Techniques for particle identification and energy measurement of helium ions in the intermediate energy range

NUCLEAR

INSTRUMENTS

AND

METHODS

I33

(1976) 475-483;

©

NORTH-HOLLAND

PUBLISHING

CO.

TECHNIQUES FOR PARTICLE IDENTIFICATION AND ENERGY MEASUREMENT OF HELIUM IONS IN THE INTERMEDIATE ENERGY RANGE* N. C H I R A P A T P I M O L t , J. C. F O N G +, M. M. G A Z Z A L Y , G. IGO, A. D. L I B E R M A N § , R. J. R I D G E , S. L. V E R B E C K +, C. A. W H I T T E N , Jr.

University of California, Los Angeles, CaliJbrnia 90024, U.S.A. J. A R V I E U X * * a n d V. P E R E Z - M E N D E Z

Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720, U.S.A. Received 29 D e c e m b e r 1975 Techniques used in the particle identification a n d energy m e a s u r e m e n t o f helium ions with several h u n d r e d MeV o f kinetic energy are described. In a two a r m array, magnetic rigidity, specific ionization, a n d time-of-flight m e a s u r e m e n t s were employed in the forward arm, while time-of-flight, specific ionization a n d total energy m e a s u r e m e n t s were employed in the recoil arm. Very clean particle identification was obtained for hydrogen and helium isotopes.

1. Introduction In a recent measurement of quasi-elastic knockout of alpha particles by 0.65 and 0.85 GeV alpha particles 1), two methods of particle identification and energy measurement with some unique features have been employed. One method, used in the forward arm of a coincidence array, employed a combination of magnetic rigidity, specific ionization, and time-of-flight measurements to identify the particle type and measure the kinetic energy. The second method was used in the recoil arm where high quality particle identification, accurate energy measurement (3 TIT = 0.025) and high count rate capabilities were required over a large solid angle. The detector array employed a large (3.8 × 104 cm 3) plastic scintillator to measure kinetic energy and thin plastic scintillators to measure specific ionization and time-of-flight.

2. Forward arm: particle identification using the measurement of magnetic rigidity, specific ionization, and time-of-flight The experimental setup for the quasi-elastic knockout (~, 2~) measurements is shown in fig. 1. In the forward arm, the magnetic rigidity of the particles was measured * Supported in part by the United States Energy Research Development Agency. t Supported by the Royal Thai G o v e r n m e n t . + Recipient o f support f r o m Associated Western Universities, Inc., 136 East South Temple, Salt Lake City, U t a h 98411, U.S.A. § Present address: High Physics Laboratory, Stanford University, Stanford, California, U.S.A. ** Present address: Institut des Sciences Nucl6aires, Grenoble, France.

by a spectrometer system which consists of a " C " type magnet and three multiwire proportional counters (MWPC). The magnet had an effective length of 1.0 m and a bending angle of 23 ° for the central momentum, while the geometrical acceptance of the system was 4-4.5 ° horizontally and +_2 ° vertically. The momentum acceptance was AP/P= + 2 0 % . All three MWPCs measured both a horizontal (1") and vertical (Y) position. The front MWPC, measuring X - 1 and Y-I coordinates, and the back MWPC, measuring X-2 and Y-2 coordinates, were of the individual amplifier typeZ), while VPM-MWPC, measuring X-3 and Y-3 coordinates, was of the delay line type3). Specific ionization and time-of-flight (TOF) measurements were obtained using the plastic scintillators D1 and D2. The relevant characteristics of the forward arm were: momentum resolution, 5P/P=O.OI5 (fwhm); and time-of-flight resolution, 5 ( T O F ) < 1 ns (fwhm). AI absorber

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Fig. 1. Schematic d i a g r a m o f the experimental setup. The event trigger was D I 2 2 ' D E D X .

476

N.

CHIRAPATPIMOL

rigidity (P/Z) of the particle was calculated by comparison with representative trajectories which had been integrated through the magnetic field and expanded using Chebyshev polynomialsa). Particle identification in the forward arm proceeded in the following manner. First a scatter plot of the quantity (1-T/To) versus TOF for accepted trajectories through the spectrometer system was generated. Such a scatter plot from the experimental data is shown in fig. 2. TOF is the time-of-flight between the plastic scintillators DI and D2. T is the kinetic energy of the particle as calculated from its measured magnetic rigidity (P/Z) under the assumption that it is an alpha particle, while TO is the kinetic energy expected for an alpha particle undergoing alpha-alpha elastic scattering. For nonrelativistic kinematics (complete relativistic calculations produce only small corrections in the

The first step in the data analysis was to select events which formed good trajectories in the forward arm magnetic spectrometer system. The particle trajectory was determined from the coordinates X-I, Y-1 and X-2, Y-2. The intersection of this line with the plane of the target determined the point of interaction and was required to lie within the beam spot size at the target. The slopes in the x and y directions of the scattered particle's trajectory were calculated with respect to the central trajectory of the magnetic spectrometer. This central trajectory came from the target center at an angle equal to the angular setting of the spectrometer. These slopes were required to lie within the limits of the spectrometer's angular acceptance; that is, __+4° for the slope in the x direction and _ 2 ° for the slope in the y direction. Combining these data with the coordinates X-3 and Y-3, the magnetic

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290

295

300

0

265

270

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TO F (0.2 ns/channel) Fig. 2. Scatter plot o f (1 -

T/To) vs

7 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 4 14 23 20 13 24 3~

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0

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T O F f r o m the experimental data. Three particle groups are identified.

PARTICLE

energy region studied)

T/To is given by:

T/To = (Z~ L:/2 ToM,) (M/Z) 2 (I/TOF):,

(I)

where L is the flight path, and M and Z are the actual mass and charge of the particle. Thus a plot of (1-T/To) vs T O F will generate a family of curves characterized by the particle type with the ratio of separation determined by the factor (M/Z)2; that is, p : d : t : 3He:~= 1:4:9:9/4:4. When the scatter plot shown in fig. 2 is rotated by 45 ° and the data are projected on to one of the new axes, which is designated, because of the rotation as TITs, the spectrum shown in fig. 3 results. The next step is to separate the deuterons from the alpha particles. In order to do this, another quantity, MOM, is calculated M O M = (AE2) ( P / Z ) 1"4 ,

(2)

d,e 600

He3j

40(

o

477

IDENTIFICATION

where AE2 is extracted from the calibrated pulse height response of the D2 counter. The factor 1.4 in eq. (2) was obtained by fitting plots of (P/Z) versus specific ionization, AE/AX, for Z = 1 and Z = 2 isotopesS). Such plots are presented in fig. 4. The lines in fig. 4 correspond to constant values for the quantity (P/Z)"(AE/AX)with a equal to 1.45 for alpha particles, 1.32 for helium-3 particles, 1.32 for tritons, 1.14 for deuterons, and 0.90 for protons. The quantity ( P / Z ) 1"4 (AE/AX) is plotted versus (P/Z) in fig. 5 for these particle types. For each particle type this quantity is fairly constant over the range of P/Z measured in this experiment in relation to the spacings between the average values for each particle type. The points in fig. 5 have been calculated with data obtained from the specific ionization tables of JanniS). A scatter plot of the T/T~ variable versus the MOM variable obtained in an experimental run is shown in fig. 6. Most of the protons traversing the spectrometer system have been eliminated by a pulse height threshold in the D 1 counter and therefore do not appear in figs. 2, 3 and 6. Separation of particle types can be easily done with cuts on M O M and T/Tr (fig. 6).

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Fig. 4. P l o t s o f m o m e n t u m d i v i d e d b y charge, P/Z, vs specific i o n i z a t i o n , AE/AX, for v a r i o u s Z = / a n d Z = 2 i s o t o p e s . T h e AE/AX v a l u e s w e r e o b t a i n e d f r o m ref. 5. T h e solid c u r v e s o f c o n s t a n t (P/Z)" (AE/AX) are best fits to the d a t a p r e s e n t e d . T h e p a r a m e t e r a e q u a l s 0.90 for p r o t o n s , 1.14 for d e u t e r o n s , 1.32 for t r i t o n s a n d h e l i u m - 3 ions, a n d 1.45 for a l p h a particles.

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478

N. C H I R A P A T P I M O L 200 198 196 194 192 190 188 186 184 182 180 178 176 174 172 170 168 166 164 162 160 158 156 154 152 150 148 146 144 142 140 138 136 134 132 130 128 126 124 122

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et al.

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340

360

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9 i O--O 0N o OCt 0 0 I 0 0 0 320

0 0 0 0 0 0 0 0 0 0 0 0 0 2 0 0 0 i i 0 0 2 I 2 2 2 5 9 4 0 0

MOM Fig. 6. Scatter plot o f T/T, vs M O M from the experimental data. F o u r particle groups are identified.

3. Recoil arm: particle identification using the measurement of time-of-flight, specific ionization, and kinetic energy In the (~, 2~) reaction studies 1) a larger solid angle was needed in the recoil arm than in the forward arm in order to subtend a sizable portion of quasi-elastic events. A detection system involving a large (60 c m x 60 cm x 10.65 cm) plastic scintillator (total energy counter) to measure the kinetic energy of the recoil particles was used to provide a large solid angle. Besides the kinetic energy measurement, T', obtained from the total energy counter, time-of-flight and specific ionization measurements were obtained using thin plastic scintillators (DEDX) and a delay line type MWPC, measuring X-4 and Y-4 coordinates, which were positioned between the target and the total energy counter (see fig. 1). If, as in the work reported here,

time-of-flight and kinetic energy are measured (the measurement of the specific ionization is necessary to separate out singly charged particles), the fractional uncertainty in the mass, AM/M, is (to order f14): A M / M = [A T '/T') 2 + (4 + 6fl 2) (ATOF/TOF)Z] 6 .

(3) Because of the coefficient (4 + 6fl2), where fl is the ratio of the velocity of the particle to that of light,careful attention must be paid to the minimization of the uncertainty in the time-of-flight measurement, since the recoil arm configuration has to be designed to provide sufficient resolution in AM/M to separate helium-3 and -4 ions. In conjunction with the counter D1 on the forward arm the time-of-flight for particles in the recoil arm was measured using four large

PARTICLE

counters (DEDX) placed directly in front of the total energy counter (see fig. 1). The dimensions of each of these four plastic scintillators are approximately 30 cm × 30 cm × 0.3 cm ; and together they covered the surface area of the total energy counter. The scintillators were viewed by RCA 8575 photomultipliers6). The anode signals were clipped with 2 ns lines; and they triggered zero-crossing discriminators. In testing with a

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479

IDENTIFICATION

c

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beta source, the time delay due to light propagation in the plastic scintillator was found to vary, mainly along the longitudinal axis (the axis defined by the center line of the scintillator and photomultiplier). The difference in time delay from end to end of the scintillator was about 1.5 ns. The time delay versus position along the longitudinal axis is shown in fig. 7. In order to correct for this effect, the counter was mapped with a 450 MeV alpha beam. The time-of-flight spectrum after the time delay correction from the map was applied is shown in fig. 8. The resolution is seen to be 0.6 ns fwhm. The total energy counter was designed and constructed by Fredrickson et al. v) (see fig. 9). The plastic was Pilot-Y 8) scintillator. This counter had a surface area of 60 cm x 60 cm, and its thickness of 10.65 cm stopped alpha particles up to 480 MeV. It was viewed by four 13 cm diameter RCA 4522 photomultiplier tubes6). The energy resolution obtained by Fredrickson et al. 7) was 6 % fwhm, not sufficient for the quasielastic experiment described in ref. 1. To improve the energy resolution, the following modifications were made: I) An air gap coupling between the lucite light pipe Intercolibrotion tube viewing Am source and light pulser,}

0.2

0

\ [ IO

0

I 20 Distance (cm)

I 30

Luc" --l-

Fig. 7. The time delay o f the response o f a D E D X counter vs distance relative to the light pipe.

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400 -

|

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300 u)

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200

2 I0

220

TOF

Fig. 8. T h e time-of-flight s p e c t r u m obtained using the recoil arm. T h e time-of-flight channel width is 0.2 ns.

I.

-

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Fig. 9. Detailed diagram showing the total energy counter.

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N. C H I R A P A T P I M O L et al. 0.78 089

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Fig. 10. Contours of equal pulse height response obtained from the four RCA 4522 photomultipliers. The total energy counter was mapped with 450 MeV alpha particles. The contour plots (1) and (4) are for the outside photomultipliers, (3) and (4) for the inner photomultipliers.

and the photocathode was filled with a transparent resing). 2) The focusing field for the photocathode in each tube was adjusted to maximize the gain, keeping the high voltage fixed. 3) The pulse height response as a function of position on the scintillator was measured. This was accomplished by setting the forward spectrometer to zero degrees in a beam of reduced intensity. The AI absorber and the D2' plastic scintillator were removed and the DEDX counters and the total energy counter were placed as a unit 1 m behind D2 in such a way that the entire surface area of the D E D X counters and the total energy counter could be illuminated by momentum analyzed beam particles whose spatial locations on the DEDX counters and the total energy counter were defined by X-l, 2, 3 and Y-l, 2, 3. The surface area of the total energy counter was divided into 144 zones. For each phototube the mean value hSi and the width cr~ of the pulse height distribution in each zone was

determined; and a linear interpolation between zones was made. Contours of ~i for the four phototubes, shown in fig. 10, can be understood in terms of the spatial location of each phototube. 4) The high voltages were optimized to maintain linearity. This was checked by using variable energy alpha particle beams. To minimize the uncertainty introduced by the analog to digital converter, the pulses from each phototube were first passed through by ac-coupled fast amplifiers. The final high voltage settings for all four photomultipliers were - 1 7 5 0 V for the cathode and +280 V for the anode. The pulse height response of the total energy counter was calibrated using an alpha ranging in energy from 100 MeV to 450 MeV in 50 MeV steps. A chi-square fitting program was applied to the data to obtain the energy calibration curve as a function of pulse height for each of the photomultipliers. An energy calibration curve of one of the photomultipliers is shown in fig. 11. 5) The gains of the 13 cm photomultipliers were

481

PARTICLE IDENTIFICATION

monitored by an argon glow lamp 1o). The stability of the argon lamp, in turn, was checked against a 241Am source dissolved in a scintillator which was placed near the photocathode of an RCA 8575 photomultiplier tube. The 8575 photomultiplier was situated (see fig. 9) in such a way that photons from both the argon lamp and the plastic scintillator containing 241Am could be detected, thus making a direct comparison possible. The gain shifts of the RCA 4522 photomultipliers were found to be as large as 7 %. During the run, the argon lamp was turned on and the electronic gate for the 241Am source was opened during two time intervals between beam pulses once every 100 spills. The pulse height response was recorded on magnetic tape along with the data from the experiment and the correction due to gain shifts was made off-line. The argon lamp was occasionally turned on during the beam pulse to ascertain if there were beam related effects which would alter this resolution. These were always negligible at the beam level used, 3 × 108 alpha particles/s. The trajectory of the recoil particle was determined by a line connecting the point of interaction at the target as defined by X-l, Y-1 and X-2, Y-2 and the coordinates/'-4 and Y-4 (see fig. 1). The intersection of the recoil particle's trajectory with the planes of the DEDX counter and total energy counter located the points of impact on these counters. The DEDX counter and total energy counter maps were used to correct the time-of-flight and pulse height response. In the latter case the correction due to the apparent gain shift of the 13 cm photornultiplier was taken into account. The energy of the recoil particle was deter-

mined from the energy calibration curve (see fig. 11) assuming that all recoil particles were alpha particles. The point of interaction and the point where the recoil particle hit a DEDX counter determined the flight path. Other calculated quantities were the opening

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Pulse h e i g h t ( c h o n n e l )

E N E R G Y (MeV) Fig. l I. Energy calibration curve for one of me K C A 4522 photornultipliers viewing the total energy counter in the recoil arm,

Fig. 12. Energy spectra for the four large photomultipliers o f the total energy counter from a calibration run.

N. C H I R A P A T P I M O L

482

angle, coplanarity, and the sum of the energies of the

I000

scattered and recoil particles. A correction to the timeof-flight and energy measurements due to energy losses in the target, air paths, plastic scintillators, and WMPCs for both outgoing particles was applied. 3.1.

m

== - FWHM

M E A S U R E M E N T OF ENERGY OF THE RECOIL ARM

For each event, four measurements of the kinetic energy T/were obtained (see fig. 12) and the weighted mean of the energy was calculated,

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215 210 205 200 195 190 185 180 175 170 165 160 155 150 145 140 135 130 125 120

115 ii0 105 i00 95 90 85 80 75 70 65 60 55

50 45 40 35 30 25 20

i 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

1 0 0 0 0 0 0 0 0 0 0 0 0 0 i 0 0 0

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Fig. 13. Weighted m e a n energy spectrum for the total energy counter from a calibration run.

t'~

et al.

0 0 0 0 0 0 0 i 0 0 0 i I 0 0 0 0 0

i 0 0 i 0 0 0 0 0 3 0 2 i 0 0 0 i I

The spectrum of the weighted mean energy from the 450 MeV alpha beam data is shown in fig. 13. For this

0 0 0 I 0 i 0 0 1 0 0 0 i 0 2 1 i 5

i 0 0 1 2 3 0 0 0 0 0 0 0 i 0 0 0 0 0 0 0 2 i i 0 0 0 0 0 0 2 2 0 0 0 0 0 0 0 6 3 0 0 0 0 0 0 i 2 0 i/0 0 0 0 0 6 7 i 4 / 0 0 0 0 i 0 8 4 /2 i i 0 0 0 2 9 7// 4 0 0 0 0 2 i0 12 9 4 0 0 1 0 2 i0 2/5 3 2 0 0 0 3 9 16 ~ 7 4 0 0 i i 4 12 12 6 3 0 0 0 i 12 i0/ 6 3 2 i 0 0 2 16 13/17 4 i I 0 0 8 19 13 9 i i 0 i 0 15 11/17 8 5 0 0 0 0 14 13/15 3 4 i i 2 0 0 1 3 0 2 7 16 18 12 7 1 1 0 0 2 0 i 1 1 1 18 26/'10 6 2 1 2 i i i 0 0 0 0 7 18 17/ 13 12 1 2 2 4 4 2 0 0 2 2 I0 31/36 19 7 2 3 3 1 i 2 1 0 1 5 16 32/17 18 4 3 6 3 0 3 0 0 0 i ii 35 29 21 0 2 1 6 2 3 4 3 0 0 i 18 33~31 ii 8 1 5 1 4 2 1 8 0 0 5 23 35 24 ii 3 2 I 3 4 5 3 16 i 0 0 34~52 23 3 4 4 7 4 3 6 i0 12 0 i 14 36 33 12 3 2 5 8 6 7 ii 23 28 i 4 24~28 20 3 4 0 2 9 12 24 ii 37 40~ i 4 29 37 18 6 2 3 5 8 33 44 29 53~56 1 6~30 17 9 5 4 5 5 19 51 56 46~50 30 I/ii 23 6 4 5 5 ii 15 30 78 93~85 60 22 0 13 i0 i 0 2 9 ii 23 103 1 7 7 ~ 1 4 3 89 49 23 1 3 4 2 7 ii i0 16 76 202-"201 148 90 32 ii 2 4 I 3 8 13 53.,...226""315 243 iii 49 7 9 8 42....-198 382 325 182 70 29 0 IZ~ z 6 18 7 0 0 1 8 31"~i~9 392 320 189 62 32 18 9 6 0 0 0 1 17 73 149 114 30 14 6 5 7 2 0 0 0 0 4 7 13 0 4 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0

oZ=l

80

90

i00

ii0

120

130

140

150

160

170

180

190

200

210

220

0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0

1

0

0

1 0 2 1 1 0 0 2 2 0 3 1 6 1 lO 2 15 4 20/4 19 12 21 9 28 3 I0 2 8 3 9 3 3 4 7 4 7 7 3 3 0 0 0 0

0 0 0 1 1 1 4 4 8

230

240

TOF (0.2 ns/channel) Fig. 14. Scatter plot o f the pulse height response o f the D E D X counters vs the time-of-flight in the recoil arm.

8 5 2 4 2 2 i 5 7 9 2 0 0 250

PARTICLE IDENTIFICATION Z=l

483 3H e

),d,t

150

IOOC

Z=2

"3

He,3 a

E

~5o

2

,J,

-150

-I00

-50

(

0

ZRE

i r 2500

Fig. 15. Projected events on the (rotated) axis ZRE; Z R E = 0.749 DEDX-0.664 TOF.

i 3000 3500 Mass (MeV)

4000

Fig. 16. The 3He-'~He mass separation in the recoil arm. c a l i b r a t i o n d a t a no c o r r e c t i o n s were made for energy losses in the a p p a r a t u s . The energy resolution o b t a i n e d was 2.4 % (fwhm). 3.2. PARTICLE IDENTIFICATION IN THE RECOIL ARM

A scatter plot o f the pulse height response in the D E D X counters versus time-of-flight extracted from the experimental d a t a is shown in fig. 14. The two groups o f particles characterized as singly a n d d o u b l y charged particles are widely separated. A g a i n it is convenient to rotate the scatter plot a n d to p r o d u c e a new variable Z R E . The projection o f Z R E on the abscissa is shown in fig. 15. A cut was m a d e on this spectrum in o r d e r to remove the singly charged particles f r o m the d o u b l y charged particles. Helium-3 and -4 ions were separated in the recoil arm using time-of-flight and T' measurements a n d the relativistic mass-energy relation: Mc

2 =

T'

I-(1-,6~) - ~ -

I]

(5)

Fig. 16 illustrates the s e p a r a t i o n o f 3He a n d 4He particles obtained. 4. S u m m a r y

The experimental techniques discussed a b o v e for the particle identification a n d energy m e a s u r e m e n t o f helium ions in the intermediate energy range quite

effectively met the experimental criteria for the study o f the quasi-elastic k n o c k o u t o f a l p h a particles by the (~, 2~) reaction. They are particularly useful when large solid angle acceptance, high counting rate capability, m o d e r a t e l y g o o d energy resolution and positive identification o f light nuclear fragments are required. The authors express their a p p r e c i a t i o n to the 184" synchrocyclotron staff under the direction o f L. K a n s t e i n for p r o v i d i n g an excellent b e a m o f a l p h a particles for these measurements. References

i) N. Chirapatpimol, Ph.D. Thesis (University of California, Los Angeles, 1975). 2) J.-L. Pellegrin, SLAC-TN-70-22 (Sept. 1970). 3) A. W. Stetz, V. Perez-Mendez, J. Geaga and H. Spinka, Nucl. Instr. and Meth. 120 (1974) 17. 4) C. Lechanoine, M, Martin and H. Wind, Nucl. Instr. and Meth. 69 (1969) 122. 5) j. F. Janni, Tech. Rep. No. AFWL-TR-65-150 (Sept. 196,% 6) Technical Manual PT-60, Radio Corp. of America, Lancaster. 7) D. H. Fredrickson et al., Nucl. Instr. and Meth. 107 (1973) 205. 8) Nuclear Enterprises, 935 Terminal Way, San Carlos, Calif. 94070, U.S.A. 9) Sylgard 184, Dow-Corning Corp., Coming, New York 14830, U.S.A. ~o) Q. A. Kerns and R. F. Tusting, UCRL-10895 (July 1963).