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A high efficiency proton polarimeter using a liquid helium target
450
Nuclear Instruments and Methods in Physics Research A270 (1988) 450-455 North-Holland, Amsterdam
A HIGH EFFICIENCY PROTON POLARIMETER USING A LIQUID HELIUM TARGET K. S A G A R A , K. M A E D A , H. N A K A M U R A , K. A I T A *, M. I Z U M I , M. N A K A S H I M A , T. N A K A S H I M A a n d A. I S O Y A * *
Department of Physics, Kyushu University, Hakozaki, Fukuoka, 812 Japan Received 19 August 1987 and in revised form 29 February 1988
A high efficiency ( - 1 x 1 0 - 3 at ,4y = -0.45) polafirneter used for a secondary proton beam of 14-18 MeV energy is described. The polarimeter target is liquid helium held in a conical cell. Protons scattered at 52 ° +8 ° are detected by twelve Si detectors placed symmetrically around the beam axis. This polarimeter has been used for polarization transfer studies.
1. Introduction Recently, the intensity of polarized proton and deuteron beams has been increased to the order of/~A and polarization transfer coefficients have been measured in various reactions by a double scattering method. The double scattering experiment, however, takes much more time than a single scattering experiment for analyzing powers and cross sections. If one requires the same resolution and statistical accuracy, it takes about a factor 104 or more time. To aid in the study of the polarization transfer reaction, the efficiency of the secondary beam polarimeter must also be increased to as great an extent as possible. Here we report a high-efficiency secondary proton polarimeter. At the Kyushu University Tandem Accelerator Laboratory, high-intensity polarized proton and deuteron beams are produced by a Lamb-shift-type ion source [1] and accelerated up to 20 MeV. The beam intensity on target is about 0.5 fLA. The polarization of the secondary protons from (p, p) or (d, p) reactions is measured by the present polarimeter. In this proton energy range, 4He is the best target for a proton polarimeter because the p - 4 H e scattering has a high figure of merit and its analyzing power varies only slightly with energy and angle [2]. Proton polarimeters using a high-pressure 4He gas target have been reported by several authors [3-6]. The present polarimeter uses a thick target of liquid helium. The target thickness is about 1 cm ( - 1 4 0 mg/cm2), equivalent to gaseous helium at about 20 atm × 40 cm. Owing to the thick
* Present address: Seiko Instruments Inc., Kameido, Tokyo, 136 Japan. ** Institute of Research and Development, Tokai University, Hiratsuka, Kanagawa, 259-12 Japan. 0168-9002/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
target and the large solid angle obtained by using many detectors, the efficiency of the polarimeter is very high, - 1 × 10-3, with an effective analyzing power of about - 0.45.
2. Setup for polarization transfer experiments Fig. 1 shows the layout of the polarization transfer experiment. A polarized proton or deuteron beam from the tandem accelerator is incident on a primary target in a scattering chamber 6 in. in diameter. During the experiment, the polarization of the primary beam is measured downstream from the scattering chamber with a beam polarimeter [7] by making use of the 4He(p, p) scattering or the 3He(d, p) reaction. The secondary beam protons, emitted from the primary target with horizontal and vertical angular spreads of + 1° and + 2 ° , respectively, are momentum-analyzed by QDQ magnetic analyzer system and focused on a polarimeter target. Protons scattered by the polarimeter target are detected by twelve detectors placed symmetrically around the beam axis, and the polarization of the secondary beam protons in a plane normal to the beam axis is determined. The secondary proton beam is bent 50 ° by the magnetic field. This causes the proton spin to be rotated by nearly 140 ° in the horizontal plane. Then we can measure two components of the secondary beam proton polarization, one normal to the reaction plane and the other along the direction in which the secondary beam protons are emitted. The QDQ analyzer and the polarimeter can be rotated around the primary target. The secondary beam protons go out of the vacuum scattering chamber through a 1 8 / t m thick mylar window, passing through the atmosphere for about 5 mm and entering into the
K. Sagara et al. / A high efficiency proton polarimeter
451
ognet
'~o~arirneter b e ~~a
r
n/ ~ v
.5
1m
~ potarime~er
Fig. 1. Setup for a polarization transfer experiment. The QDQ magnetic analyzer system and the secondary proton polarimeter can be rotated around the primary target.
vacuum duct of the magnetic analyzer through another 18/xm thick mylar window. The focusing property of the QDQ analyzer was tested by passing a fine primary beam of 1 mm diameter directly through the polarimeter. The beam was swung at the position of the primary target and the corresponding movement of the beam spot at the position of the polarimeter target was measured. From this measurement the beam was estimated to be focused to about 5 × 5 mm 2 area if it is not spread by kinematics, by multiple scattering in the primary target or by multiple scattering in the mylar foils and the air described above.
3. Liquid helium cryostat
The target of the polarimeter for the secondary beam protons is liquid helium kept in the superfluid state to avoid bubbles in the target. Fig. 2 shows the cross section of the liquid helium cryostat. The liquid helium held in a reservoir tank is dropped through a leak valve into the target cell, where the hehum is brought into the superfluid state by decreasing the vapor pressure below 37 mm Hg using a rotary pump. The leak valve is controlled from outside the cryostat. Small carbon resistors are used to monitor the level of the liquid helium. To reduce heat input to the helium, the helium tank and the target cell are completely surrounded by walls cooled to the liquid nitrogen temperature. The helium tank is supported by thin stainless steel pipes from the Walls. Moreover, radiation shields of thin aluminumcoated mylar foils are inserted between the tank and the walls. The whole heat input is about 30 mW so there is
~)
110
2i0
3~0cm
Fig. 2. Liquid helium cryostat. The liquid helium in the target cell is kept in a superfluid state.
452
K. Sagara et al. / A high efficiency proton polarimeter
enough liquid helium in the 5.5 1 reservoir tank to last seven days.
E3
4. Target shape and energy spectra E0 . The polarimeter target a n d detectors are s h o w n in fig. 3. A n aperture of 6 m m diameter is placed in front of the target. Secondary b e a m p r o t o n s which have passed through the helium target are detected b y a Si detector (center detector) whose sensitive area has been divided into four sections in order to m o n i t o r b e a m position (described in the next section). Protons scattered b y the target in the laboratory angle region of 52 ° + 8 ° are detected b y twelve Si detectors (side detectors) placed symmetrically a r o u n d the b e a m axis. The scattering angle was chosen to maximize the figure of merit [4]. The present liquid helium target is very thick, a b o u t 1 cm (140 m g / c m 2 ) , a n d 30-40% of the incident energy is lost as the b e a m passes t h r o u g h the target. If a flat target of the same thickness is used, some of the p r o t o n s scattered at 52 ° stop in the target. T h e conical shape of the target in fig. 3 was designed to minimize the energy spread of the scattered protons. W e shall consider only elastic scattering within a conical target as s h o w n in fig. 4. T h e b e a m is supposed to pass along the axis of the cone. The energy of a particle, initially E0, is decreased to Ea due to the target thickness L 1, to E 2 due to elastic scattering a n d finally to E 3 due to the thickness L 2. In the case of elastic scattering, E 2 is p r o p o r t i o n a l to El, E 2 = k E 1. By using a n approximate relation E i = a R 7 between the particle
Liq. He
absorber
EI t-1
Fig. 4. Elastic scattering within a conical target. The particle energy, initially E0, is decreased to E 1 due to the target thickness L1, to E 2 by elastic scattering and to E 3 due to the thickness L 2. The energy E 3 becomes a constant for any value of L 1 if the angle/3(0) is chosen properly.
energy E i a n d the c o r r e s p o n d i n g range R i in the target material, the following relation is obtained, R 3 = k Y R o - k~'L1 _ L 2 = k r R o - k r L 1 - ( L o - L 1 ) sin f l / s i n ( f l + 0 ) ,
(1)
where y is 1 / a a n d L 0 is the height of the cone. W h e n the cone a n g l e / 3 is chosen so as to satisfy the relation k Y = sin f l / s i n ( f l + 0 ) ,
(2)
R 3 becomes i n d e p e n d e n t of La a n d the scattered particles have a m o n o c h r o m a t i c energy. T h e present target has the cone angle /3 thus determined. The conical exit window was shaped by pressing a 25 ~tm thick m y l a r foil, which was heated to a b o u t 2 0 0 ° C in a n oven, into a metal mold. The e n t r a n c e w i n d o w is merely m a d e from a flat piece of 25 ktm mylar foil. This target cell has b e e n used for more t h a n two years w i t h o u t trouble. Typical energy spectra m e a s u r e d b y one of the side detectors are s h o w n in fig. 5. T h e b a c k g r o u n d at low energy in the single s p e c t r u m (left), which might be induced b y neutrons, is almost completely rejected (right) by accepting signals from the side detectors only w h e n a p r o t o n passes a A E counter in front of the target b u t does n o t c o m e to the center detector. The p r o t o n energy a n d time of flight are recorded on a floppy disc. T h e A E counter, the side detectors a n d the center detector have thicknesses of 0.17, 0.9 a n d 2.0 m m , respectively. All of t h e m were m a d e in our laboratory. They are o p e r a t e d at liquid nitrogen temperature.
5. False asymmetry Fig. 3. Proton polarimeter with liquid hefium target. The windows of the target cell are made of mylar foil. Scattered protons are detected by twelve detectors placed symmetrically around the beam axis.
As s h o w n in fig. 3 the distance between the target a n d the side detectors is r a t h e r short ( a b o u t 6 cm). It is necessary to o b t a i n a large cross section. However, if there is even the slightest misalignment, a nonnegligible
K. Sagara et al. / A high efficiency proton polarimeter 10 4 :
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false asymmetry among the proton yields measured by the side detectors will result. For example, a 2% false asymmetry is caused by about a 1 m m displacement between the beam axis and the symmetrical axis of the side detectors. The slits of the side detectors are fixed on a thick metal flange as indicated in fig. 2. The symmetrical axis of this flange and the beam slit are aligned to an accuracy of 0.1 mm. The target cell is positioned to an accuracy of 0.15 m m with respect to the detector flange. The adjustment is made at room temperature and atmospheric pressure. It has been ascertained, after the cryostat was evacuated and filled with liquid nitrogen and liquid helium, that the target cell held the initially aligned position within an accuracy of 0.15 mm. Another possible source of false asymmetry is the shift of the center of the beam distribution from the aligned axis. As shown in fig. 3, the center detector behind the target is used for monitoring the b e a m position. The sensitive area (Au-evaporated surface) of the center detector is divided into four sections as shown in fig. 6. F r o m the proton yields (a, b, c and d ) in these four sectors the l e f t / r i g h t and u p / d o w n ratios are derived as (a + b ) / ( c + d) and (a + d ) / ( b + c), respectively.
The effect of beam path shift was investigated by swinging the secondary proton beam in the horizontal plane with the dipole magnet. The results are shown in fig. 7. The relation between left-right asymmetries measured by the center detector (ACR) and by the side detectors (ALR) s shows that the center detector is about
-0.Smm
I 1
I 2 cm
Fig. 6. The center detector (Si) with four sensitive areas. Proton counts in the shaded areas are used to monitor the left-right and up-down asymmetry of the beam distribution. Thin gold wires are bonded on the gold-evaporated surfaces by indium metal.
K. Sagara et aL / A high efficiency proton polarimeter
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eter, Ninc is the number of protons incident on the polarimeter target and Px is the proton polarization in the horizontal direction normal to the beam axis. The efficiency ~ is defined as
i
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e = 3Npair/Ninc,
i
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-.05
Fig. 7. Asymmetries (lower) measured by the center detector (ALCR) and by the side detectors (ASR). The solid line has a slope of 1/8. The beam path is adjusted so as to maximize yC (upper), the yield in the center detector normalized by the primary beam charge.
8 times more sensitive to such a shift than the side detectors. Then the asymmetry ACLR, which is statistically accurate enough, is used to correct the false asymmetry in ASg. In the upper part of fig. 7, the total yields in the center detector normalized by the primary beam charge, yC, are shown. The secondary beam path is adjusted so as to maximize yC. Similar procedures are followed for the up-down asymmetry. The overall false asymmetry remaining is estimated to be less than 0.5%. Since most of the polarization transfer coefficients are derived from the difference between two proton polarizations (weighted by some factors) measured for different spin states of the primary beam, the false asymmetry tends to be canceled out.
(4)
where Npair is the number of protons detected by one diagonal pair of side detectors. The statistical error for Px is expressed similarly. The effective analyzing power of the present polarimeter has been determined by making use of the Si(~, ~) elastic scattering reaction. The vector polarization transfer coefficient K~. is 1.0 assuming time reversal invariance. The polarization of the primary proton beam has been measured accurately during the experiment by using the beam polarimeter [4]. The experimental results are shown in the upper graph of fig. 8. The incident proton loses about 1 MeV in passing through the AE counter positioned in front of the helium target. The proton energy just before the target is shown in the figure. The solid curve indicates the results of computer simulation using the p - 4 H e phase shifts in ref. [2]. Both results are in good agreement.
- 0,6
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- 0.4
- 0,2
0 xl0 3
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The present polarimeter has twelve side detectors placed symmetrically around the beam axis. The statistical error Apy of the proton polarization in the vertical direction Py is expressed by the following equation, (Apy) 2 =
1 - A2(ap2 + p 2 ) / 4 A2eNinc '
(3)
where A is the effective analyzing power of the polarim-
0.4
t 12
i
i
i 15
t
t
Ep(MeV)
i 18
Fig. 8. Effective analyzing power and efficiency of the secondary proton polarimeter. Curves are results of computer simulation.
K. Sagara et al. / A high efficiency proton polarimeter The experimental results for the efficiency are shown in the lower part of fig. 8. They also coincide with the simulation. The decrease of the efficiency below 14 MeV is caused by the fact some of the scattered protons are stopped in the target material. If none of them were stopped, the dashed curve would be obtained. For the incident beam along the axis of the conical target, all of the scattered protons come out of the target with a monochromatic energy as described in section 4. However, this does not hold for the beam off the axis. A thinner target is necessary for protons below 14 MeV. It is to be noted that a very high efficiency (about 10 3) is obtained owing to the thick target of liquid helium and many detectors. The effective analyzing power is fairly high, about - 0 . 4 5 , and varies smoothly with the incident energy. The analyzing power and the efficiency decrease as the incident energy increases. If the incident proton energy is higher than 18 MeV, it is decreased to the 14-18 MeV region by an absorber shown in fig. 3.
455
The present polarimeter and the Q D Q magnetic analyzer have been used to measure polarization transfer coefficients in (d', P) reactions. Due to the high efficiency of the polarimeter and the high intensity ( - 500 nA) deuteron b e a m which has been polarized to about 65%, fairly accurate data can be obtained in a short time. For example, the statistical error of the vector polarization transfer coefficient, A K y, is +4% for a one hour measurement with a 28Si target of 10 m g / c m 2 and a cross section of 1 m b / s r .
Acknowledgements The authors wish to thank Mr. H. Koga and Mr. T. Maeda for their skilful work in machining numerous parts of the polarimeter and in preparing electronic circuits, respectively.
References 7. Summary A polarimeter for the secondary proton beam has been described. Very high efficiency and fairly high analyzing power are obtained by using a thick liquid helium target in a conical cell and placing twelve Si detectors symmetrically around the beam axis. In order to correct false asymmetry, the beam position is monitored behind the target by a Si detector whose sensitive area has been divided into four sections. The polarimeter is used for secondary beam protons within an energy range of 14-18 MeV. Any proton energy higher than 18 MeV is decreased by an absorber in front of the polarimeter. By preparing a thinner target cell, it is even possible to measure the polarization of protons with energies as low as about 10 MeV.
[1] A. Isoya, T. Nakashima, K. Sagara and H. Nakamura, Proc. 6th Int. Symp. on Polarization Phenomena in Nuclear Physics, Osaka (1985) p. 1054. [2] P. Schwandt, T.B. Clegg and W. Haeberli, Nucl. Phys. A163 (1971) 432. [3] R.A. Hardecopf, D.D. Armstrong and P.W. Keaton, Jr., Nucl. Instr. and Meth. 114 (1974) 17. [4] J.F. Clare, Nucl. Instr. and Meth. 116 (1974) 525. [5] F. Sperisen, W. Griiebler, V. KSnig, P.A. Schmelzbach, K. Elsener, B. Jenny and C. Schweizer, Nucl. Instr. and Meth. 190 (1981) 301. [6] W.G. Weitkamp, I. Halpern, T.A. Trainer, S.K. Lamoreaux and Z.Y. Liu, Nucl. Phys. A417 (1984) 405. [7] K. Sagara, K. Maeda, H. Nakamura, M. Izumi, T. Yamaoka, Y. Nishida, M. Nakashima and T. Nakashima, Nucl. Instr. and Meth. A270 (1988) 444.
Nuclear Instruments and Methods in Physics Research A270 (1988) 450-455 North-Holland, Amsterdam
A HIGH EFFICIENCY PROTON POLARIMETER USING A LIQUID HELIUM TARGET K. S A G A R A , K. M A E D A , H. N A K A M U R A , K. A I T A *, M. I Z U M I , M. N A K A S H I M A , T. N A K A S H I M A a n d A. I S O Y A * *
Department of Physics, Kyushu University, Hakozaki, Fukuoka, 812 Japan Received 19 August 1987 and in revised form 29 February 1988
A high efficiency ( - 1 x 1 0 - 3 at ,4y = -0.45) polafirneter used for a secondary proton beam of 14-18 MeV energy is described. The polarimeter target is liquid helium held in a conical cell. Protons scattered at 52 ° +8 ° are detected by twelve Si detectors placed symmetrically around the beam axis. This polarimeter has been used for polarization transfer studies.
1. Introduction Recently, the intensity of polarized proton and deuteron beams has been increased to the order of/~A and polarization transfer coefficients have been measured in various reactions by a double scattering method. The double scattering experiment, however, takes much more time than a single scattering experiment for analyzing powers and cross sections. If one requires the same resolution and statistical accuracy, it takes about a factor 104 or more time. To aid in the study of the polarization transfer reaction, the efficiency of the secondary beam polarimeter must also be increased to as great an extent as possible. Here we report a high-efficiency secondary proton polarimeter. At the Kyushu University Tandem Accelerator Laboratory, high-intensity polarized proton and deuteron beams are produced by a Lamb-shift-type ion source [1] and accelerated up to 20 MeV. The beam intensity on target is about 0.5 fLA. The polarization of the secondary protons from (p, p) or (d, p) reactions is measured by the present polarimeter. In this proton energy range, 4He is the best target for a proton polarimeter because the p - 4 H e scattering has a high figure of merit and its analyzing power varies only slightly with energy and angle [2]. Proton polarimeters using a high-pressure 4He gas target have been reported by several authors [3-6]. The present polarimeter uses a thick target of liquid helium. The target thickness is about 1 cm ( - 1 4 0 mg/cm2), equivalent to gaseous helium at about 20 atm × 40 cm. Owing to the thick
* Present address: Seiko Instruments Inc., Kameido, Tokyo, 136 Japan. ** Institute of Research and Development, Tokai University, Hiratsuka, Kanagawa, 259-12 Japan. 0168-9002/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
target and the large solid angle obtained by using many detectors, the efficiency of the polarimeter is very high, - 1 × 10-3, with an effective analyzing power of about - 0.45.
2. Setup for polarization transfer experiments Fig. 1 shows the layout of the polarization transfer experiment. A polarized proton or deuteron beam from the tandem accelerator is incident on a primary target in a scattering chamber 6 in. in diameter. During the experiment, the polarization of the primary beam is measured downstream from the scattering chamber with a beam polarimeter [7] by making use of the 4He(p, p) scattering or the 3He(d, p) reaction. The secondary beam protons, emitted from the primary target with horizontal and vertical angular spreads of + 1° and + 2 ° , respectively, are momentum-analyzed by QDQ magnetic analyzer system and focused on a polarimeter target. Protons scattered by the polarimeter target are detected by twelve detectors placed symmetrically around the beam axis, and the polarization of the secondary beam protons in a plane normal to the beam axis is determined. The secondary proton beam is bent 50 ° by the magnetic field. This causes the proton spin to be rotated by nearly 140 ° in the horizontal plane. Then we can measure two components of the secondary beam proton polarization, one normal to the reaction plane and the other along the direction in which the secondary beam protons are emitted. The QDQ analyzer and the polarimeter can be rotated around the primary target. The secondary beam protons go out of the vacuum scattering chamber through a 1 8 / t m thick mylar window, passing through the atmosphere for about 5 mm and entering into the
K. Sagara et al. / A high efficiency proton polarimeter
451
ognet
'~o~arirneter b e ~~a
r
n/ ~ v
.5
1m
~ potarime~er
Fig. 1. Setup for a polarization transfer experiment. The QDQ magnetic analyzer system and the secondary proton polarimeter can be rotated around the primary target.
vacuum duct of the magnetic analyzer through another 18/xm thick mylar window. The focusing property of the QDQ analyzer was tested by passing a fine primary beam of 1 mm diameter directly through the polarimeter. The beam was swung at the position of the primary target and the corresponding movement of the beam spot at the position of the polarimeter target was measured. From this measurement the beam was estimated to be focused to about 5 × 5 mm 2 area if it is not spread by kinematics, by multiple scattering in the primary target or by multiple scattering in the mylar foils and the air described above.
3. Liquid helium cryostat
The target of the polarimeter for the secondary beam protons is liquid helium kept in the superfluid state to avoid bubbles in the target. Fig. 2 shows the cross section of the liquid helium cryostat. The liquid helium held in a reservoir tank is dropped through a leak valve into the target cell, where the hehum is brought into the superfluid state by decreasing the vapor pressure below 37 mm Hg using a rotary pump. The leak valve is controlled from outside the cryostat. Small carbon resistors are used to monitor the level of the liquid helium. To reduce heat input to the helium, the helium tank and the target cell are completely surrounded by walls cooled to the liquid nitrogen temperature. The helium tank is supported by thin stainless steel pipes from the Walls. Moreover, radiation shields of thin aluminumcoated mylar foils are inserted between the tank and the walls. The whole heat input is about 30 mW so there is
~)
110
2i0
3~0cm
Fig. 2. Liquid helium cryostat. The liquid helium in the target cell is kept in a superfluid state.
452
K. Sagara et al. / A high efficiency proton polarimeter
enough liquid helium in the 5.5 1 reservoir tank to last seven days.
E3
4. Target shape and energy spectra E0 . The polarimeter target a n d detectors are s h o w n in fig. 3. A n aperture of 6 m m diameter is placed in front of the target. Secondary b e a m p r o t o n s which have passed through the helium target are detected b y a Si detector (center detector) whose sensitive area has been divided into four sections in order to m o n i t o r b e a m position (described in the next section). Protons scattered b y the target in the laboratory angle region of 52 ° + 8 ° are detected b y twelve Si detectors (side detectors) placed symmetrically a r o u n d the b e a m axis. The scattering angle was chosen to maximize the figure of merit [4]. The present liquid helium target is very thick, a b o u t 1 cm (140 m g / c m 2 ) , a n d 30-40% of the incident energy is lost as the b e a m passes t h r o u g h the target. If a flat target of the same thickness is used, some of the p r o t o n s scattered at 52 ° stop in the target. T h e conical shape of the target in fig. 3 was designed to minimize the energy spread of the scattered protons. W e shall consider only elastic scattering within a conical target as s h o w n in fig. 4. T h e b e a m is supposed to pass along the axis of the cone. The energy of a particle, initially E0, is decreased to Ea due to the target thickness L 1, to E 2 due to elastic scattering a n d finally to E 3 due to the thickness L 2. In the case of elastic scattering, E 2 is p r o p o r t i o n a l to El, E 2 = k E 1. By using a n approximate relation E i = a R 7 between the particle
Liq. He
absorber
EI t-1
Fig. 4. Elastic scattering within a conical target. The particle energy, initially E0, is decreased to E 1 due to the target thickness L1, to E 2 by elastic scattering and to E 3 due to the thickness L 2. The energy E 3 becomes a constant for any value of L 1 if the angle/3(0) is chosen properly.
energy E i a n d the c o r r e s p o n d i n g range R i in the target material, the following relation is obtained, R 3 = k Y R o - k~'L1 _ L 2 = k r R o - k r L 1 - ( L o - L 1 ) sin f l / s i n ( f l + 0 ) ,
(1)
where y is 1 / a a n d L 0 is the height of the cone. W h e n the cone a n g l e / 3 is chosen so as to satisfy the relation k Y = sin f l / s i n ( f l + 0 ) ,
(2)
R 3 becomes i n d e p e n d e n t of La a n d the scattered particles have a m o n o c h r o m a t i c energy. T h e present target has the cone angle /3 thus determined. The conical exit window was shaped by pressing a 25 ~tm thick m y l a r foil, which was heated to a b o u t 2 0 0 ° C in a n oven, into a metal mold. The e n t r a n c e w i n d o w is merely m a d e from a flat piece of 25 ktm mylar foil. This target cell has b e e n used for more t h a n two years w i t h o u t trouble. Typical energy spectra m e a s u r e d b y one of the side detectors are s h o w n in fig. 5. T h e b a c k g r o u n d at low energy in the single s p e c t r u m (left), which might be induced b y neutrons, is almost completely rejected (right) by accepting signals from the side detectors only w h e n a p r o t o n passes a A E counter in front of the target b u t does n o t c o m e to the center detector. The p r o t o n energy a n d time of flight are recorded on a floppy disc. T h e A E counter, the side detectors a n d the center detector have thicknesses of 0.17, 0.9 a n d 2.0 m m , respectively. All of t h e m were m a d e in our laboratory. They are o p e r a t e d at liquid nitrogen temperature.
5. False asymmetry Fig. 3. Proton polarimeter with liquid hefium target. The windows of the target cell are made of mylar foil. Scattered protons are detected by twelve detectors placed symmetrically around the beam axis.
As s h o w n in fig. 3 the distance between the target a n d the side detectors is r a t h e r short ( a b o u t 6 cm). It is necessary to o b t a i n a large cross section. However, if there is even the slightest misalignment, a nonnegligible
K. Sagara et al. / A high efficiency proton polarimeter 10 4 :
453
1Of:
103:
10~
Jz z u
10 2
1(
z CD
lC l,l tJ
10 ~ :
6~a
'
7~
8~0
I
6ee
880
7fl8
CHANNEL CHANNEL Fig. 5. Single (left) and gated (fight) energy spectra of protons scattered by the liquid helium target.
false asymmetry among the proton yields measured by the side detectors will result. For example, a 2% false asymmetry is caused by about a 1 m m displacement between the beam axis and the symmetrical axis of the side detectors. The slits of the side detectors are fixed on a thick metal flange as indicated in fig. 2. The symmetrical axis of this flange and the beam slit are aligned to an accuracy of 0.1 mm. The target cell is positioned to an accuracy of 0.15 m m with respect to the detector flange. The adjustment is made at room temperature and atmospheric pressure. It has been ascertained, after the cryostat was evacuated and filled with liquid nitrogen and liquid helium, that the target cell held the initially aligned position within an accuracy of 0.15 mm. Another possible source of false asymmetry is the shift of the center of the beam distribution from the aligned axis. As shown in fig. 3, the center detector behind the target is used for monitoring the b e a m position. The sensitive area (Au-evaporated surface) of the center detector is divided into four sections as shown in fig. 6. F r o m the proton yields (a, b, c and d ) in these four sectors the l e f t / r i g h t and u p / d o w n ratios are derived as (a + b ) / ( c + d) and (a + d ) / ( b + c), respectively.
The effect of beam path shift was investigated by swinging the secondary proton beam in the horizontal plane with the dipole magnet. The results are shown in fig. 7. The relation between left-right asymmetries measured by the center detector (ACR) and by the side detectors (ALR) s shows that the center detector is about
-0.Smm
I 1
I 2 cm
Fig. 6. The center detector (Si) with four sensitive areas. Proton counts in the shaded areas are used to monitor the left-right and up-down asymmetry of the beam distribution. Thin gold wires are bonded on the gold-evaporated surfaces by indium metal.
K. Sagara et aL / A high efficiency proton polarimeter
454 • yC ,
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i
1
eter, Ninc is the number of protons incident on the polarimeter target and Px is the proton polarization in the horizontal direction normal to the beam axis. The efficiency ~ is defined as
i
1
o\ i
1
e = 3Npair/Ninc,
i
o
i
i t l I -.50 -.50 ~ / ~
"
I/_~
0
-.05
Fig. 7. Asymmetries (lower) measured by the center detector (ALCR) and by the side detectors (ASR). The solid line has a slope of 1/8. The beam path is adjusted so as to maximize yC (upper), the yield in the center detector normalized by the primary beam charge.
8 times more sensitive to such a shift than the side detectors. Then the asymmetry ACLR, which is statistically accurate enough, is used to correct the false asymmetry in ASg. In the upper part of fig. 7, the total yields in the center detector normalized by the primary beam charge, yC, are shown. The secondary beam path is adjusted so as to maximize yC. Similar procedures are followed for the up-down asymmetry. The overall false asymmetry remaining is estimated to be less than 0.5%. Since most of the polarization transfer coefficients are derived from the difference between two proton polarizations (weighted by some factors) measured for different spin states of the primary beam, the false asymmetry tends to be canceled out.
(4)
where Npair is the number of protons detected by one diagonal pair of side detectors. The statistical error for Px is expressed similarly. The effective analyzing power of the present polarimeter has been determined by making use of the Si(~, ~) elastic scattering reaction. The vector polarization transfer coefficient K~. is 1.0 assuming time reversal invariance. The polarization of the primary proton beam has been measured accurately during the experiment by using the beam polarimeter [4]. The experimental results are shown in the upper graph of fig. 8. The incident proton loses about 1 MeV in passing through the AE counter positioned in front of the helium target. The proton energy just before the target is shown in the figure. The solid curve indicates the results of computer simulation using the p - 4 H e phase shifts in ref. [2]. Both results are in good agreement.
- 0,6
Aef f
- 0.4
- 0,2
0 xl0 3
Efficiency
""'-.
1.2
° ~.o.~.
o..
0.8 6. Efficiency and effective analyzing power
The present polarimeter has twelve side detectors placed symmetrically around the beam axis. The statistical error Apy of the proton polarization in the vertical direction Py is expressed by the following equation, (Apy) 2 =
1 - A2(ap2 + p 2 ) / 4 A2eNinc '
(3)
where A is the effective analyzing power of the polarim-
0.4
t 12
i
i
i 15
t
t
Ep(MeV)
i 18
Fig. 8. Effective analyzing power and efficiency of the secondary proton polarimeter. Curves are results of computer simulation.
K. Sagara et al. / A high efficiency proton polarimeter The experimental results for the efficiency are shown in the lower part of fig. 8. They also coincide with the simulation. The decrease of the efficiency below 14 MeV is caused by the fact some of the scattered protons are stopped in the target material. If none of them were stopped, the dashed curve would be obtained. For the incident beam along the axis of the conical target, all of the scattered protons come out of the target with a monochromatic energy as described in section 4. However, this does not hold for the beam off the axis. A thinner target is necessary for protons below 14 MeV. It is to be noted that a very high efficiency (about 10 3) is obtained owing to the thick target of liquid helium and many detectors. The effective analyzing power is fairly high, about - 0 . 4 5 , and varies smoothly with the incident energy. The analyzing power and the efficiency decrease as the incident energy increases. If the incident proton energy is higher than 18 MeV, it is decreased to the 14-18 MeV region by an absorber shown in fig. 3.
455
The present polarimeter and the Q D Q magnetic analyzer have been used to measure polarization transfer coefficients in (d', P) reactions. Due to the high efficiency of the polarimeter and the high intensity ( - 500 nA) deuteron b e a m which has been polarized to about 65%, fairly accurate data can be obtained in a short time. For example, the statistical error of the vector polarization transfer coefficient, A K y, is +4% for a one hour measurement with a 28Si target of 10 m g / c m 2 and a cross section of 1 m b / s r .
Acknowledgements The authors wish to thank Mr. H. Koga and Mr. T. Maeda for their skilful work in machining numerous parts of the polarimeter and in preparing electronic circuits, respectively.
References 7. Summary A polarimeter for the secondary proton beam has been described. Very high efficiency and fairly high analyzing power are obtained by using a thick liquid helium target in a conical cell and placing twelve Si detectors symmetrically around the beam axis. In order to correct false asymmetry, the beam position is monitored behind the target by a Si detector whose sensitive area has been divided into four sections. The polarimeter is used for secondary beam protons within an energy range of 14-18 MeV. Any proton energy higher than 18 MeV is decreased by an absorber in front of the polarimeter. By preparing a thinner target cell, it is even possible to measure the polarization of protons with energies as low as about 10 MeV.
[1] A. Isoya, T. Nakashima, K. Sagara and H. Nakamura, Proc. 6th Int. Symp. on Polarization Phenomena in Nuclear Physics, Osaka (1985) p. 1054. [2] P. Schwandt, T.B. Clegg and W. Haeberli, Nucl. Phys. A163 (1971) 432. [3] R.A. Hardecopf, D.D. Armstrong and P.W. Keaton, Jr., Nucl. Instr. and Meth. 114 (1974) 17. [4] J.F. Clare, Nucl. Instr. and Meth. 116 (1974) 525. [5] F. Sperisen, W. Griiebler, V. KSnig, P.A. Schmelzbach, K. Elsener, B. Jenny and C. Schweizer, Nucl. Instr. and Meth. 190 (1981) 301. [6] W.G. Weitkamp, I. Halpern, T.A. Trainer, S.K. Lamoreaux and Z.Y. Liu, Nucl. Phys. A417 (1984) 405. [7] K. Sagara, K. Maeda, H. Nakamura, M. Izumi, T. Yamaoka, Y. Nishida, M. Nakashima and T. Nakashima, Nucl. Instr. and Meth. A270 (1988) 444.