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A position sensitive counter-telescope with large angular acceptance for study of fast-neutron induced reactions
Nuclear Instruments and Methods 206 (1983) 413-419 North-Holland Publishing Company
413
A POSITION SENSITIVE COUNTER-TELESCOPE STUDY
OF FAST-NEUTRON
N. KOORI,
T. G O T O
INDUCED
*, H. K O N I S H I
WITH REACTIONS
LARGE ANGULAR
ACCEPTANCE
FOR
** a n d I. K U M A B E
Department of Nuclear Engineering, Kyushu University, Fukuoka, 812 Japan Received 4 December 1981 and in revised form 5 July 1982
A new position sensitive counter-telescope with a large angular acceptance has been developed for measurements of charged particles from neutron induced reactions. A high efficiency of measurement with a wide angular acceptance of 35 °, a large solid angle of 40.0 msr and a good angular resolution of 5 ° was established by the introduction of two position sensitive proportional counters and a large rectangular Si(Li) detector into a d E / d x - E type counter-telescope. The performances were studied by the n - p scattering at 14 MeV.
1. Introduction M e a s u r e m e n t s of n e u t r o n induced reactions consume a considerably long time because of the very low intensity of incident n e u t r o n beams. In most measurements of charged particles emitted from neutron induced reactions, d E / d x - E type counter-telescopes have been used with solid angles of a b o u t 5 - 1 0 msr corres p o n d i n g to their fairly good angular resolutions of a b o u t 10 ° fwhm [1 5]; the angular resolution is determined essentially by the geometry of counter-telescopes that is chosen to achieve the solid angle required for measurements. A counter-telescope with larger solid angle is desirable for efficient m e a s u r e m e n t of the reaction. However, the increase in dimensions of the target, the detectors or b o t h of the counter-telescope makes worse the angular resolution which is i m p o r t a n t for precise m e a s u r e m e n t of the angular distributions. Valkovic et al. [6] tried to improve the angular resolution of their counter-telescope (the solid angle was a b o u t 11 msr) by dividing the E detector into several sections with a multiwire proportional counter; their ameliorated angular resolution was still limited by the size of the target. Two position sensitive parts that give the information on the particle trajectory are necessary for further i m p r o v e m e n t of the angular resolution in the case of enlarged target and detectors. Recent developments of the position sensitive counters make it possible to assemble a position sensitive counter-telescope which consists of two position sensi* Present address: Nippon Atomic Industry Group Co. Ltd., Kawasaki, 210 Japan. ~* Present address: Nittetsu Plant Engineering Co. Ltd., KitaKyushu, 805 Japan. 0 1 6 7 - 5 0 8 7 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 c,) 1983 N o r t h - H o l l a n d
tive proportional counters (PSPCs), d E / d x proportional counters and an E detector, with a good angular resolution a n d high m e a s u r e m e n t efficiency. The position information from the two PSPCs placed directly b e h i n d the target a n d in front of the E detector can be used to determine the particle trajectory and the precise reaction angle in the target. This article describes a developed position sensitive counter-telescope with both a large angular acceptance of 35 ° a n d a good angular resolution of a b o u t 5 ° to measure efficiently reactions with small differential cross sections. The following properties are also required of the position sensitive counter-telescope as in the cases of conventional counter-telescopes. These are (1) the possibility of particle identification, (2) good energy resolution and (3) efficient rejection of b a c k g r o u n d events. In the present counter-telescope system, the particle identification was performed by a two dimensional d E / d x - E spectrum. A lithium drifted silicon [Si(Li)] detector with a large sensitive area which was suitable to the dimensions of the PSPCs was developed to obtain good energy resolution. The multiple coincidence between the two PSPCs, d E / d x proportional counter and E detector was adopted for the rejection of background events.
2. Description of the position sensitive counter-telescope 2.1. Determination of the reaction angle The reaction angle of particles from a reaction can be determined precisely by the position information from the two PSPCs of the counter-telescope. The w i n ciple of the position sensitive counter-telescope is schematically shown in fig. 1, where the geometry is
N. Koori et a L / Position sensitive counter-telescope
4]4
considered only in the counter plane that consists of the n e u t r o n source and the anode wires of the two PSPCs. The reaction angle 0 for each emitted particle is given by the following formula using the geometry defined in fig. 1; 0 = tan-
12
[
j - tan
1
10 c-os O0
PSPC1 AE counter
:hdmber
telescope (lxis
'
/ i
where 00 is the setting angle of the telescope with respect to the direction of the central beam of incident neutrons. The reaction point r is determined by the position information of xj and x 2 from the two PSPCs; Ii
r=x2-~2(x:-xl). counter
ptane
2.2. The position sensitive proportional counters Two PSPCs (PSPC1 and PSPC2), a d E / d x proportional counter and an E detector are placed in a counter c h a m b e r as depicted in fig. 2. The PSPCs (PSPCI 45 m m a n d PSPC2 65 m m in length) are set at distances l~ = 98.0 m m and l 2 = 56.5 mm. The depth and height of the PSPCs are 10 m m and 12 mm, respectively. The anode wire of the PSPC is a highly resistive (8 k g 2 / m m ) carbon-coated quartz-fibre (25 p,m in diameter). The cathode of the PSPC consists of two parallel plates placed 6 m m from the central a n o d e wire. A t a n t a l u m plate of 0.3 m m thickness backed with 4 m m thick graphite is used as this cathode plate for realizing a low counting rate of b a c k g r o u n d events. The position d e p e n d e n t charge signal Qp from the PSPC is derived from one of the anode terminals a n d the total charge signal Qt from the cathode plate. T h e position information i n d e p e n d e n t of the a m o u n t of energy losses can be obtained from the following equation;
Fig. 2. Schematic views of the position sensitive counter-telescope,
where x is the distance of the incident point from the other anode terminal and L is the length of the PSPC. Each module of the PSPC was tested by 21°po 5.3 MeV c~-particles collimated to 0.2 m m wide on the anode. Operating gas of Ar + 5% CO 2 was flowed into the counter c h a m b e r at a pressure of 300 Torr. A high voltage of 800 V was supplied to the anode. The posi-
x / L = Qp/Qt,
PSPC2 E-DETECTOR
PSPCl +
0'~'~'.,~.
TARGET
NEUTRON~"~'-~
SOURCE
In
L
I-
"-C.''<..
POINT
-
~ ~
t,
[2
\'1 \'1 \'1
\'t
Fig. 1, The reaction position r and reaction angle 0 calculated from the trajectory of the emitted particle that passes through the positions x I and x 2 in the position sensitive proportional counters PSPC1 and PSPCZ respectively.
N. Koori et al. / Position sensitive counter- telescope
tion resolution obtained is 0.5 mm in fwhm, which includes the beam spread, noise of the preamplifier and the resolution of the analog division circuit. The linearity of the position determination is good enough for the purpose except in the region within 5 mm from both sides of the PSPC. 2.3. The dE / dx proportional counter
The d E / d x counter of 55 mm length is employed for the measurement of energy losses of particles and for rejection of background events. The counter of 40 mm in depth is divided into two similar parts of 20 mm with five field defining wires; similar field defining wires are used also at the entrance and exit of the counter in order to define the sensitive volume of the counter. A tungsten wire (50 /~m in diameter) plated with gold is used as the anode wire. The cathodes have a similar structure to the PSPCs and are connected electronically. Signals of the energy losses from the cathodes are used for the particle identification. Two signals from the anodes can be used as coincidence signals to reduce the background in severe cases. Measured distributions of energy losses for 14 MeV protons and 12.5 MeV deuterons are compared with the calculated results in fig. 3; the figure indicates a good separation of protons and deuterons. 2.4. The E detector
The E detector for this counter-telescope must have a large rectangular sensitive area and a thickness correF
I
f
I
415
sponding to the range of 14 MeV protons. An Si(Li) detector with an area of 14 × 56 mm 2 and a thickness of 1.8 mm was fabricated in the laboratory. A rectangular p-type silicon wafer was sliced along the axis of the crystal rod of the ~ 1, 1, 1) orientation for use as starting material. The fabrication method was similar to that described by Shirato et al. [7]. The energy resolution obtained is about 70 keV fwhm for 21°po a-particles, and the counting rate is uniform in the sensitive area within a statistical error of less than 2%. The solid angle subtended by this detector through a slit (40 mm x 12 mm) is 40.0 msr, which is larger by a factor of 4 - 8 than those used in most neutron experiments. The energy range for protons can be covered up to 17 MeV with this detector; it is possible to extend the range by replacing the E detector by a thicker one which would be prepared up to 5 mm in the thickness. 2.5. Electronics
Signals from the counter-telescope were processed by a small on-line computer (Toshiba USC-3). The block diagram of the electronics is shown in fig. 4. The gate signal from the a-monitor of T(d, a)n neutrons was used for the coincident events in the counter-telescope when the low background was strictly required. The signals of Qp (PP1 and PP2) and Qt (PTI and PT2) from the PSPCs were fed to the computer through the ADCs. The position of the detected particle was calculated by the computer to obtain a wider dynamic range than that of the analog division circuit used in the test I
I
I
PROTON 14 M e V
--
100 -
100
DEUTERON 12,5 I~eV
,._1 hl Z Z
A
T U
U')
ut,) o
Z 0
o
50
--
0
-
*.-*~+ --
20
L
60 ENERGY
<,.9 n." Ill Z t.,t.I
+
I
40
50
80
100
ch
0
LOSS
Fig. 3. Comparison of measured distribution of energy lost by 14 MeV protons and 12.5 MeV deuterons with the calculated distributions.
416
N. Koori et al. / Position sensiti~e counter-telescope
USC-3 COMPUTER I
I
I
]~ gate
I
[ADClIADcSlADCTIA ADC2ADC6IADC8I I
I
I
I
I
I
I
I
iDA Jo I iDA J[DA IDA IDA ] I
ILA ILIA I L1 I LAI'A L ILAI '
l
t El
tar;t"~O"~n. 2 _source ~ -monitor
Fig. 4. Block diagram of the electronics. PA: preamplifier, TFA: timing filter amplifier, CFD: constant fraction discriminator, GDG: gate and delay generator, TSC: timing single channel analyzer, COINC: coincidence circuit, FD: fast discriminator, SD: scaler driver, PS: pre-scaler, S: scaler, R: rate meter, LA: linear amplifier, DA: delay amplifier, ADC: analog-to-digital converter.
of the PSPC modules. The energy signal and the d E / d x signal were also sent to the computer through the ADCs. Six parameters for each event were acquired with the c o m p u t e r in the mode of event-by-event recording on magnetic tapes. The single spectrum for each parameter and the position spectrum for each PSPC were monitored on an oscilloscope during the experiment. After the experiment, the data recorded on magnetic tapes were analysed even by event to determine the trajectory and reaction angle and to obtain the precise angular distribution with another larger computer ( F A C O M M-200 at the computer center in Kyushu University).
3. Measurements and results 3.1. Measurement of the n - p scattering at 14 M e V The performance of the counter-telescope was studied by the n - p scattering at 14 MeV, whose differential cross sections could be used as the reference of the measurement. The counter-telescope was set at a distance l 0 of 21.7 cm from the neutron source, and was turned around the vertical axis of the target for measurement of the angular distribution. The position resolutions of the PSPCs were measured for recoiled protons that were emitted from a
polyethylene target of 20 m g / c m 2 defined by a slit of 2.8 mm wide and detected through four slits of 0.5 mm wide placed just in front of the E detector. The observed resolutions in fwhm were 1.8 mm for PSPCI and 2.2 mm for PSPC2, which were worse than those measured for 5.3 MeV s-particles. This deterioration is attributable to much smaller energy losses of 14 MeV protons compared to that of 5.3 MeV a-particles in the PSPCs, The angular resolution due to the position resolutions was estimated to be about 4 ° by a numerical calculation of the aperture function which is a frequency distribution of the real reaction angles in the 3-dimensional space for a finite angular interval defined in the counter plane [8]. The calculation also showed that the contribution of the finite geometry effect to the angular resolution varies from 3 ° to 5 ° with the reaction angles defined in the counter plane. The overall performance of the counter-telescope was studied by measurement of the angular distribution of n p scattering at 14 MeV. The target for this measurement was a thin plastic scintillator sheet (NEI04) of 41.9 m g / c m 2 thick. The target was enlarged to 20 mm wide and the narrow slits in front of the E detector were replaced by a large slit of 40 m m × 12 mm for the measurement of the angular distribution. The measurement of the n - p scattering was done at the setting
N. Koori et al.
/
Position sensitive counter- telescope
angles of 0 0 = - 2 0 ° , 0 °, 18 ° a n d 38 ° . T h e scattering angles c o v e r e d were - 37.8 ° to - 2.2 ° for 00 = - 20 °, - 1 7 , 9 ° to 17.9 ° for 0 0 = 0 °, 0.1 ° to 35.8 ° for 0 0 = 18 ° , a n d 20.5 ° to 55.3 ° for 00 = 38 °. T h e m e a s u r e d energy s p e c t r a s h o w n in fig. 5 are r e a s o n a b l y b r o a d for the wide scattering angle range covered. Fig. 6 s h o w s the p o s i t i o n s p e c t r a of the P S P C s m e a s u r e d at the s a m e setting angles as t h o s e for the energy s p e c t r a in fig. 5. T h e p o s i t i o n s p e c t r a at 00 = 0 ° are e x p e c t e d to be s y m m e t r i c b e c a u s e the a n g u l a r d i s t r i b u t i o n of the n - p scattering at 14 M e V is s y m m e t r i c a r o u n d the scattering angle o f 0 °. Small d e v i a t i o n f r o m s y m m e t r y a n d n o n u n i f o r m i t i e s o b s e r v e d in the p o s i t i o n s p e c t r a m a y be d u e to the m i s a l i g n m e n t o f c o u n t e r s in the telescope. A n e x a m p l e o f the f r e q u e n c y d i s t r i b u t i o n of the r e a c t i o n p o s i t i o n (i.e. the scattering p o s i t i o n in this case) on the target is s h o w n for the case o f 00 = 0 ° in fig. 7. It was o b t a i n e d by the calculation of the reaction p o s i t i o n for each event with the f o r m u l a given in sect.
417
20001 10001 O-
g
r
PSPC1
PSPC2
•.-~'~ ~ •".
,
-,
N~= 10!°
..
.-.:..
. .~....~/
%
,'._
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i
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0o 2000
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....~,.,~~:,,,,.,,~,,,~;...~....,.........,.
1ooo
o
~ 2000 18' z
1000
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00 =-20"
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N,t: 1010
1000
.,..:. 1000
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.-...-'.'u"."~...,
,
k I'---
,
00 = 0*
, I J
4000
00
I
20
40
60
20
xl
POSITION
40
60
h
x2
(chdnnet)
Fig. 6. Position spectra of the PSPCs for the n - p scattering, which are compared with the calculated results by the Monte Carlo stimulation.
3OOO Z
• "I
2000
I u
~I i5 ~,~z,
1000 Z 0 u 0
2000
eo=18"
•,,
""
2.1. As s h o w n in the figure, the d i s t r i b u t i o n d e f i n e s clearly the target region. It suggests the possibility of exclusion o f b a c k g r o u n d events that c o m e from regions o t h e r than the target. This possibility is very effective for the low b a c k g r o u n d m e a s u r e m e n t o f the n e u t r o n i n d u c e d reactions.
..
., 00= 0*
1000
j w Z z
0
2000 1000
o;
3000
O0= 3B*
u
.t-;.-----.-,.,,,. ENERGY(channel)
2000 --
•1
;-20am--4.
z 1 0 0 0 -0
100
Fig. 5. Energy spectra of recoiled protons in the n - p scattering at 14 MeV. Since the counter-telescope covers a wide angular range at each setting angle 0u, the peak of the recoiled protons is reasonably broad.
0
-i 0
i
i'~ i
20 40 60 80 POSITION(¢hclnnet)
Fig. 7. Distribution of the reaction position in the target for 00 = 0 ° derived by the calculation of the particle trajectory which is determined with the position information from PSPCI and PSPC2 for each particle.
N. Koori et al. / Positron sensttwe counter- telescope
418
3.2. The angular distribution
I
I
+
By using the derived reaction (scattering) angle for each event, the events were classified into six small angular divisions (about 6 ° ) to obtain the precise angular distribution of the scattering and the energy spectra for these angular divisions. Fig. 8 shows the variation of the classified energy spectra for the case of 0o = 18 °. The angular distribution of the n - p scattering was o b t a i n e d from the classified energy spectra for the small angular divisions. In this classification, the events that came from places other than the target were rejected a n d the background events were subtracted. The elastic parts in the derived energy spectra were translated into the differential cross sections. Fig. 9 shows the angular distribution c o m p a r e d with the expected one. A fairly good agreement is obtained within the statistical errors. At extreme angles for each setting angle, the agreement is not so good because of the non-uniformity of the counting efficiency of PSPC2.
I
I
I
[
I
%:o °
+ %:18 ° 250
K, % = 3 B °
200
15C
.J,
100
50
o
I
I
I
1
I
[
[
50
40
30
20
10
-10
-20
ANGLE ( deg )
Fig. 9. Angular distribution of the n - p scattering measured by the position sensitive counter-telescope and the expected distribution.
3.3. C o m p a r i s o n with the simulation
50f 0=0'13%6'10° 0
"'M"""N"" - - ' ~ • "
50-o
"~'~.80=" 18°; t " " " ~ " ""~""~"%"
0=6,10~12,02
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50
0 Z
o
o.
o
•
d Onp
1 1 -#"
•
--
0=2391%29,85"
5 0 --
."
/ I
.- ""
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"~A ~ ' ~5" v v ' ~ " ENERGY
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= 4 COS 0p' 52.9[ 1 + 0.0639 COS(20p) + 0.0288 COS(20p)] m b / s r ,
", °,,,,-~.° •
" ""~''~";:"="•
The energy and position spectra were simulated by a calculation with the M o n t e Carlo method that included the finite geometry effect caused by the spread of the counter-telescope in 3-dimensional space. The differential cross section of the n - p scattering in the lab. system used in the calculation was obtained by the following formula transformed from the least-squares-fit formula given by T a n a k a et al. [9];
I
4
15)
1
where 0p is the recoil angle of the proton. The calculated results for the position spectrum shown in fig. 6 agree with those of the measurements except at both extreme regions of the spectrum of PSPC2. The energy spectra calculated for small angular divisions in the case of 00 = 18 ° reproduce well the classified spectra, as shown in fig. 8. Broadening of the peak in the energy spectra is explained by the finite geometry effect of the counter-telescope and the effect of the energy losses in the target that are taken into account in the calculation. The agreement between the measured and calculated spectra suggests that the position sensitive counter-telescope was successful},' operated a n d the data reduction method was sufficiently reliable.
(MeV)
Fig. 8. Classified energy spectra into the six angular divisions at 00 = 18° by the scattering angle derived for each event. The solid line is the result of the simulation which takes into account the effects of the finite geometry and energy loss in the target.
4. Conclusions The most difficult challenges of high measurement efficiency and good angular resolution were overcome by the introduction of two PSPCs and a large rectangu-
N. Koori et al. / Position sensitive counter-telescope
lar Si(Li) detector into a d E / d x - E type counter-telescope. The total solid angle of the telescope is 40.0 msr a n d the covered angular interval is 35 ° . The angular resolution of the telescope is a b o u t 5 ° which includes the position resolution (2 m m fwhm for protons) of the PSPCs and the finite geometry effect. This angular resolution is better than those of good conventional counter-telescopes. The large rectangular Si(Li) detector was successfully adopted to this counter-telescope with an energy resolution of a b o u t 250 keV. A m e a s u r e m e n t of the n - p scattering at 14 MeV reproduced well its angular distribution. The simulation with the M o n t e Carlo m e t h o d agrees well with the position spectra of the PSPCs and the energy spectra classified into small angular divisions. Near the reaction angle of 0 °, the finite geometry effect is still the most i m p o r t a n t c o n t r i b u t i o n to the angular resolution of the position sensitive counter-telescope with two single-wire PSPCs. A n i m p r o v e m e n t left to be achieved in the telescope is the uniformity of the counting efficiency of PSPC2 which may be worsened by the misalignment of the counters. This telescope is suitable to measure efficiently precise angular distributions of fast n e u t r o n induced reactions with small cross sections, and is being successfully used in the m e a s u r e m e n t of (n, p) reactions for nuclei in the region of mass n u m b e r A >~ 100.
419
The authors wish to t h a n k Drs. M. M a t o b a and M. H y a k u t a k e for m a n y valuable discussions, and Messrs. Y. M a t s u m o t o and M. M a r u b a y a s h i for their help with the electronics. This work was supported in part by a G r a n t - i n - A i d for Scientific Research from the Ministry of Education, Science and Culture, Japan.
References [1] F.L. Ribe and J.D. Seagrave, Phys. Rev. 94 (1954) 934. [2] L.G, Kuo, M. Petravic and B. Turko, Nuct. Instr. and Meth. 10 (1961) 53. [3] V. Voitovetskii, I.L., Korsunskii, A.I. Novikov, Yu.F. Pazhin and R.S. Silakov, Nucl. Instr. and Meth. 33 (1965) 19. [4] S. Shirato and N. Koori, Nucl. Instr. and Meth. 57 (1967) 325. [5] J. Niidome, M. Hyakutake, N. Koori, I. Kumahe and M. Matoba, Nucl. Phys. A245 (1975) 509. [6] V. Valkovic, K., Kovacevic and S. Vidic, Nucl. Instr. and Meth. 79 (1970) 13. [7] S. Shirato, M. Tanaka, N. Koori, 1. Ogawa, M. Tsukuda and E. Tajima, SJC-T-68-4 (1968), Institute for Nuclear Study, Tokyo. [8] D.J. Franz, Nucl. Instr. and Meth. 67 (1969) 323. [9] M. Tanaka, N. Koori and S. Shirato, J. Phys. Soc. Jpn 28 (1970) 11.
413
A POSITION SENSITIVE COUNTER-TELESCOPE STUDY
OF FAST-NEUTRON
N. KOORI,
T. G O T O
INDUCED
*, H. K O N I S H I
WITH REACTIONS
LARGE ANGULAR
ACCEPTANCE
FOR
** a n d I. K U M A B E
Department of Nuclear Engineering, Kyushu University, Fukuoka, 812 Japan Received 4 December 1981 and in revised form 5 July 1982
A new position sensitive counter-telescope with a large angular acceptance has been developed for measurements of charged particles from neutron induced reactions. A high efficiency of measurement with a wide angular acceptance of 35 °, a large solid angle of 40.0 msr and a good angular resolution of 5 ° was established by the introduction of two position sensitive proportional counters and a large rectangular Si(Li) detector into a d E / d x - E type counter-telescope. The performances were studied by the n - p scattering at 14 MeV.
1. Introduction M e a s u r e m e n t s of n e u t r o n induced reactions consume a considerably long time because of the very low intensity of incident n e u t r o n beams. In most measurements of charged particles emitted from neutron induced reactions, d E / d x - E type counter-telescopes have been used with solid angles of a b o u t 5 - 1 0 msr corres p o n d i n g to their fairly good angular resolutions of a b o u t 10 ° fwhm [1 5]; the angular resolution is determined essentially by the geometry of counter-telescopes that is chosen to achieve the solid angle required for measurements. A counter-telescope with larger solid angle is desirable for efficient m e a s u r e m e n t of the reaction. However, the increase in dimensions of the target, the detectors or b o t h of the counter-telescope makes worse the angular resolution which is i m p o r t a n t for precise m e a s u r e m e n t of the angular distributions. Valkovic et al. [6] tried to improve the angular resolution of their counter-telescope (the solid angle was a b o u t 11 msr) by dividing the E detector into several sections with a multiwire proportional counter; their ameliorated angular resolution was still limited by the size of the target. Two position sensitive parts that give the information on the particle trajectory are necessary for further i m p r o v e m e n t of the angular resolution in the case of enlarged target and detectors. Recent developments of the position sensitive counters make it possible to assemble a position sensitive counter-telescope which consists of two position sensi* Present address: Nippon Atomic Industry Group Co. Ltd., Kawasaki, 210 Japan. ~* Present address: Nittetsu Plant Engineering Co. Ltd., KitaKyushu, 805 Japan. 0 1 6 7 - 5 0 8 7 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 c,) 1983 N o r t h - H o l l a n d
tive proportional counters (PSPCs), d E / d x proportional counters and an E detector, with a good angular resolution a n d high m e a s u r e m e n t efficiency. The position information from the two PSPCs placed directly b e h i n d the target a n d in front of the E detector can be used to determine the particle trajectory and the precise reaction angle in the target. This article describes a developed position sensitive counter-telescope with both a large angular acceptance of 35 ° a n d a good angular resolution of a b o u t 5 ° to measure efficiently reactions with small differential cross sections. The following properties are also required of the position sensitive counter-telescope as in the cases of conventional counter-telescopes. These are (1) the possibility of particle identification, (2) good energy resolution and (3) efficient rejection of b a c k g r o u n d events. In the present counter-telescope system, the particle identification was performed by a two dimensional d E / d x - E spectrum. A lithium drifted silicon [Si(Li)] detector with a large sensitive area which was suitable to the dimensions of the PSPCs was developed to obtain good energy resolution. The multiple coincidence between the two PSPCs, d E / d x proportional counter and E detector was adopted for the rejection of background events.
2. Description of the position sensitive counter-telescope 2.1. Determination of the reaction angle The reaction angle of particles from a reaction can be determined precisely by the position information from the two PSPCs of the counter-telescope. The w i n ciple of the position sensitive counter-telescope is schematically shown in fig. 1, where the geometry is
N. Koori et a L / Position sensitive counter-telescope
4]4
considered only in the counter plane that consists of the n e u t r o n source and the anode wires of the two PSPCs. The reaction angle 0 for each emitted particle is given by the following formula using the geometry defined in fig. 1; 0 = tan-
12
[
j - tan
1
10 c-os O0
PSPC1 AE counter
:hdmber
telescope (lxis
'
/ i
where 00 is the setting angle of the telescope with respect to the direction of the central beam of incident neutrons. The reaction point r is determined by the position information of xj and x 2 from the two PSPCs; Ii
r=x2-~2(x:-xl). counter
ptane
2.2. The position sensitive proportional counters Two PSPCs (PSPC1 and PSPC2), a d E / d x proportional counter and an E detector are placed in a counter c h a m b e r as depicted in fig. 2. The PSPCs (PSPCI 45 m m a n d PSPC2 65 m m in length) are set at distances l~ = 98.0 m m and l 2 = 56.5 mm. The depth and height of the PSPCs are 10 m m and 12 mm, respectively. The anode wire of the PSPC is a highly resistive (8 k g 2 / m m ) carbon-coated quartz-fibre (25 p,m in diameter). The cathode of the PSPC consists of two parallel plates placed 6 m m from the central a n o d e wire. A t a n t a l u m plate of 0.3 m m thickness backed with 4 m m thick graphite is used as this cathode plate for realizing a low counting rate of b a c k g r o u n d events. The position d e p e n d e n t charge signal Qp from the PSPC is derived from one of the anode terminals a n d the total charge signal Qt from the cathode plate. T h e position information i n d e p e n d e n t of the a m o u n t of energy losses can be obtained from the following equation;
Fig. 2. Schematic views of the position sensitive counter-telescope,
where x is the distance of the incident point from the other anode terminal and L is the length of the PSPC. Each module of the PSPC was tested by 21°po 5.3 MeV c~-particles collimated to 0.2 m m wide on the anode. Operating gas of Ar + 5% CO 2 was flowed into the counter c h a m b e r at a pressure of 300 Torr. A high voltage of 800 V was supplied to the anode. The posi-
x / L = Qp/Qt,
PSPC2 E-DETECTOR
PSPCl +
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TARGET
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SOURCE
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L
I-
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POINT
-
~ ~
t,
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Fig. 1, The reaction position r and reaction angle 0 calculated from the trajectory of the emitted particle that passes through the positions x I and x 2 in the position sensitive proportional counters PSPC1 and PSPCZ respectively.
N. Koori et al. / Position sensitive counter- telescope
tion resolution obtained is 0.5 mm in fwhm, which includes the beam spread, noise of the preamplifier and the resolution of the analog division circuit. The linearity of the position determination is good enough for the purpose except in the region within 5 mm from both sides of the PSPC. 2.3. The dE / dx proportional counter
The d E / d x counter of 55 mm length is employed for the measurement of energy losses of particles and for rejection of background events. The counter of 40 mm in depth is divided into two similar parts of 20 mm with five field defining wires; similar field defining wires are used also at the entrance and exit of the counter in order to define the sensitive volume of the counter. A tungsten wire (50 /~m in diameter) plated with gold is used as the anode wire. The cathodes have a similar structure to the PSPCs and are connected electronically. Signals of the energy losses from the cathodes are used for the particle identification. Two signals from the anodes can be used as coincidence signals to reduce the background in severe cases. Measured distributions of energy losses for 14 MeV protons and 12.5 MeV deuterons are compared with the calculated results in fig. 3; the figure indicates a good separation of protons and deuterons. 2.4. The E detector
The E detector for this counter-telescope must have a large rectangular sensitive area and a thickness correF
I
f
I
415
sponding to the range of 14 MeV protons. An Si(Li) detector with an area of 14 × 56 mm 2 and a thickness of 1.8 mm was fabricated in the laboratory. A rectangular p-type silicon wafer was sliced along the axis of the crystal rod of the ~ 1, 1, 1) orientation for use as starting material. The fabrication method was similar to that described by Shirato et al. [7]. The energy resolution obtained is about 70 keV fwhm for 21°po a-particles, and the counting rate is uniform in the sensitive area within a statistical error of less than 2%. The solid angle subtended by this detector through a slit (40 mm x 12 mm) is 40.0 msr, which is larger by a factor of 4 - 8 than those used in most neutron experiments. The energy range for protons can be covered up to 17 MeV with this detector; it is possible to extend the range by replacing the E detector by a thicker one which would be prepared up to 5 mm in the thickness. 2.5. Electronics
Signals from the counter-telescope were processed by a small on-line computer (Toshiba USC-3). The block diagram of the electronics is shown in fig. 4. The gate signal from the a-monitor of T(d, a)n neutrons was used for the coincident events in the counter-telescope when the low background was strictly required. The signals of Qp (PP1 and PP2) and Qt (PTI and PT2) from the PSPCs were fed to the computer through the ADCs. The position of the detected particle was calculated by the computer to obtain a wider dynamic range than that of the analog division circuit used in the test I
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PROTON 14 M e V
--
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DEUTERON 12,5 I~eV
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20
L
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+
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50
80
100
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LOSS
Fig. 3. Comparison of measured distribution of energy lost by 14 MeV protons and 12.5 MeV deuterons with the calculated distributions.
416
N. Koori et al. / Position sensiti~e counter-telescope
USC-3 COMPUTER I
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Fig. 4. Block diagram of the electronics. PA: preamplifier, TFA: timing filter amplifier, CFD: constant fraction discriminator, GDG: gate and delay generator, TSC: timing single channel analyzer, COINC: coincidence circuit, FD: fast discriminator, SD: scaler driver, PS: pre-scaler, S: scaler, R: rate meter, LA: linear amplifier, DA: delay amplifier, ADC: analog-to-digital converter.
of the PSPC modules. The energy signal and the d E / d x signal were also sent to the computer through the ADCs. Six parameters for each event were acquired with the c o m p u t e r in the mode of event-by-event recording on magnetic tapes. The single spectrum for each parameter and the position spectrum for each PSPC were monitored on an oscilloscope during the experiment. After the experiment, the data recorded on magnetic tapes were analysed even by event to determine the trajectory and reaction angle and to obtain the precise angular distribution with another larger computer ( F A C O M M-200 at the computer center in Kyushu University).
3. Measurements and results 3.1. Measurement of the n - p scattering at 14 M e V The performance of the counter-telescope was studied by the n - p scattering at 14 MeV, whose differential cross sections could be used as the reference of the measurement. The counter-telescope was set at a distance l 0 of 21.7 cm from the neutron source, and was turned around the vertical axis of the target for measurement of the angular distribution. The position resolutions of the PSPCs were measured for recoiled protons that were emitted from a
polyethylene target of 20 m g / c m 2 defined by a slit of 2.8 mm wide and detected through four slits of 0.5 mm wide placed just in front of the E detector. The observed resolutions in fwhm were 1.8 mm for PSPCI and 2.2 mm for PSPC2, which were worse than those measured for 5.3 MeV s-particles. This deterioration is attributable to much smaller energy losses of 14 MeV protons compared to that of 5.3 MeV a-particles in the PSPCs, The angular resolution due to the position resolutions was estimated to be about 4 ° by a numerical calculation of the aperture function which is a frequency distribution of the real reaction angles in the 3-dimensional space for a finite angular interval defined in the counter plane [8]. The calculation also showed that the contribution of the finite geometry effect to the angular resolution varies from 3 ° to 5 ° with the reaction angles defined in the counter plane. The overall performance of the counter-telescope was studied by measurement of the angular distribution of n p scattering at 14 MeV. The target for this measurement was a thin plastic scintillator sheet (NEI04) of 41.9 m g / c m 2 thick. The target was enlarged to 20 mm wide and the narrow slits in front of the E detector were replaced by a large slit of 40 m m × 12 mm for the measurement of the angular distribution. The measurement of the n - p scattering was done at the setting
N. Koori et al.
/
Position sensitive counter- telescope
angles of 0 0 = - 2 0 ° , 0 °, 18 ° a n d 38 ° . T h e scattering angles c o v e r e d were - 37.8 ° to - 2.2 ° for 00 = - 20 °, - 1 7 , 9 ° to 17.9 ° for 0 0 = 0 °, 0.1 ° to 35.8 ° for 0 0 = 18 ° , a n d 20.5 ° to 55.3 ° for 00 = 38 °. T h e m e a s u r e d energy s p e c t r a s h o w n in fig. 5 are r e a s o n a b l y b r o a d for the wide scattering angle range covered. Fig. 6 s h o w s the p o s i t i o n s p e c t r a of the P S P C s m e a s u r e d at the s a m e setting angles as t h o s e for the energy s p e c t r a in fig. 5. T h e p o s i t i o n s p e c t r a at 00 = 0 ° are e x p e c t e d to be s y m m e t r i c b e c a u s e the a n g u l a r d i s t r i b u t i o n of the n - p scattering at 14 M e V is s y m m e t r i c a r o u n d the scattering angle o f 0 °. Small d e v i a t i o n f r o m s y m m e t r y a n d n o n u n i f o r m i t i e s o b s e r v e d in the p o s i t i o n s p e c t r a m a y be d u e to the m i s a l i g n m e n t o f c o u n t e r s in the telescope. A n e x a m p l e o f the f r e q u e n c y d i s t r i b u t i o n of the r e a c t i o n p o s i t i o n (i.e. the scattering p o s i t i o n in this case) on the target is s h o w n for the case o f 00 = 0 ° in fig. 7. It was o b t a i n e d by the calculation of the reaction p o s i t i o n for each event with the f o r m u l a given in sect.
417
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2.1. As s h o w n in the figure, the d i s t r i b u t i o n d e f i n e s clearly the target region. It suggests the possibility of exclusion o f b a c k g r o u n d events that c o m e from regions o t h e r than the target. This possibility is very effective for the low b a c k g r o u n d m e a s u r e m e n t o f the n e u t r o n i n d u c e d reactions.
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Fig. 7. Distribution of the reaction position in the target for 00 = 0 ° derived by the calculation of the particle trajectory which is determined with the position information from PSPCI and PSPC2 for each particle.
N. Koori et al. / Positron sensttwe counter- telescope
418
3.2. The angular distribution
I
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By using the derived reaction (scattering) angle for each event, the events were classified into six small angular divisions (about 6 ° ) to obtain the precise angular distribution of the scattering and the energy spectra for these angular divisions. Fig. 8 shows the variation of the classified energy spectra for the case of 0o = 18 °. The angular distribution of the n - p scattering was o b t a i n e d from the classified energy spectra for the small angular divisions. In this classification, the events that came from places other than the target were rejected a n d the background events were subtracted. The elastic parts in the derived energy spectra were translated into the differential cross sections. Fig. 9 shows the angular distribution c o m p a r e d with the expected one. A fairly good agreement is obtained within the statistical errors. At extreme angles for each setting angle, the agreement is not so good because of the non-uniformity of the counting efficiency of PSPC2.
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3.3. C o m p a r i s o n with the simulation
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The energy and position spectra were simulated by a calculation with the M o n t e Carlo method that included the finite geometry effect caused by the spread of the counter-telescope in 3-dimensional space. The differential cross section of the n - p scattering in the lab. system used in the calculation was obtained by the following formula transformed from the least-squares-fit formula given by T a n a k a et al. [9];
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1
where 0p is the recoil angle of the proton. The calculated results for the position spectrum shown in fig. 6 agree with those of the measurements except at both extreme regions of the spectrum of PSPC2. The energy spectra calculated for small angular divisions in the case of 00 = 18 ° reproduce well the classified spectra, as shown in fig. 8. Broadening of the peak in the energy spectra is explained by the finite geometry effect of the counter-telescope and the effect of the energy losses in the target that are taken into account in the calculation. The agreement between the measured and calculated spectra suggests that the position sensitive counter-telescope was successful},' operated a n d the data reduction method was sufficiently reliable.
(MeV)
Fig. 8. Classified energy spectra into the six angular divisions at 00 = 18° by the scattering angle derived for each event. The solid line is the result of the simulation which takes into account the effects of the finite geometry and energy loss in the target.
4. Conclusions The most difficult challenges of high measurement efficiency and good angular resolution were overcome by the introduction of two PSPCs and a large rectangu-
N. Koori et al. / Position sensitive counter-telescope
lar Si(Li) detector into a d E / d x - E type counter-telescope. The total solid angle of the telescope is 40.0 msr a n d the covered angular interval is 35 ° . The angular resolution of the telescope is a b o u t 5 ° which includes the position resolution (2 m m fwhm for protons) of the PSPCs and the finite geometry effect. This angular resolution is better than those of good conventional counter-telescopes. The large rectangular Si(Li) detector was successfully adopted to this counter-telescope with an energy resolution of a b o u t 250 keV. A m e a s u r e m e n t of the n - p scattering at 14 MeV reproduced well its angular distribution. The simulation with the M o n t e Carlo m e t h o d agrees well with the position spectra of the PSPCs and the energy spectra classified into small angular divisions. Near the reaction angle of 0 °, the finite geometry effect is still the most i m p o r t a n t c o n t r i b u t i o n to the angular resolution of the position sensitive counter-telescope with two single-wire PSPCs. A n i m p r o v e m e n t left to be achieved in the telescope is the uniformity of the counting efficiency of PSPC2 which may be worsened by the misalignment of the counters. This telescope is suitable to measure efficiently precise angular distributions of fast n e u t r o n induced reactions with small cross sections, and is being successfully used in the m e a s u r e m e n t of (n, p) reactions for nuclei in the region of mass n u m b e r A >~ 100.
419
The authors wish to t h a n k Drs. M. M a t o b a and M. H y a k u t a k e for m a n y valuable discussions, and Messrs. Y. M a t s u m o t o and M. M a r u b a y a s h i for their help with the electronics. This work was supported in part by a G r a n t - i n - A i d for Scientific Research from the Ministry of Education, Science and Culture, Japan.
References [1] F.L. Ribe and J.D. Seagrave, Phys. Rev. 94 (1954) 934. [2] L.G, Kuo, M. Petravic and B. Turko, Nuct. Instr. and Meth. 10 (1961) 53. [3] V. Voitovetskii, I.L., Korsunskii, A.I. Novikov, Yu.F. Pazhin and R.S. Silakov, Nucl. Instr. and Meth. 33 (1965) 19. [4] S. Shirato and N. Koori, Nucl. Instr. and Meth. 57 (1967) 325. [5] J. Niidome, M. Hyakutake, N. Koori, I. Kumahe and M. Matoba, Nucl. Phys. A245 (1975) 509. [6] V. Valkovic, K., Kovacevic and S. Vidic, Nucl. Instr. and Meth. 79 (1970) 13. [7] S. Shirato, M. Tanaka, N. Koori, 1. Ogawa, M. Tsukuda and E. Tajima, SJC-T-68-4 (1968), Institute for Nuclear Study, Tokyo. [8] D.J. Franz, Nucl. Instr. and Meth. 67 (1969) 323. [9] M. Tanaka, N. Koori and S. Shirato, J. Phys. Soc. Jpn 28 (1970) 11.