Single-detector particle identification in conjunction with coincidence measurements

Nuclear Instruments and Methods 206 (1983) 403-412 North-Holland Publishing Company

SINGLE-DETECTOR MEASUREMENTS J. K A N T E L E

PARTICLE

*, M . L U O N T A M A ,

403

IDENTIFICATION

W. TRZASKA

IN CONJUNCTION

WITH

COINCIDENCE

** a n d A. P A S S O J A

Department of Physics, University of JyviiskylgL Jyv?iskylii, Finland Received 6 September 1982

Thin semiconductor or scintillation detectors function essentially as d E / d x detectors in reactions involving charged particles. Therefore, such detectors can in some cases be used as particle identifiers (with no energy resolution) in coincidence experiments, when it is important to separate y-rays or conversion electrons belonging to different reaction channels. We have tested the feasibility of a number of coincidence arrangements in this respect and have found that the separation of at least protons, deuterons and tritons is possible with thin Si(Au) and NE 102A plastic scintillation detectors in conjunction with y-particle coincidence experiments. Simple cylindrical plastic scintillators based on light collection by total reflection, and exhibiting detection solid angles of about 30%, were constructed and found useful both in y-particle and electron-particle coincidence experiments. In the latter case, the detector was placed in a Si(Li)+ magnetic lens combination conversion-electron spectrometer.

1. Introduction In studies of y-rays and conversion electrons associated with nuclear reactions, the separation of the different reaction (particle) channels is often necessary. This is especially true in cases where weak channels involving charged-particle emission are of special interest but tend to be masked by radiations from strong reactions of the (projectile, x n ) type. The identification of the emitted charged particles can be carried out using e.g. magnetic spectrographs or semiconductor detector telescopes which also yield a good or moderate energy resolution on the particle side. However, since a good particle identification rather than a good energy resolution of the particle detector is often sufficient, we have tested the feasibility of a m u c h simpler method, based on the use of thin, single Si(Au) or plastic scintillation detectors which act essentially as d E / d x detectors. The idea is based on the following factors: (1) the stopping powers of the different particles (like protons, deuterons and tritons) at energies typically encountered in practice differ strongly from each other, (2) a suitable absorber may be used occasionally for degrading the particle energies, which further amplifies the particle-separation effect. In this paper, the m e t h o d is d e m o n s t r a t e d for some proton-induced reactions, with certain emphasis on the (p, t) * Temporary address: Los Alamos National Laboratory, Los Alamos, New Mexico, U.S.A. ** On leave from Institute of Nuclear Research, Warsaw, Poland. 0 1 6 7 - 5 0 8 7 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d

reaction which is of special interest to our project aimed at studies of 0 + states and E0 transitions [1]. In addition to surface-barrier silicon detectors giving the best energy resolution and particle separation, we also tried thin plastic scintillators which were expected to feature possibilities for high-efficiency geometries, high counting-rate capability, simplicity of construction a n d operation, and low cost. These expectations turned out to be justified reasonably, in spite of the fact that the differences of the light outputs from the plastic for the different particles [2] cancel partially the differences of the stopping powers. A cylindrical plastic scintillation detector having a detection solid angle of a b o u t 30% was found useful in conjunction with our Si(Li) plus magnetic c o m b i n a t i o n conversion-electron spectrometer [3], although the particle identification capability of this more complicated setup was rather modest.

2. The equipment

2.1. Standard detectors and coincidence arrangements In experiments involving the detection of y-rays, a 100 cm 3 Ge(Li) detector was placed at right angles to the beam, the distance between the target center and the cryostat entrance window being 2 to 3 cm. For particle detection, the standard detectors used were a 200 ~tm thick 300 m m 2 Si(Au) detector and a 500 btm thick ~ 40 m m N E 102A planar plastic scintillator. The particle detector was placed opposite to the y-ray detector with respect to the target; in the case of the Si(Au) detector,

J. Kantele et al. / Single-detector particle tdentification

404

the distance to the target was 30 mm, whereas the plastic scintillator could be placed at a 40 m m distance. The solid angles of detection in the two cases were approximately 2.5% and 6% respectively. The plastic scintillator was covered by a 0.01 m m thick AI foil; a 1 m m thick Cu absorber was placed in front of the ",{-ray detector to attenuate X-rays produced in the b o m b a r d ments. In the face-to-face geometry, the targets were placed at an angle of 45 ° to the beam. Conventional electronics served to store coincident events on-line on a magnetic tape for subsequent analysis.

2.2. Cylindrical plastic' scintillation detectors In order to achieve a high detection efficiency for charged particles emitted in nuclear reactions, we constructed nearly cylindrical detectors made of 0.5 m m thick N E 102A plastic scintillator foils. These detectors are essentially based on an idea similar to that presented by Aihara et al. [4], but represent a greatly simplified version of the device. Our construction is sketched in fig. 1. The 0.5 m m thickness of the foil was chosen on the basis of the following two facts: 1) It is sufficiently thin to act as a transmission counter for most of the particles and energy ranges of interest; it almost stops e.g. 10 MeV tritons but lets protons of > 7 MeV to pass through. 2) It is much easier to fabricate detectors having good optical properties using 0.5 m m thick foils than using 0.2 or 1.0 m m foils also tested. After cutting a suitably dimensioned strip of the scintillation foil, all the edges of the strip were polished. The strip was bent to its final shape after warming it up to a temperature of about 85°C. The ends of the strip were glued together with optical cement and clamped between two Perspex pieces which form the support --'RONT VIEW

of the detector [item (4) in fig. 1]. The ends of the strip coincide with the ends of the support, and the lower surface of the foil + support system is carefully polished and glued to the light guide (5). Since the light is collected from the scintillator via total reflection [4], the pulse height is essentially i n d e p e n d e n t of the position where the particle hits the detector. To protect the detector from light, it was covered by a loose layer of 0.01 m m thick a l u m i n i u m foil. Fig. 1 also shows that the projectile beam traverses along the axis of the cylinder, and the target can, in principle, be placed at any position along the axis. The position indicated in fig. 1 is suited to cases in which a significant fraction of the particles of interest are emitted between angles of about 55 ° to 90 ° . The solid angle of detection is in this case about 30% of 4v. In the measurements of coincidences between y-rays and charged particles, the Ge(Li) detector is placed as in most of the runs at right angles to the beam direction.

2.3. Geometry' for electron-particle coincidence measurements In measurements requiring coincidences between conversion electrons and heavier particles, it is desirable to use a magnetic device for transporting the electrons to a cooled semiconductor detector. In the case of the

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J. Kantele et al. / Single-detector particle identification Siegbahn Sl/~tis type of intermediate-image electron transporter of our laboratory [3], the heavy-particle detector must be placed in a strong magnetic field near the target position inside the electron spectrometer. In order to achieve the highest possible detection efficiency, we have used a cylindrical plastic scintillator described in subsect. 2.2, at the expense of losing the superior particle-identification capability of a Si(Au) detector. A sketch of the setup is presented in fig. 2. Because of the strong magnetic field and limited space inside the spectrometer, a long light pipe made of acrylic resin was constructed, which allows the placement of the photomultiplier tube outside the system. The optical properties were not as good as in the case illustrated in fig. 1, neither was the dependence of the pulse height on the magnet current insignificant. Nevertheless, the detector has been found quite useful in coincidence experiments. Since the mean angle of acceptance of the electron spectrometer is about 40 ° , the target must not be moved upstream beyond the position indicated in fig. 2, which corresponds to an angular range of 50 ° to 90 ° and a solid angle of over 30% of 4~r. The target can be moved along the axis of the detector to also allow the studies of cases characterized by backward-peaking angular distributions. On the electron side, cooled Si(Li) detectors of different sizes can be used which allow total-absorption peak transmissions of typically 6% (with anti-positron baffles included).

3. Test measurements

The performance of the various coincidence setups was tested with the aid of reactions induced in a 1 m g / c m 2 thick enriched (98.4%) metallic 152Sm target by 17 to 19 MeV protons from the Jyv/~skyl~i 90 cm cyclotron. One reason for choosing this target was that it is known [5] to exhibit fairly large (p, t) cross sections leading to certain states in tS°Sm. Since our geometries in the different cases were fixed, the observed relative intensities of the various lines are in general not proportional to the absolute total intensities, due to angular distribution and correlation effects. The Q-values of some proton-induced reactions in 152Sm are as follows [6]: Q ( p , n ) = - 2 . 6 7 MeV, Q(p, 2 n ) = - 8 . 9 7 MeV, Q(p, 3 n ) = - 16.96 MeV, Q(p, d ) = - 6 . 0 4 MeV, Q(p, t) = - 5 . 3 8 MeV, Q(p, 3 H e ) = - 8 . 8 6 MeV, Q(p, 4 H e ) = +6.16 MeV. The proton beam intensity was usually one or a few nanoamperes, and the running time for the y-particle coincidence spectra about 10 h. The counting rates of the detectors were typically 10000 to 20000 cps.

405

3.1. y-rays in coincidence with particles recorded by a Si(A u) detector In order to optimize the geometry for the detection of the tritons in the energy region of interest, a 150/~m thick A1 absorber was placed in front of the Si(Au) detector. As a result of this, tritons were completely stopped in the detector, whereas both deuterons and protons penetrated it with considerably smaller energy losses and consequently with d E / d x values lowered with respect to that of tritons. Thus the use of an absorber can amplify the particle-separation effect. At the same time, both 3He and 4He particles were removed, but they were of no particular interest in this case. The particle and y-ray spectra obtained are illustrated in figs. 3 and 4. Figs. 3 c - e show the various particle groups obtained by gating with y-lines characteristic of the different product nuclei. After revealing the particle groups in this way, gates were placed on them to yield the corresponding coincident y-ray spectra of figs. 4 b - d . As one can clearly see from fig. 3, the triton, deuteron and proton groups are well separated from each other. That the particles in fig. 3d indeed are predominantly deuterons [and not protons from the (p, pn) reaction] is apparent from the consideration of the Q-value of the (p, pn) reaction ( = - 8 . 3 MeV) and the height of the Coulomb barrier ( ~ 9 MeV). The coincident y-ray spectra of figs. 4b d are also very "clean" from interfering lines. The fact that none of the lines in 15°Sin due to the (p, t) reaction are even weakly discernible in the singles spectrum in fig. 4a, clearly demonstrates how powerful the technique employed is. Due to a narrow time gate in the analysis, the intensities of the lowest-energy lines are somewhat attenuated. 3.2. y-rays in coincidence with particles recorded by scintillation detectors The general qualities of the particle spectra recorded by the planar and cylindrical plastic scintillation detectors are demonstrated in fig. 5 which shows the singles planar spectrum (a) the spectrum of the planar detector in coincidence with all y-ray events (b), and the spectrum of the cylindrical detector in coincidence with all y-rays (c). A comparison of (b) and (c) indicates already that the structure of the particle spectrum is beginning to be smeared out in the latter case. This is even more clearly demonstrated in fig. 6 where the y-ray gating described in subsect. 3.1 is used to obtain the triton, deuteron and proton groups recorded by the two detectors. In the case of the planar detector, the centers of gravity of the three particle groups differ clearly from each other but the overall particle separation is deft-

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nitely worse t h a n in the Si(Au) case. F o r the cylindrical detector, the situation is still worse, a n d the s e p a r a t i o n o f tritons a n d d e u t e r o n s is c o m p l e t e l y impossible. W e have not t h o r o u g h l y investigated to w h i c h e x t e n t the

d i f f e r e n c e o f the qualities o f the two s p e c t r a is d u e to the different effective angular ranges associated with the detectors, a n d w h a t the actual role of the s o m e w h a t w o r s e resolution of the cylindrical d e t e c t o r is.

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As can be expected on the basis of figs. 1 and 6, the best particle identification is obtained with the silicon detector. The performance of all three detectors is compared in fig. 7 which shows the y-ray spectra due to the (p, t) reaction obtained by gating with different detectors. (Note that the gate positions for the scintillators are indicated in fig. 6.) The spectrum (b) is practically a pure Y t coincidence spectrum. In (c) the same spectrum is strongly dominating, with weak admixtures from other sources; the quality of (c) is not much worse than that of (b). Spectrum (c), however, is a fifty-fifty mixture of ,/-t and y d spectra, with a contribution from the y - p coincidence spectrum, as well. Thus all of the three spectra show all the observed lines attributed to the (p, t) reaction, but there are dramatic differences in the qualities of the spectra. On the other hand, although the quality of the spectrum is worsened when going from (b) to (d), the overall detection efficiency is increased by one order of magnitude.

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3.3. Conversion electrons in coincidence with heavier charged particles The coincidence setup described in subsect. 2.3 was used in mesurements of conversion electrons in coincidence with charged particles from 152Sin + p reactions at Ep = 19 MeV. In this case, the beam current was 6 n A and the m e a s u r e m e n t time 18 h. The counting rate of the scintillator was a b o u t 70000 cps and that of the Si(Li) detector varied between 4000 cps and 11 000 cps, depending on the magnet current which was swept in the usual manner. The gross particle spectrum in coincidence with all electron events is illuminated in fig. 8. This spectrum is essentially identical to that in fig. 4c, although in this case the high counting rate gave rise to noticeable pile-up summing of pulse amplitudes. Three gate positions used in sorting the coincident electron spectra are also indicated in fig. 8. The conversion electron spectra corresponding to these gates are shown in fig. 9. The singles spectrum (a), again containing mainly lines due to the (p, 2n) reaction, is too complex for most lines to be identified. The electron spectrum is more complicated than the "g-ray spectrum of fig. 4a because of the multitude of lines associated with each transition (K, L, M .... lines) and because of the lines due to the (p, 3n) channel is open at Ep = 19 MeV. In all of the three coincidence spectra, strong lines in ]52Sm corresponding to the (p, p') reaction are clearly seen. The case (b) seems to represent the (p, p') reaction fairly well. In (c) and (d), lines in 15°Sm due to the (p, t) reaction are discernible, in (d) even the 515.1 keV 0 J ~ 0 f E0 transition. This shows that the coincidence

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m e t h o d employed may b e useful, in spite of a rather poor particle-identification capability, Lines due to the (p, d) reaction are apparently too n u m e r o u s and weak to be clearly identified in any of the spectra.

4. Discussion and conclusions The examples given in the present work d e m o n s t r a t e the feasibility of single-detector particle identification in

conjunction with reactions associated with charged-particle emission. It is clear, however, that some care must be excercised in generalizing the conclusions from the present experiments, because other projectile-target c o m b i n a t i o n s may give very different results. Furthermore, it must be pointed out that ~52Sm represents a favourable case for the (p, t) reaction populating 0 + states. Nevertheless, we believe that the following conclusions are justified on the basis of this work: (1) A thin Si(Au) detector can give an excellent

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separation for p, d, t, 3He and 4He. We have demonstrated this for the hydrogen nuclei in the case of (y)(particle) coincidence experiments; however, a consideration of the d E / d x values of the different particles and the Q-values of the reactions clearly shows that detectors of suitable thicknesses will be able to separate the He nuclei, as well. In principle, Si(Au) detectors can also be employed in e -particle coincidence experiments, although efficiency problems may arise in these inherently more difficult experiments. (2) Thin planar plastic detectors seem to have a reasonably good particle separation capability at least for p, d and t. Problems associated with the light output from the plastic scintillator [2] makes plastic a less suitable detector material than silicon. This appears to be particularly true for the He nuclei. On the other hand, plastic scintillators are simple, cheap and rugged detectors exhibiting flexibility in geometrical constructions and high counting-rate capability. (3) Cylindrical plastic scintillators based on light collection via total reflection appear to have a more limited particle-identification capability than the planar ones. This is true at least in the geometries of the present experiments; a device made of a narrower strip would probably improve the resolution, due to a better uniformity of the lengths of the distances travelled by the particles in the detector foil. At any rate a cylindrical plastic detector can be helpful in coincidence experiments with y-rays and electrons, due to its high detection solid angle. (4) It is important to stress that the use of a coincidence (particle) detector of almost an)' type is able to take the most important step in the studies of the charged-particle channels, which usually is the removal of the dominating lines in the singles spectrum due to the (projectile, x n) reactions. The observation of a particular particle channel of

interest can often be facilitated by optimizing the parameters of the experiments, such as projectile energy, angles associated with the detectors, absorbers etc., and by considering also the Q-values of the reactions and the Coulomb-barrier heights. The angular distributions of the particles should also be taken into account, if possible. The angular correlations between ~'-rays/electrons and heavy particles would affect the setting up of the experiment in a manner too complicated for practical purposes; however, the influence of the correlations on the observed relative intensities may sometimes have to be taken into account. If proton is the outcoming particle, the use of a suitable absorber usually removes all other heavy charged particles. This has previously been demonstrated at least for the (d, p) and (t, p) reactions [7,8]. If no absorbers are employed, then the channel associated with a single alpha-particle emission is easy to identify, due to the Q-value which usually is the highest. More generally, it is obvious that the use of absorbers can be helpful in detecting particles lighter than the projectile. In experiments of this type, a cylindrical plastic scintillator can be of utmost value. Because of the complexity of the spectrum due to the multitude of lines associated with each trnasition (K. L, M .... lines) and # activities often present in the target, coincidence experiments may be particularly important for conversion-electron studies. The example shown in the present work clearly demonstrates this. The inherently very poor energy resolution of the particle spectra is a characteristic feature of the singledetector particle-identification method. Obviously, no information is obtained about the cross sections for direct population of the different levels. In the studies of many transfer reactions, this is undesirable, but the information is often available from other investigations.

J. Kantele et aL / Single-detector particle identification

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Fig. 9. Conversion-electron spectra from reactions induced in 1528m by 19 MeV protons, recorded with a combination spectrometer c o m p r i s i n g an i n t e r m e d i a t e - i m a g e type m a g n e t i c lens plus a Si(Li) detector. (a) Singles spectrum, m a i n l y due to the 152Sm(p, 2n) reaction. (b) to (d) Spectra in coincidence with particle gates i n d i c a t e d in fig. 8.

O n the other hand, it is also clear that the present coincidence technique allows a more detailed study of electromagnetic transitions between nuclear levels, and offers a n o t h e r possibility even for lifetime measurements.

This work has been supported in part by the National Research Council for Sciences. We wish to t h a n k Mr. V. N i e m i n e n for valuable assistance with the conversion-electron spectrometer.

412

J. Kantele et al. / Single-detector particle identification

References [1] For a review article, see J. Kantele, Dept. of Physics, Univ. of Jyv~iskyl~i, Research Report 18/1981 (unpublished); Nukleonika, to be published. [2] T.J. Gooding and H.G. Pugh, Nucl. Instr. and Meth. 7 (1960) 189. [3] J. Kantele, JYFL Annual Report 1981, p. 15 (unpublished); a nearly similar instrument is described by M. Luontama et al., Nucl. Instr. and Meth. 159 (1979) 339. [4] H. Aihara, H. FujiL T. Kamae, K. Nakamura and T.

Sumiyoshi, Nucl. Instr. and Meth. 184 (1981) 359. [5] W. Oelert, G. Lindstr~Sm and V. Riech, Nucl. Phys. A233 (1974) 237 and references therein. [6] N.B. Gove and A.H. Wapstra, Nucl. Data Tables 11 (1972) 127. [7] R. Julin, J. Kantele, M. Luontama, A Passoja, T, Poikolainen, A. B~.cklin and N.-G. Jonsson, Z. Physik A296 (1980) 315. [8] .I. Kantele, E. Hammaren, D.G. Burke and J.C. Waddington, Nucl. Instr. and Meth. 93 (1982) 495.