum of 'He particles . The resulting position spectrum along the focal plane is shown in fig . 12. The reaction Q value and the atomic mass of the residual 193Au nucleus are reported in ref. [161.

9°Zr + a REACTION

E=65 MeV, 0=8*

8. Further reduction of the background Since there remained no background particle in the 197Au(a, s He)193Au spectrum, we applied the means of the further background reduction to the other case. The magnetic rigidity of gHe 2+ particles from the 9° Zr(a, 8He)86Zr reaction was smaller than that of the incident alpha beam. A large number of 'He 2 + particles were scattered by the wall of the vacuum chamber and hit the counter causing many background counts. The selection of gHe2+ particles was difficult . Fig. 13 shows a contour plot of the energy and the energy loss for the 9°Zr + a reaction after the background reduction by the incident angle and the TOF . The peak of g He2 + is not clearly isolated. Moreover there is a remarkable locus (hyperbola) that should not appear. Besides the incident angle and the TOF, we utilized the energy losses through the two position counters and 90Zr + a REACTION

E = 65 MeV,

e=8'

CHANNEL NUMBER OF ENERGY

Fig. 14. A contour plot of the energy (abscissa) and the energy loss (ordinate) for the 9°Zr+ a reaction after the background reduction by all the available means. the vertical position on the focal plane as well for the background reduction . Resolution of the energy-loss signals from the position counters was a little worse than that of the exclusive energy-loss counter. The gates of these energy-loss signals were set so as to pass the 4 He' particles . A wide gate of the vertical position. was set because the spectrum was composed of a broad single peak. Fig. 14 shows a contour plot of the energy and the energy loss after the background reduction by all the available means . Although many counts whose energy loss was different from g He2+ passed the selection, the background was fairly reduced . A peak of 8 He2 + is clearly isolated. The high resolution of the energy counter brought us a background-free spectrum of 8 He particles . The reaction Q value and the atomic mass of 86 Zr will be reported elsewhere [171. 9. Discussion

CHANNEL NUMBER OF ENERGY

Fig . 13. A contour plot of the energy (abscissa) and the energy (ordinate) for the 9°Zr+ a reaction after the background reduction by the incident angle and the TOF.

loss

For the selection of 'He particles. the energy and the energy loss played very important roles. They were positively used for assigning nuclear species. The highresolution energy signals from the silicon detector was very powerful because we could set a narrow gate for the particle selection. This is a contrast to the low-resolution energy signals from the scintillation counecr which required a special device for the background reduction [91. The incident angle, the TOF, the energy losses from the position counters and the vertical position were

S. Kato et al. / A focalplane counter

useful for the reduction of many background particles. It is desirable that one uses as many means of background reduction as possible. By the use of seven such means, we succeeded to obtain background-free spectra of s He particles in an environment of a large number of background particles . The counter proved to be useful for measuring rarely-happening particles whose cross section is less than 0.3 nb/sr (one count in the spectrum) . The sensitive size of the silicon detector (11 cm) was longer than any other reported silicon detectors for the particle-identification use [1-51 . By the length of the silicon detector, and by the setting of the counter perpendicular to the particle trajectory, the counter covered quite a wide energy range. From the known dispersion of the spectrometer (4 m) [151, the covered energy range was 5.~% . This energy range is broad enough to measure unknown Q values by a single setting of the magnetic field strength since the uncertainty of most mass prediction is less than 1 MeV . 10. Summary The present counter system consisted of a large, silicon detector, a proportional counter, and two position counters . Each element supplied us with the signals of the energy, the energy loss, the position and the angle. The vertical position and the time of flight were also obtained using the timing signals . Multi-parameter measurement of these signals enabled us to detect rare particles in an environment of a large background . Among them, the high-resolution energy signals from the silicon detector was very important for the particle identification. By the use of a large silicon detector, we covered a wide energy range retaining its high resolution. The covered energy range was broad enough to determine unknown Q values by a single setting of the magnetic field strength . Acknowledgements The authors are grateful to Dr. l. Sugai for his aid in preparing the target foils. They are also grateful to Dr.

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O. Nitoh, Prof. Y. Yoshida and Prof. T. Suehiro for their useful discussions . The aid in the procedure of fabrication of the silicon detector by Mr. Y. Ikeda, Mr. T. Kobayashi and Mr. H. Matsuzawa of Shin'etsu Handoutai Co., and Mr. I . Ogura and Mr. H. Onabe of Tokyo Denshi Yakin Co. is also highly acknowledged.

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