Identification and spectrometry of charged particles produced in reactions induced by 14 MeV neutrons (II)

NUCLEAR

INSTRUMENTS

AND

METHODS

128

(1975) 495-503;

~c3 N O R T H - H O L L A N D

PUBLISHING

CO.

I D E N T I F I C A T I O N AND S P E C T R O M E T R Y OF C H A R G E D P A R T I C L E S P R O D U C E D IN R E A C T I O N S I N D U C E D BY 14 MeV N E U T R O N S (II)* CH. S E L L E M , J.P. P E R R O U D and J . F . L O U D E

Institut de Physique Nucl(aire de I' Universit#, B.S.P., Dorigny, CH- 1015 Lausanne, Switzerland Received 15 May 1975 A counter telescope, consisting o f gas proportional counters, a thin semiconductor detector and a thick one has been built and used for the study of the angular differential cross sections o f (n, charged particles) reactions induced by 14 MeV neutrons. Detection o f the ~t-particles emitted in the neutron production reaction 3H(d,n)4He gives a time reference for the measurement of the time o f flight o f the charged particles and allows a precise monitoring of the intensity of the neutron beam. High energy protons, deuterons and tritons are identified by their energy losses

1. Introduction Measurements of the angular differential cross sections of I°B (n, charged particles) reactions induced by 14 MeV neutrons have been carried out by one of us1). The previously described spectrometer 2-4) had to be modified, on account of the difficulty in handling the large energy loss dynamics in the proportional counters, resulting from the kinematics of the studied reactions: deuterons with energy up to 9.5 MeV and protons, tritons and ~-particles with energies up to 14 MeV have to be identified simultaneously, while the ~t spectrum goes down to less than 3 MeV. With a proportional counter filling of 0.6 mg/cm 2 of argon, the ratio of the mean energy losses between 3 MeV ~t-particles and 14 MeV protons is about 36, as appears in table 1. This difficulty has been circumvented by inserting a thin semiconductor detector between the proportional counters and the thick semiconductor detector. This thin detector acts as an energy loss detector for high energy protons, deuterons and tritons, whereas it works as a residual energy detector for all or-particles and for other low energy particles. The spectrometric information of the proportional counter is used in the latter case only; the energy loss dynamics in this counter is therefore considerably reduced. An associated identification method has been developed. 2. Description of the spectrometer The experimental set-up is shown in fig. 1. A beam of 150 keV deuterons (diameter 2.5 mm) impinges on a T i - T target, producing 14 MeV neutrons and 3.6 MeV * Continuation of the work presented in ref. 2.

495

in the thin semiconductor detector and in the thick one and by their time of flight. Low energy protons, deuterons, tritons and all 0t-particles stop in the thin semiconductor detector and are identified by their energy losses in this detector and in one gas proportional counter as well as by their time of flight. It is possible to identify and to measure the energy of all charged particles in the energy range of 2 to 15 MeV; a very low background results from the use o f the time o f flight.

TABLE I Limit energies EL and energy losses DE in PPI for particles leaving the target with an energy E, at gas pressures o f 30 and 60 torr; as can be seen, EL does not vary much with the pressure. EL(MeV) 30 torr 60 torr

P

4.1

4.2

d t

5.5 6.4 16.6

5.6 6.5 16.9

E(MeV)

14 El, El,

Et, 3 14

DE (keV)

DE (keV)

30 torr

60 torr

13.6 35 46 54 450 154

27.3 71 94 108 1057 311

a-associated particles, whose detection by a thin scintillator coupled to a fast phototube (XP 1020) determines a coincident neutron beam with known characteristics 5) and provides a time reference for the measurement of the time of flight of the charged particles through the telescope. The thin semiconductor detector delivers a signal for each charged particle going through the telescope; the time of flight information is therefore taken from it. Considering the time resolution, a flight path of 190 mm has been selected. A basic limitation to the data acquisition rate of the spectrometer is the counting rate of the phototube, which cannot exceed 1-2 x 106 s- 1 2.1. THE TELESCOPE The telescope consists of six gas proportional counters, electrically coupled three by three in two sections PPl and PP2, a thin semiconductor detector

496

CH. SELLEM et al.

.-

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Fig. 1. Experimental set-up. Scintillator: NE 102A, 12.7/~m thick, 11.4 x 30 m m '~. 10B target: II .4 x 30 mm 2. The I°B target and the whole telescope are immersed in a mixture of argon and CO2; they can be rotated around the center o f the target.

JM and a thick one JE. Its anguiar position relative to the neutron beam axis is given by 0 L. After going through PP2 and PPI, the particles with a charge z~>2 will stop in detector JM. They are identified by their energy losses EjM in JM and DE in PPI as well as by their time of flight DT. The situation is the same for the slow singly charged particles which stop in J M. Faster singly charged particles go through J M and stop in JE. They are identified by their energy loss EjE in JE and EjM in JM as well as by their time of flight DT, D E being advantageously replaced by EjM. It is useful to define a limit energy E L depending on the type of particles, as the energy at which the range of a particle in JM is equal to the thickness of this detector. The proportional counter PP2 is only a trigger counter. The thicknesses of the detectors are chosen according to the following considerations: (1) maximum energy protons, whose energy loss in the gas is negligible, must stop in the total thickness of J M and JE; (2) the desire to identify most particles with EjE and EjM leads to the use of a detector JM as thin as is technically feasible, with a concomitantly small value of EL; (3) the energy loss in JM of highest energy protons must be well above the electronic threshold of the time of flight (see sect. 2.2). As a compromise, a thickness of 157/~m has been chosen for JM, in which 16 MeV ~-particles stop and 14 MeV protons lose about 1 MeV. JM is a 300 mm z totally depleted surface barrier detector; a diaphragm of 18 mm diameter in front of it defines the solid angle. JE is a 300mm 2 partially depleted surface barrier detector, 1000 pm thick, that can stop 12 MeV protons. The proportional counters have a total length of

180 mm; they are separated by tantalum windows and have an inner cladding of tantalum foil. The whole telescope is immersed in the working gas of the proportional counters; argon with 5% COz has been chosen on account of its low (n, charged particles) cross sections. The choice of the gas pressure is a compromise; on the one hand, it ought to be as low as possible, in order to minimize the number of the parasitic reactions in the gas around the target, which cannot be rejected by the time of flight, as well as to lower the energy threshold of the telescope, that is to say the energy of particles leaving the target, going through PP2 and PPI and losing in J Mjust enough energy to trigger the time of flight. On the other hand, the pressure must be high enough (1) to ensure the stability of the gain of the proportional counters, (2) so that highest energy protons lose in the trigger counter PP2 enough energy to produce a signal above the noise level and (3) so that identification be made possible by the energy resolution of PPI, which is proportional to D E - ~ (DE being the energy loss in PPI). Moreover, taking into account the kinematics of the studied reactions, one has to reduce the pressure when the particle energy decreases with increasing angle 0L. We have chosen to work with only two pressures: 60 torr (1.2 mg/cm 2 of gas between the target and the JM detector) at the forward angles (0 L< 80 °) and 30 torr at the backward angles (0L >/80°)The resulting limit energies and energy losses are given in table 1. When PP1 has to be used in the identification process, the range of mean energy losses in PPI is the ratio between the losses for 3 MeV ~-particles and protons at the limit energy; this ratio is now about 15. Although several similar targets have been used, all

IDENTIFICATION

OF C H A R G E D

diagrams and spectra are here given for a I°B target 550/~g/cm 2 thick, deposited under vacuum on a tantalum foil 0.1 mm thick. 2.2. ELECTRONICS

Each event is described by at least three parameters

DE, E~M and DT or EjM, E~E and DT. The analog signals DE, EjM and E~E are obtained by amplification and shaping of the corresponding detector signals, as shown in fig. 2. For the measurement of the time of flight a fast component of the thin junction detector pulse is picked up by the method proposed by Sherman et al.6). After amplification by a high input impedance fast amplifier, the observed rise time of the JM pulses is 4 ns and the noise level corresponds to 850 keV; after integration of the rise time to l0 ns, the upper level of the noise has been reduced to 500 keV, which is the value Eet of the electronic threshold of the time of flight. A leading edge discriminator has been used. A four-parameter analyser, consisting of 4 ADCs, a display and an interface with a paper tape puncher

PP2

laP1

497

PARTICLES

digitalizes, when required by an external gate signal, the four parameters DE, EjM, EjE and DT in 200, 400, 400 and 100 channels respectively and punches their values for an off-line analysis on a big computer. Only events whose parameter values are between predetermined limits are considered. Single channel analysers deliver the logical signals PPI, Jm, Je and Dt when these conditions are met. The only purpose of PP2 is the reduction of the accidental rate of coincidences, for which an integral discriminator is adequate. The recording of an event depends on the realization of the logical function G A r E = { [ ( P P 2 x Jm) X Je] + [-(PP2 x Jm) x PP~]} x Dt. Upper and lower thresholds are set so that the detection efficiency be 100% and the two terms of the function GATE overlap. 2.3. PERFORMANCEOF THE TELESCOPE At a given pressure, the detection threshold of a given type of particle is the sum of the electronic threshold Let of the time of flight and of the energy loss DEtot of these particles between the target and J M, as shown in fig. 3. For protons, deuterons and tritons, the measured energy resolution of PPI is equal to its theoretical value as given by the Bohr formula (30 keVrwhm at 60 torr); for ~'s, it is between 40 and 60keV. The energy resolution of JE is 25 keV; the JM resolution is only 80 keV because of the degradation due to the time pick-off; neither depends on the type or the energy of the particles. The energy resolution of the telescope is eventually limited by the target thickness and by the kinematical effects associated with its spatial extension. The range of the time of flight is about 30 ns. With '

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20

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40

60

Pressure(Torr)

80

100

Fig. 3. Energy threshold Eth o f the telescope vs pressure o f the A + 5% CO2 gas mixture. The time o f flight imposes an clectronic threshold Eet.

498

CH. SELLEM et al.

the rather slow rise time of the pulses at the input of the leading edge discriminator, the time walk cannot be neglected; it is minimum near the limit energy EL. The time resolution is approximately constant (about 1 nsfwhm) when EjM~>3 MeV and deteriorates at lower energy (2.5 ns at EjM = I MeV); it is slightly better than the resolution for ct's given by Perroud et al.:); this improvement can be ascribed to the use of a totally depleted detector. 3. Particle identification

3.1. GI-NERALMETHOD The measurement of DE, EjM and DT and (for higher energy particles) of EjE completely describes an event. Introducing the new parameter E a = EjE+ EjM (residual energy), each event is then represented by a point of the four-dimensional space (DE, EjM, Ea, DT). These four parameters are not independent; it is theoretically possible to find an identification function

°°.°.°.°.°.°.°.°.(E ~ . R , EjM, DE, DT), which takes a different determined value for each kind of particle, independent of its energy. When building ~-, one has to know a priori and precisely the relations between parameters, which is the case neither with the ER-DE relation nor with Ea-DT; in fact, especially for low energy ~t-particles, the stopping power tables are rather unreliable and it is not possible to calculate the ER-DE relation with a high enough precision; in the calculated ER-DT, it would be very difficult to include

experimental effects as the time walk of a leading edge trigger. The identification function method has therefore been rejected; another technique has been preferred, according to which it is possible to adjust the relations between parameters to the experimental data themselves, to determine the resolution on each parameter, to control step by step the identification process and the validity of the chosen criteria and finally to detect and correct possible calibration drifts of the proportional counters. The method adopted uses the graphical representation of biparametric diagrams, projections on the plane of the four-dimensional space or of its threedimensional projections (EjM, ER, DE) and (EjM, ER, DT). Neglecting the detector resolution, representative points of each kind of particle are along different characteristic lines. The situation in the subspace (EjM, Ea, DE) is shown in detail in fig. 4. All orparticles stop in JM; for them, Ea=EjM and their characteristic line is entirely in the so delined bissecting plane. The situation is the same for all other particles with an energy below the limit energy E L, to which corresponds, in the J M detector, a limit energy EjM ' L = EL-DEtot; the intersection of the bissecting plane with the (ER, EjM) plane is the EjM,L axis of the tigure. Particles with an energy above EL have their representative point above the bissecting plane, along a characteristic line in the three-dimensional subspace. Ea being always greater than EjM, no event has its representative point under the bissecting plane. Projections of the characteristic lines on the (EjM, ER) and

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IDENTIFICATION

OF CHARGED

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EIt-DE;all events measured at 0L = 2 0 , 4 0 ,

60 ' have been added• 10B target. The ct strip can be clearly seen on the upper right side of the diagram.

(ER, DE) planes have no intersection at the considered energies. These two biparametric diagrams ER-DE and ER--EjM (the latter equivalent to EjE-EjM) are essential for the identification process. The situation in the subspace (EjM, ER, DT) is very similar, although not quite so favourable. The projections of the characteristic lines on the (ER, DT) plane merge indeed slowly above EL, where fortunately the

time of flight is not essential for identification (although it still contributes very much to the b a c k g r o u n d reduction). The identification technique must take into account the energy resolution of the detectors and the time resolution of the time of flight• By these resolutions, the characteristic identification lines in the parameter subspaces broaden in tubes of elliptical cross-section,

500

CH. S E L L E M e t al.

whose projections are strips in the biparametric diagrams. The particular technique used for identification is dependent on the kind and on the energy of the particles (namely above or below EL). For ~-particles and for high energy protons, deuterons and tritons, a "visual'" technique has been used; a more sophisticated "'analytical" technique has been necessary for protons, deuterons and tritons with an energy lower than Et. Because of the detectors resolution, EL is not a sharp limit and both methods have been concurrently applied within a certain energy domain on both sides of E L.

3.2. "VISUAL" IDENTIFICATION This technique is appropriate when, initially, an identification strip does not overlap with others in at least one biparametric diagram (for instance ER-DE for ~-particles). It is then possible to draw generous limits of the strips with eye guidance only, to introduce these limits in the identification program and to request from the computer another biparametric diagram (for instance ER-DT) for events with their parameters between these limits. In the latter diagram, new limits can be chosen and the former diagram can be printed out again. A tube in the identitication space

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IDENTIFICATION

OF C H A R G E D

[in the example case (ER, DE, DT)] has so been selected; corresponding biparametric diagrams contain only interesting and background events inside the tube. New narrower limits are then chosen, taking into account the resolutions; the process can be repeated, till it is no longer possible to reduce the background without losing good events. The or-particles have been identified by means of the diagrams ERIDE (tig. 5) and ER-DT. High energy protons, deuterons and tritons have been identified by means of the diagrams EjM-Ej~ (fig. 6) and Ert-DT; the EjM resolution, above EL, is indeed good enough (fig. 7a). The proportional counter provides in this case no useful information. Under EL, proportional counter resolution (fig. 7b) precludes the use of this simple "visual" method, except for or-particles.

501

PARTICLES

with the probability P = 1-exp(-½)2). An event is identified with quasi-certitude if it belongs to one ellipse only; should it belong to more than one, it is ambiguous (practically, less than 1% of the measured events); if it does not belong to any ellipse, it is considered as a background event. Tables by Williamson et al. 7) have been used for the tabulation of DE; the result has been fitted to the experimental data by a least-squares program, expanding ~ in a polynomial of degree :(. For the fit, calibration' points obtained with a Z26Ra source in place of the target as well as prominent peaks in the diagram ER-DE have been used. A coefficient set A~ is so determined for each kind of particles, such that

3.3. " A N A L Y T I C A L " IDENFIFICATION

DE = A o+ ~ A,E~.

This method is a development of the method used and described by Perroud et al.2), for the separation of ct and 6He produced by neutrons hitting 9Be nuclei. The process begins with the determination, using the experimental data, of the mean values of the parameters DE and DT as functions of E R and of the kind ~ of particle:

DE = DE(E R, ~ ) ,

i-= I

¢o

DT = DT(ER, ~ ) ,

:2o.

and of the statistical fluctuations ._1

GOT = ao.r(ER, .:;/).

O'OE = O'DF(ER, Off),

A measured point with the parameters ER, DE, D T is to be found in the ellipse

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b)E R =7.0 MeV

Fig. 7. Encrgy resolutions of Jrv~ and PPI. (a) 93.8 keV/channel; (b) 8.7 keV/channel.

Fig. 8. Energy spectrum in detector JE of the identified deuterons from the reaction ]°B(n,d)gBe, at 0 L = 2 0 % From the total spectrum (solid lines) must be subtracted the background (dotted surface). The do peak( do'/do9 = 7.48 mb/sr) corresponds to the ground state of 9Be and the d2 one (da/dto = 4.6 mb/sr) to its second excited level at 2.43 MeV. Total duration of the measurement: 130 h.

502

CH. S E L L E M et al.

degree greater than 5. Finally, with the knowledge of Ai, Bi, Li, Mi for each kind of particles, the criterion (1) can be applied to the identification of particles.

In order to determine DT and aor, one has to select in the diagram ER-DE events which are certainly of type .# and to look at these events in the diagram

ER-DT. For the sake of convenience, the criterion (1) has been applied for this selection by choosing a small value for ).~ around the now known function DE and by taking coarse trial functions for DT and trOT, e.g. DT

3.4.

ENERGY SP['CTRA

After

identification, the biparametric diagrams EjE-EjM or EjM-DE are reduced to energy spectra by summation along the EjM (respectively DE) axis. Measurements with the ~°B target have been alternated at short intervals with background measurements, where a tantalum foil is used as a target. For each kind of measurement, the identification of the events has been carried out separately but according to the same criteria ; the true events spectrum is finally the difference (weighted according to the relative duration of both measurements) of the energy spectra with and without t oB target. Differential angular reaction cross sections are evaluated from the energy spectra, counting the number of events in the peaks which correspond to the excited levels of the residual nucleus. Examples of energy spectra are shown in figs. 8 and 9, one for deuterons and the other for protons. The latter spectrum, where the background is rather high, represents one of the most unfavourable cases; when the proton cross sections are low, as is the case here, one has nevertheless to choose the target thickness according to the required ~-particle energy resolution; as for the background, it does not depend on the target thickness.

= half of maximum time of flight,

;-%T ~- maximum time of flight. All events in this diagram ER-DT are of type .#. A least-squares program determines the mean lines and the statistical fluctuations around it: 2

/ ) T = L o + E L;E~, i=l

trot = M o + ~ MiE L. The last unknown is aoe; putting into the criterion (l) the known values of DT, %T and D--E, a small value for 2 and a large one for % e , the diagram ER-DE contains only .#-type events and aOE is obtained by a least-squares fit: /t

aor. = Bo + ~ BIER. i I

It has not been necessary to use polynomials of a

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EjE (150keY/channel) Fig. 9. Energy spectrum in detector Jg of the identified protons from the reaction t°B(n,p)l°Be, at 01, = 60L ]'he total spectrum and the background contribution are shown. The po peak (da/dt~ = 0.48 mb/sr) corresponds to the ground state of X°Be, the Pt peak (de/dto = 1.69 mb/sr) to its first excited level at 3.37 MeV; P2, 3, 4 ( d o / d o = 1.67 mb/sr) is the superposition of the contributions from the levels at 5.96 MeV, 6.18 and 6.26 MeV. Total duration of the measurement: 170 h.

IDENTIFICATION

OF C H A R G E D

With the help of the time of flight, all cross sections, except for protons, can be measured at 0L=0 °, the telescope being in the neutron beam. In the case of protons, the time of flight difference between, on the one hand, neutrons reacting in JE and producing a charged particle detected by JM and PP and, on the other hand, protons emitted by the target, is too low for the discrimination to be possible.

4. Conclusions With the combination of two semiconductor detectors and gas proportional counters, the simultaneous measurement of differential angular cross sections of reactions induced by 14 MeV neutrons and leading to the emission of protons, deuterons, tritons and ct's has been made possible; even at small forward angles, the background is not so high as to impede a safe subtraction. In typical conditions, it is possible to measure differential cross sections lower than 0.5 mb/ sr, in an energy range of 2 to 15 MeV, for all kinds of particles. The laboratory angle can be varied from 0 ° to 137 :. No more than 1% of the events are found to be ambiguous in the identification process. The detection efficiency is the same for all particles and is practically 100%; using a target containing hydrogen or deuterium in precisely known amounts, it is therefore possible to perform an overall control of the

PARTICLES

503

precision of all cross section evaluations; this determination must be in agreement with the value calculated from the known number of or-associated particles detected by the phototube. Systematic errors can consequently be reduced to 5% or less. As each point of the angular distribution requires at least a 120 h measurement and as about ten points are necessary, a large time-saving results from the simultaneity of the measurements for the light as well as for the heavy (i.e. ct) particles. We are grateful to Prof. Ch. Haenny and Prof. C. Joseph for their unfailing interest in this work; our thanks go also to the Swiss National Fund for Scientific Research for its financial support. References 1) Ch. Sellem, Thesis 196, l~cole Polytechnique Fdddrale de Lausanne (1974), unpublished. 2) J. P. Perroud, Ch. Sellem and J. F. Loude, Nucl. Instr. and Meth. 115 (1974) 357. a) j. F. Loude, J. P. Perroud and Ch. Sellem, Helv. Phys. Acta 44 {1971) 33. 4) j. p. Perroud and Ch. Sellem, Nucl. Phys. A227 (1974) 330. 5) j. F. Loude and J. P. Perroud. Nucl. Instr. and Meth. 88 (1970) 261. 6) 1. S. Sherman, R. G. Roddick and A. J. Metz, IEEE Trans. Nucl. Sci. NS-15 (1968) 500. 7) C. F. Williamson, J. P. Boujot and J. Piccard, Tables of range and stopping power, Rapport CEA-R3042 (Saclay 1966).