A time-of-flight spectrometer for heavy ions

:NUCLEAR

INSTRUMENTS

AND

M E T H O D S 95

©

(I97I) 397-402;

NORTH-HOLLAND

PUBLISHING

CO.

A TIME-OF-FLIGHT S P E C T R O M E T E R FOR HEAVY IONS C. K. G E L B K E , K. D. H I L D E N B R A N D and R. B O C K

Max-Planck-lnstitut fiir Kernphysik, Heidelberg, Germany Received 7 May 1971 A time-of-flight spectrometer for the identification of complex nuclei produced by nuclear reactions with heavy ions has been constructed. The transmission detector consists of a thin scintillator foil; the stop signal and the energy information are

derived from a surface barrier detector. A time resolution of better than 0.3 ns has been obtained. The system is able to separate masses up to A = 40 at energies below 3 MeV/amu.

1. Introduction

detectors the inhomogeneity and particle loss caused by multiple scattering are considerable. The area of these detectors is very small and only particles of sufficiently high energy can pass through the A E counter. The applicability of this method, therefore is limited to light nuclei only. The use of an open photomultiplier to detect secondary electrons emitted by the particle's passing through a thin 12C-foil gives a time resolution of

Several methods for particle identification in nuclear reactions induced by heavy ions have been used up to now, in particular d E / d x - E telescopes, magnetic analysis and time-of-flight techniquesl-S). This paper describes a time-of-flight spectrometer optimized for the mass separation of heavy ions in the range of tandem energies and for masses up to A = 40 which allows much better mass resolution than the d E / d x - E telescope. According to the relation Et 2 = m s 2 ~ 2 the mass resolution of a time-of-flight spectrometer is determined by

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( A m / m ) 2 = (AE/E) 2 + (2At/t) 2 + (2As/s) 2 ,

SEPTEMBER 1971

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where A E / E is the intrinsic energy resolution of the energy detector and A t / t is the time resolution of the spectrometer at a given energy. The design of the spectrometer is based on the following considerations. Because the relative energy resolution of a solid state detector for heavy ions is about 0.5%, the relative time resolution should be at least of the same magnitude, requiring a long flight path or a good absolute time resolution. On the other hand, the flight path should be as short as possible in order to obtain a reasonable solid angle. For several ,;ets of parameters s, At and A E / E , fig. 1 presents the calculated mass resolution as a function of the energy per nucleon. Because of the requirement for subnanosecond time information and for the limitation of the strong multiple ,;cattering with heavy ions, the development of a suitable transmission detector is the most difficult problem. For investigations with heavy ions several methods have been used until now. With thin solid state detectors a time resolution of about 100 ps has been obtained2'1°). With these

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Fig. 1. The mass resolution as a function of the energy per nucleon for various parameter sets. The quantity s is the length of the flight path; As/s is assumed to be negligible; dE/E, the intrinsic energy resolution of the detector, is assumed to be 0.5% for all energies and curves; At is the time resolution of the spectrometer. Parameters Curve number

s

At

(cm)

(ps)

1 2 3 4 5 6 7

100 100 100 100 150 150 150

500 400 300 200 500 400 300

398

c . K . GELBKE et al.

0.8 ns. These open multiplier tubes have the disadvantage that they can only be used in very good vacuum and that they need high voltages for the acceleration and focussing of the electrons4'7). By the application of a thin foil made of NE 102A and connected optically to an AVP 56 phototube a time resolution of 0.9 ns has been obtained6). The spectrometer described here is based on this principle. Systematic investigations have been undertaken for finding the best conditions for this kind of transmission detector. We have found that one can dispense with the light guide of the latter system and use a scintillator foil that is surrounded by a hemispherical mirror and get better time resolutions at 100% efficiencf). This system is very resistive and easy to handle. The scintillator foil can be changed quickly in order to adapt it to the special requirements of the experiment regarding the compromise between desirable time resolution and minimum energy loss and energy spread.

is made by varying the external delay in the stop branch. The time resolution can be determined from the width of a peak in the TAC spectrum. Fig. 3 shows a typical TAC spectrum in linear and logarithmic display. The separation of the peaks is 1 ns and 0,5 ns respectively. The pulses of the phototube were triggered with a constant fraction timing discriminator (CFTD). The properties of two CFTD units (Ortec Model No. 453 and Berthold and Frieseke No. 2005 B) were found to be about the same. A comparison of various fast scintillators showed that the timing properties of N e l l l were superior to other scintillators commonly usedS). We compared NE 111 with the NE 102A used by Muga et al. Table 1 shows the time resolution of these two scintillators obtained under identical experimental conditions.

2. Timing properties The transmission detector consists of an XP 1021 phototube and a thin scintillator foil in the arrangement of fig. 2. The dependence of the time resolution on the energy loss of the particles in the scintillator foil, on the diameter of the hemispherical mirror and on the scintillator material has been studied. The timing properties of various surface barrier detectors with two different trigger systems have been compared. The time resolution A t was determined with monoenergetic heavy ions obtained by elastic scattering on a thin gold target. In these measurements start and stop detector were only 5 cm apart. Variations of the flight time caused by the energy spread in the scintillator foil therefore can be neglected. The calibration of the time-to-amplitude converter (TAC) Mir

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Fig. 2. The start detector. The angle between the scintillator foil and the beam can be varied.

Fig. 3. A time spectrum in linear and logarithmic displays. The separation of the peaks is Ins and 0.5 ns respectively. The time resolution is At = 260 ps.

TIME-OF-FLIGHT SPECTROMETER FOR HEAVY IONS TABLE 1

TABLE

A comparison of the time resolutions obtained with scintillator foils made of NE 111 and NE 102A; the foils are chosen very thin in order to show the effect more clearly. Beam

Scintillator

160 (50 MeV)

A E in foil

(ps)

375 425

480 635

TABLE 2 The time resolution obtained with two hemispherical mirrors of different size. For thinner foils the better time resolution with the smaller mirror becomes more apparent. Diameter of the hemisphere

A E in foil

A t (fwhm)

(MeV)

(ps)

= 38 mm

1.9 0.375

225 480

= 48 mm

1.9 0.375

255 580

The time resolution depends on the energy loss in the scintillator foil (see fig. 4). A n energy loss of 1 MeV already yields a good time resolution, which c a n n o t be i m p r o v e d very m u c h by thicker foils. W i t h smaller energy losses the time resolution is getting worse. One obtains a better time resolution when the b e a m has n o r m a l incidence to the foil. This can be explained as a kinematic effect. W h e n the b e a m axis is inclined with respect to the foil by a n angle of 45 ° the variation in the length of the flight path is the same as the diameter of the b e a m spot on the scintillator At(psec) 600 SO0

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Time resolutions obtained with a leading edge trigger and a constant fraction trigger for several semiconductor counters. The quantity F is the active area, d is the depletion depth of the detector.

A t (fwhm)

(keY)

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399

I 2500

I 3000

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Time pickoff (ps)

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230

250

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320

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F = 400 mm2 Heavy-ion-det. 70 V bias (nominal) 150 V bias

345

460

340

360

foil; for particles with 3 M e V / a m u this causes a difference in flight time of a b o u t 120 ps for a beam spot of 3 m m diameter. This difference has to be added quadratically to the time resolution obtained with n o r m a l incidence. The size of the hemispherical mirror has some influence on the time resolution. Table 2 shows the results o b t a i n e d with two different mirrors. It can be seen that the smaller m i r r o r provides a better time resolution, especially if the energy loss in the scintillator foil is small. To derive fast time signals from the semiconductor c o u n t e r two different systems were c o m p a r e d : an Ortec time pickoff u n i t (model No. 260) triggering in the leading edge m o d e a n d a c o n s t a n t fraction timing system following the design of S h e r m a n et a19). The time resolution obtained depends o n the rise time of the s e m i c o n d u c t o r pulses. F o r all types of counters the c o n s t a n t fraction trigger gives better results t h a n the time pickoff system a n d the difference of the two trigger modes becomes more significant with increasing rise time of the counters (see table 3).

A E(keV)

3. Mechanical setup of the spectrometer Fig. 4. The time resolution obtained with various energy losses in the scintillator foil. Open points: angle of 45 ° between the foil and the beam axis; full points: beam axis is normal to the scintillator.

The spectrometer (fig. 5) is placed at the Heidelberg E m p e r o r V a n de G r a a f f accelerator. The target c h a m b e r (No. 1 in fig. 5) is connected with the

400

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G E L B K E et al.

Fig. 5. T h e time-of-flight spectrometer at the Heidelberg M P t a n d e m accelerator; (I) target chamber, (2) target, (3) c h a m b e r for the start detector, (4) photomultiplier with base, (5) flight tube, (6) rear flange with energy detector, (7) turbo-molecular p u m p , (8) circular rail.

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Fig. 6. (E,t) matrix of the reaction products obtained by b o m b a r d i n g a target containing 7Li and 12C with 160 at 50 MeV. A l o n g the abscissa t o - t is plotted and along the ordinate the energy E. F r o m right to left the masses A = 12, 14, 15, 16, 17, 18, 19, 20, 23, 24 can be identified.

Fig. 7. Part of the spectrum of fig. 6 expanded in both directions by the use o f biased amplifiers. The masses from right to left are 15, 16, 17, 18, 19, 20, 24.

TIME-OF-FLIGHT

SPECTROMETER

beam tube by a sliding metal b a n d and is turned with the whole spectrometer a r o u n d a fixed target (No. 2). The chamber containing the start detector (No. 3) is flanged to the exit of the target chamber. The phototube (No. 4) is introduced horizontally, the p h o t o c a t h o d e being inside in the v a c u u m and the base being outside. The distance between target and scintillator is 28 cm. The length of the flight tube (No. 5) can be changed from 1.0 to 2.5 m. The energy detector is m o u n t e d at the rear flange (No. 6). A turbo-molecular p u m p (No. 7) fixed to the tube maintains a v a c u u m o f better than 2 x 10 - 6 torr. The whole system is m o v e d on a circular rail (No. 8) o f 1.7 m diameter a r o u n d the target. 4. P e r f o r m a n c e o f the s p e c t r o m e t e r

As shown before the time resolution better than 300 ps is excellent. The thickness of the foil, however, has some influence on the energy resolution and may cause particle loss due to multiple scattering. In order to investigate these effects, the intensities of elastically scattered 160 ions o f 50 MeV in the counter at the end of a flight path of 1.5 m with and without foils were compared. At an energy loss of 3 MeV in the foil a loss of about 10% o f the particles was observed; an energy loss o f 1.3 MeV gave less than 2% particle loss. The energy straggling was found to be about 10% of the energy loss. This adds quadratically to the intrinsic energy resolution o f the counter. The best result obtained until now was a time resolution o f 200 ps with an energy loss in the foil o f 1.37 MeV and an energy resolution of 215 keV for a60. The energy resolution of the detector was 175 keV without foil. During an experiment, the energy and time infor-

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FOR

HEAVY

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IONS

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Fig. 9. Mass separation of the reaction products at one definite energy obtained by bombarding a target containing 4OCa, 42Ca and 12C with 160 at 28 MeV. 0lab = l0 °.

mation for every particle are stored in a two-dimensional matrix. In order to reduce dead time the T A C is started with the timing pulse from the energy detector. Therefore the T A C - o u t p u t corresponds to (to-t), where t o is the electronic delay and t the time-of-flight. In this case particles o f the same mass are on a hyperbola. Fig. 6 shows the reaction products that were obtained by bombarding a target containing 7Li and lzC with 160 o f 50 MeV. F r o m right to left masses from A = 12 to A = 24 can be identified. In fig. 6 the mass resolution is limited by the n u m b e r of channels. By using biased amplifiers the intermediate mass region o f the spectrum was expanded (fig. 7). The different masses are seen to be well separated. Fig. 8 shows the counts per channel o f definite energy. This corresponds to a cut parallel to the time axis. The mass resolution for A = 1 6 ( e l a s t i c a l l y scattered 160 line) is AA = 0.27 that means a AA/A o f 1.7%. Fig. 9 shows a similar plot for the reaction 160 on 42Ca, 4°Ca target on l z c backing at Ela b = 28 MeV and 0~,b = 10 °. At this energy the mass resolution in the region A = 4 0 is mainly determined by the energy resolution of the detector. It should be possible to separate Am = 1 around the mass A = 40 by using counters especially selected for g o o d heavy-ion resolution. Investigations with heavy-ion detectors are in progress. The experimental data shown in fig. 6 to fig. 9 were taken with a time resolution of 500 ps and a flight path o f 1.5m.

402

c . K . GELBKE et al.

5. Summary

References

Measurements done until now indicate that the spectrometer can separate masses up to A = 4 0 . The limiting factor for identification of higher masses is the intrinsic resolution of the energy detectors for heavy particles. The spectrometer has an excellent time resolution for small energy losses in the transmission detector. So the energy resolution is influenced very little by the presence of the transmission detector. The overall qualities of the spectrometer make it especially suited for nuclear spectroscopy of particles a r o u n d A = 3 0.

1) A.G. Artukh, V.V. Avdeichikov, J. Er~5, G.F. Gridnev, V.L. Mikheev and V.V. Volkov, Nucl. Instr. and Meth. 83 (1970) 72. 2) G.W. Butler, A. M. Poskanzer and D.A. Landis, Nucl. Instr. and Meth. 89 (1970) 189. 3) j.C. Jacmart, M. Liu, F. Mazloum, M. Riou, J.C. Roynette and C. Stephan, Rev. Phys. Appl. 4 (1969) 99. 4) W. F. W. Schneider, B. Kohlmeyer and R. Bock, Nucl. Instr. and Meth. 87 (1970) 253. 5) K. D. Hildenbrand, H.H. Gutbrod, W. yon Oertzen and R. Bock, Nucl. Phys. A157 (1970) 297. 6) M. L. Muga, D. J. Burnsed, W. E. Steeger and H. E. Taylor, Nucl. Instr. and Meth. 83 (1970) 135. 7) C.K. Gelbke, Diplomarbeit (Heidelberg, 1970). 8) R. Kunze and R. Langkau, Nucl. Instr. and Meth. 91 (1971) 667. 9) j. S. Sherman, R. G. Roddick and A. J. Metz, IEEE Trans. Nucl. Sci. NS-15 (1968) 500. 10) H. Pleyer, B. Kohlmeyer, W. F. W. Schneider and R. Bock, to be published.

We would like to express our appreciation to Mr. W. Weiss for his help during the experiments a n d Professors W. G e n t n e r a n d U. S c h m i d t - R o h r for their interest in our work. This work was partly supported by the G S I and the B u n d e s m i n i s t e r i u m fiir Bildung u n d Wissenschaft.