A fast-neutron time-of-flight spectrometer

NUCLEAR

INSTRUMENTS

AND METHODS

27 (1964) 4 1 - 4 4 ;

©

NORTH-HOLLAND

PUBLISHING

CO.

A FAST-NEUTRON TIME-OF-FLIGHT SPECTROMETER G. C. BONAZZOLA and E. CHIAVASSA Istituto Nazionale di Fisica Nucleare, Sezione di Torino*

Received 9 October 1963 A fast neutron time-of-flight spectrometer is described. It utilizes the neutrons from the reaction 3H(d, n)4He and the method of the associated ~ particle. One of its main characteristics is a fast gate situated in the start

channel before the time-to-amplitude converter. The stability over a period of many days is of the order of 10 -10 sec, i.e. equal to its electronic resolution time.

1. Introduction During the last years an ever increasing interest has been taken in the study of inelastic scattering of fast neutrons. This has stimulated the outgrowth and the refinement of the techniques of the time intervals measurements in the nanosecond range. It is possible to recognize three fundamental methods: chronotron, "vernier" and time to amplitude conversion. 1 -11) The last method has proven to be more useful and more largely employed than the other two in physics experiments. In this paper we describe an apparatus which belongs to the latter type and in which we have tried to reach a good time resolution and a particularly high stability. This apparatus was chiefly designed to perform inelastic scattering experiments using fast neutrons obtained from the 3H(d,n)*He reaction. To define the measured time intervals, we use the pulses obtained from a counter detecting the neutrons and from that detecting the associated ~ particles. There are some difficulties arising from the great number of~ particles; to overcome these, we use the pulses from the neutron detector to start the time to amplitude conversion.

gate interposed between the time to pulse converter and the pulses height analyzer. In this way all pulses above the start threshold trigger the time to pulse height conversion. In order to be able to work with a great number of pulses in the start channel (e.g. when the neutron counter is situated directly in the neutron beam) we preferred to make the pulse selection before the start of the converter (see fig. 1). To do this, it is necessary to build a fast gate apt to transmit the selected pulses without impairing their time information. In the first work performed with this apparatus 12) the gate was opened by an output pulse from a fast discriminator, so that the time to pulse height converter is triggered only by " g o o d ' pulses. The positive output of the discriminator was lengthened at the control grid of the pentode V 1 and amplified (see fig. 2); V 2 is abruptly driven beyond cutoff. At the terminals of the delay line (RG63/U cable) there occurs a square pulse 100 nsec long. This pulse is amplified (V3) and transferred at low level impedance (V4, V5) to a summing circuit at the cathode o f V 6 which is a very high transconductance tube and constitutes the gate proper. At the E input, with a suitable delay, there arrives the anode pulse from the photomultiplier; V 6 is normally cut-offand this pulse is transferred amplified to

2. Spectrometer Circuitry

1) I. A. D. Lewis and F. H. Wells, MillimicrosecondPulse Techniques (Pergamon Press 1959). 2) R. Grismore and W. C. Parkinson, Rev. Sci. Inst. 28 (1957) 245. 3) j. W. Keuffel, Rev. Sci. Instr. 20 (1949) 197. 4) C. Cottini and E. Gatti, Nuovo Cim. 4 (1956) 1950. 5) R. L. Chase and W. A. Higinbotham, Rev. Sci. Instr. 24 (1957) 448. 6) A. A. Kurashov, A. F. Linev, B. V. Ribakov and V. A. Sidorov, J. Atomic Energy 5 (1958) 135. 7) W. Weber, C. W. Johnstone and L. Cranberg, Rev. Sci. Instr. 26 (1956) 166. 8) G. C. Neilson and D. B. James, Rev. Sci. Inst. 26 (1955) 1018. 9) j. B. Garg, Nucl. Instr. and Meth. 14 (1961) 131. 10) p. Huber, S. Lewandoski, R. Plattner, C. Poppelbaum and R. Wagner, Nucl. Instr. and Meth. 14 (1961) 131. 11) C. F. Cook, Nucl. Instr. and Meth. 15 (1962) 137. 12) G. C. Bonazzola, Nucl. Physics (to be published).

Our time to pulse height converter is a commercial modelt based on the work of Weber et al.7). The pulse to height converter is driven by a chain of fast amplifiers § and in these conditions the time resolution is partly dependent on the wideness of the incoming pulse spectrum. For this reason and for background considerations, it is necessary to introduce some pulse height selection at least in the start channel. This is a rather general situation and it is commonly resolved by discriminating the pulses taken linearly from the dynode and using the output of the discriminator to open a linear * This work has been done under the Euratom/CNEN contract. t Eldorado Electronics TH-300. § Hewlett-Paekard 460 AR and 460 BR. 41

42

G. C. B O N A Z Z O L A A N D E. CHIAVASSA

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the output only during the gate pulse. The performance of the gate has been tested comparing the electronic time resolution before and after its insertion; in both cases the time resolution has proven to be the same i.e. better than 10 -1° see.

tor 0.1 mm thick covered by a 1 mg/cm 2 aluminized Mylar film* to prevent light from attaining the scintillator. In the study of inelastic scattering the target is place d in O (see fig. 3) and the neutron detector (NE 211 liquid scintillator 2 inches of diameter and 3 inches longf) rotates around O, in the figure plane, at a distance of

3. Experimental Geometry and Shielding

112.5 cm.

The detection of the = particle defines a cone in which each neutron is identified by an electrical pulse and a surrounding zone in which there are also neutrons not accompanied by an ¢ pulse. In fig. 3 is depicted the experimental situation. The ~ detector is a plastic scintilla-

From the geometrical disposition it appears that we have a total shadow for 5040 ' < ¢p < 7°25 ', where ~0 is the angle between the neutron momentum and the cy* Supplied by the Alexander Vacuum Corporation. t Supplied by Nuclear Enterprises. EI80F

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A FAST-NEUTRON TIME-OF-FLIGHT SPECTROMETER lindrical axis of the experimental apparatus. In fig. 4 the points are the experimental counts from the neutron detector (relative to a fixed number of neutrons emitted

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particles association. in the total solid angle) in function of the angle, while the continuous line is the calculated curve. A lead cylinder wrapped in a cadmium foil and immersed in a paraffin tank shields the neutron detector.

43

Moreover, an iron rod 80 cm long and 10 cm in diameter shields the detector from direct neutrons. With this setup, the background was, for instance, 0.5 counts/ min per 10- 9 see of the examined spectrum at a total flux of 5 × 107 neutrons per second and with a bias on the energy of 3.5 MeV.

4. Performance 4.1. RESOLVINGPOWER The time resolution of our apparatus, measured with the coincidence of the ? rays f r o m a Co 6° source, is 1.2 nsec, that is, in good agreement with a theoretical calculation based on a paper by Gatti and Svelto t3) assuming for the parameters the following values, = 0.9 x 10-9sec, z = 2.4 x 10-9sec,2 = 0.9 x 10-9sec and for a bias of 100 keV in the start channel and 150 keV in the stop channel. The resolving power for 14.5 MeV neutrons varies from 3.8 to 2nsec for a bias variation from 3.5 to 10MeV, the transit time in the scintillator neutron being 1.4 nsec. Deviation from exact linearity is contained within 1 70 over 200nsec. 4.2. EFFICIENCY The spectrometer efficiency is defined 14) as the product of the detector efficiency by the solid angle that it subtends. We have experimentally measured the efficiency of the neutron counter detecting the neutrons scattered by a cylinder of paraffin I cm in diameter and 3 cm long. In this way we have a source of neutrons of energy and intensity varying in function of the angle o f scattering. Using a N E 211 liquid scintillator, we obtain the curve of fig. 5. This curve is in agreement with the values of efficiency given by

where B is the bias energy in MeV, a(E) the cross section of the (n, p) reaction in hydrogen, S the scintillator area, N i s the total number of hydrogen atoms in the scintillator, and in which it is taken for a(E) the empirical value 1+)

a(E) = 4.83/,v/Emax(MeV ) - 0.578 barns The solid angle subtended by the neutron detectors is equal to 6.3 × 10 -3 steradians and consequently the total efficiency for neutrons of about 7 MeV turns out to be 9.75 × 10 -4 per steradian for a 3.5MeV bias in energy.

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Fig. 4. Experimental check of neutron zone definition.

13) E. Gatti and V. Svelto, Nucl. Instr. and Meth. 4 (1959) 189. 14) B. V. Rybakov and V. A. Sidorov Fast Neutron Spectroscopy, (Consultants Bureau Inc., New York).

44

G. C. B O N A Z Z O L A A N D E. CHIAVASSA

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Fig. 5. Experimental scintillator efficiency. STABILITY Preliminary tests of the stability of our apparatus have shown drifts of 1 nsec over 24 hours. This instability is one order of magnitude larger then the electronic time resolution and it is perhaps due to temperature fluctuations in our laboratory. Since in our case the inelastic scattering measurements take about 50 hours for each angle, we have introduced a control system to check continuously the spectrometer stability during the run. This control must fulfill the following requirements: 4.2.

a) inject the control pulses at the beginning of the electronic system without impairing the time resolution; b) give the possibility of distinguishing between drifts due to variation in gain and those due to variations of the thresholds. For point (a) we have coupled pulses from a mercury switch generator through a capacity of 1 pF at the anode of both photomultiplier (see fig. 1). For point (b) we have the possibility of splitting our analyzer in two 256 channels analyzers and of analyzing in the second 256 channels the pulses arriving in coincidence with a suitable auxiliary pulse. Thus we have been able to obtain, in the second 256 channels, two electronic peaks 10-lo sec wide; they define a standard time interval. Drifts due to gain instability will change the number of channels in the standard time interval, while drifts due to some threshold variation will merely shift the standard time interval. In this way, acting on the window amplifier (see fig. 1) we may correct for each kind of error and maintain the drifts of the apparatus within its resolving power for an indefinite time. The authors are greatly indebted to Prof. P. Brovetto for his helpful discussion and suggestions. They wish to thank also Messrs. V. Tricomi and G. Venturello for their invaluable cooperation.