Nuclear Instruments and Methods in Physics Research 228 (1985) 349-353North-Holland, Amsterdam

A TIME-OF-FLIGHT TELESCOPE FOR CHARGED PARTICLES PRODUCED INNEUTRON-INDUCED REACTIONS

M. AHMAD, S.L . GRAHAM, S.M . GRIMES, R. LONGFELLOW *, H . SATYANARAYANA andG. RANDERS-PEHRSON **Department of Physics, Ohio University, Athens, OH 45701, USA

Received 10 April and in revised form 23 July 1984

A counter telescope has been developed for the study of neutron-induced charged-particle-producing reactions. This spectrometerdiffers from those conventionally used for such studies in that it has a flight path long enough (2 m) to allow a fairly accurate energydetermination. The use of a pulsed beam with such a spectrometer permits the particle energy to be determined in two ways : from thesignal produced by the E counter and from the time-of-flight . This redundancy allows a reduction in background relative to the typicaltelescope if we demand consistency in the two values for a pulse to be accepted . In addition, the velocity determination throughtime-of-flight makes a better energy resolution possible than would normally be possible for low-energy charged-particles detected in ascmtillator .

1. Introduction

Measurements of cross sections and spectra for neu-tron-induced charged-particle-producing reactions aredifficult because of the twin problems of low countingrates and high backgrounds. The target foil must bemade thin enough so that the charged particles canemerge and be counted; this, combined with the smallcross sections, causes the counting rate to be low. Othersources of charged particles, including structural ele-ments of the spectrometer, are thicker and subjected toa similar neutron flux . In general, they will producemany more charged particles and will cause large back-ground .

To deal with these problems, counter telescope spec-trometers have been developed. These have the im-portant advantage that backgrounds are reduced bydemanding a coincidence between two or more detec-tors for a pulse to be counted; the true counting rateremains low, however, and random coincidences causebackground .A recent review by Vonach [1] has summarized cur-

rent developments in (n, z) spectrometers . One newapproach has been to utilize magnetic focussing to build

* Ohio University Summer Research Intern . Present address :Wittenberg University, Physics Department, Springfield,OH 45501, USA.

** Present address : Radiological Research Accelerator Facil-ity, Columbia University, Nevis Laboratory, Irvington, NY10533, USA.

0168-9002/85/$03 .00 © Elsevier Science Publishers B.V .(North-Holland Physics Publishing Division)

349

a spectrometer whose telescope is 2-3 m from theneutron source and the (n, z) target . Neutrons andgamma rays are not focussed by the magnetic field sotheir contribution to background is reduced consider-ably . A magnetic quadrupole produces an image of the(n, z) target at the counter telescope, making the solidangle equivalent to that which would be subtended bythe same telescope at about 20 cm without focussingfields . The effective improvement in background is ap-proximately the square of the ratio of these distances, orabout two orders of magnitude. This type of spectrome-ter has yielded considerable data, but suffers from theproblem that only particles with a limited range ofenergies can be measured at one time . For a measure-ment of a continuum spectrum, between 5 and 10separate measurements may be required .

Higher counting rates have been obtained with twolarge solid angle spectrometers which permit the mea-surement of cross sections at many angles simulta-neously. The instrument at Geel [2] has five Si detectorsat fixed angles which are operated in coincidence withtwo wide-angle proportional counters . Each E countersubtends about 100 msr. The angular resolution of thespectrometer is poor, however, and the angles cannot bevaried . A model designed and built at Vienna [3] isarranged so that the neutron flux illuminates a targetfoil which extends halfway around the circumference ofthe reaction chamber. Charged particles which proceedtoward the center of the chamber produce a pulse inone of 16 wires in a proportional counter volume andalso a pulse in the E detector, a scintillator located in

35 0

the center of the chamber. The background can bemeasured simultaneously by recording pulses from the16 wires on the side of the chamber without a targetfoil . This spectrometer achieves counting rates compara-ble to the Geel telescope. It has somewhat better angu-lar resolution than the former device, but does require alarger target foil and has poorer charged particle energyresolution, although in many applications the neutronbeam energy width will be the dominant factor indetermining the total energy resolution .

Maxon et al . [4] have recently described a telescopewhich has the usual dE and E counters but separatesthem by a distance enough to allow a time-of-flightdetermination as an aid in particle identification andbackground reduction. This spectrometer uses a Nalscintillator as the E counter and a short flight path,making the determination of energy with time-of-flightrelatively inaccurate . Thus, the energy is deduced fromthe E pulse, which for a scintillator is more uncertainthan would be the case for a semiconductor detector .

In this paper we report on results obtained with atelescope of similar design but which achieves betterbackground rejection. The flight path has been in-creased from 50 cm to 2 m. Our telescope also utilizes ascintillator for the E detector but we have chosen plasticinstead of Nal. These two changes allow a reasonablyaccurate determination of the energy from the time-of-flight of the particle . Thus, particles which actuallytraverse the telescope and have appropriate E and AEpulses but were produced by neutrons at some pointother than the target foil can be rejected on the basis ofa time-of-flight pulse inconsistent with the known en-ergy . In addition, the longer flight path allows a largerseparation of the E and 4E counters . This reducesbackground caused by charged-particles produced byneutrons in the E counter which then enter the AEcounter. As a result of these changes we are able tooperate our spectrometer with a neutron flux which isnot tagged by counting the associated particle . For themeasurements reported here we used a neutron fluxwhich was about 50% larger than the 109 n s-tsr-1used with most of the spectrometers described in ref. [1] .This limit is set principally by the amount of beamwhich our gas cell foil will tolerate .

D

M. Ahmad et al. / Time-of-flight telescope

2. Experimental details

A diagram of the spectrometer is shown in fig . 1 .Charged particles produced in the target foil traversethe 2 m flight path and produce pulses in the propor-tional counter and scintillator . The proportional counteris a single unit physically, but is divided into twocounters electrically by separating the two halves of thecenter wire by a glass bead as shown in fig . 2. Thus,separate pulses are obtained from the two portions ofthe wire, and by demanding a coincidence, we canrequire that the particle lost energy on both sides of thecounter. The proportional counter is filled with a gasmixture of 95% Ar and 5% C02, which slowly flowsthrough the counter. Windows are foils of Formvarapproximately 100 ± 20 pg/cmz thick.

The E counter is a 2.5 mm thick Pilot-B scintillatorcoupled with a light pipe to an RCA 8854 photomulti-plier. This thickness is approximately equal to the rangeof 15 MeV protons. The scintillator has a diameter of17 .8 cm; this represents a solid angle of 6.2 msr at 2 m.A particle passing through the telescope will produce

three pulses : one in each of the dEcounters and one inthe E counter. In addition, it is characterized by atime-of-flight pulse derived by comparing the arrivaltime of the particle at the E counter with the time thatthe accelerator beam pulse reached the neutron-produc-ing target. Pulses are processed only if a triple coinci-dence occurs . As a first step in particle identification,the particle time-of-flight is compared with the 4Esignal . The time-of-flight is given by

T_ d_ dl

2m

-d

2E '

where T is the time-of-flight, d the distance of travel,and m and E the mass and energy of the particle. Forsmall energy losses in theAE counter, the 4E pulse willbe [5] .

AE __dEdx=47reaz z

NZmov 2

[In

lX 2jv

z-ln(1-ß z )-8 2

Jdx,

where z is the charge of the particle losing energy and v

Fig. 1 . The complete counter telescope showing the drift tube as well as the dE and E detectors. [T = target ladder, V = isolationvalve, (C i , C2) = two halves of proportional counter, D = drift tube, X = scintillator, and PM = photomultipher tube] .

movz=2m°

-;mvz ] =2 »t°E,

we can write this as

f-2OwSwco

aw4

zàE«2m E [1n4mIE-ln

(l-Rz

) -ß2J0

CC zzmln%4mo EE1 I m

TIME OF FLIGHT

M. Ahmad et al. / Time-of-flight telescope

Fig . 2 . Mechanical details of the proportional counter [A =anode wire, (Ni, N2 ) = high voltage connectors, (M i , M2) = gasfilling connections, (Fi , F2)=entrance and exit windows madeof thin formvar films, G = glass bead] .

is its velocity, mo is the mass of the electron, Z is thecharge on the atom of the stopping material, N is thenumber of such atoms per cubic centimeter, ß = u/c,and I is related to the ionization potential of the stop-ping material . Since

if ß << 1. Thus, both 4E and the time-of-flight are

F2

Fig . 3. Two-dimensional spectrum of AE versus TOF forz-identification using 6LiF as stopping target .

Hn.

Ot-S

11OHaHZ_Uco

3. Results and discussion

TIME OF FLIGHT

351

Fig. 4. Two-dimensional spectrum of E versus TOF for massidentification of z =1 group particles. Again thick 6LiF is usedas (n, z) target .

functions of E and m only in terms of the ratio Elm.All particles with a given z should then have dE andTOF signals which follow a universal curve as a func-tion of Elm, while particles of different z will followthe same curve multiplied by a factor of the square ofthe ratio of the two z values . This first step in particleidentification is accomplished by setting a window inthe two-dimensional AE-TOF spectrum for z = 1 and asecond window for z = 2. The pulses which satisfied oneof these conditions were then sorted on the basis of atwo-dimensional spectrum formed by plotting E againstTOF. For a given time-of-flight the energies of protons,deuterons and tritons will be in the ratio of 1 : 2 : 3 andthe energies of 3 He and ° He in the ratio of 3 : 4. Noneof the targets investigated to date have yielded 3 Heparticles at our bombarding energies, but the identifi-cation of mass for the z = 1 particles is quite effective.Fig. 3 shows a typical 4E-TOF two-dimensional spec-trum and fig . 4 a typical E-TOF spectrum . Note thatwe could not have separated alpha particles from theprotons, deuterons and tritons in one step by comparingE and TOF without first having selected the z ; thescintillator response for alpha particles is significantlylower than for the z = 1 particles, making the scintilla-tor pulse for alpha particles not very different fromprotons of the same velocity to the energy range ofinterest to us .

The spectrometer has been tested at neutron energiesbetween 8 and 11 MeV. Stopping targets of 6 LiF and

35 2

M. Ahmad et al. /

mmCc0

U

157

122

87

Table 1

Channel Number

Channel Number

Fig . 5 . a) Sample-in and sample-out spectra for 58Ni(n, xp) at9 .4 MeV and 120° . The energy calibration is 15 keV/channel .b) Background subtracted spectrum for the above energy andangle for Ss Ni(n, xp) .

Time-of-flight telescope

CH 2 were bombarded as was a SsNi target . We showtypical results in figs . 5 and 6. In each case the datawere accumulated at a neutron flux of about 1 .5 x 109 ns -1 sr -1 . Based on 3 /,A average beam current used inthis experiment, the total running time to accumulatesample-in and sample-out spectra shown in figs . 5a and6a is about 4 h.

Vonach [1] has presented a comparison between vari-ous spectrometers used for (n, z) reaction studies. Intable 1 we present a similar comparison including someof the spectrometers discussed by Vonach as well as thatof Maxon et al . [4] and our spectrometer . Variousparameters needed for calculating count rates are listed,as well as the counting rate predicted for a target withA = 90, of thickness 5 mg/cm2 and with a cross sectionof 30 nib assumed to be isotropic . The counting rate iscalculated for the actual maximum flux which has beenused with the spectrometer (Zt) and for the maximumof this value and 10 9 n s -1 sr- ' (Z2), as would beappropriate for a source strength limited to this value.

Inspection of the table indicates that a number ofspectrometers show counting rates which are substan-tially higher than those for the "typical" counter tele-scope.

For the quadrupole telescopes, the excellent back-ground suppression allows the use of a high neutronflux (presently available only at 14 MeV), which com-pensates to some degree for the need for measurementsat many gradient settings . At energies other than 14MeV, however, the lower neutron fluxes make use ofquadrupole spectrometers quite difficult.

Since the Vienna and Geel systems obtain an entireangular distribution at once, the counting rates for these

1) Counting rate for charged particles for a 5 mg/cmz target for A = 90 assuming a cross section of 30 nib and a neutron flux listedunder maximum source strength .

2) Same as Z1 except that the flux is taken to be the larger of 10 9 n s-1 sr -1 or the flux 1n Z1 .3) Since with this spectrometer the background is measured simultaneously with the sample-in spectrum, this value should be doubled

for comparison with other spectrometers.

System Source-targetdistance R

[cm]

Targetarea F

[cmz ]

Solidangle

[msr]

Maximumn-sourcestrength[n s-1 sr- 1

]

Angularresolution

[deg]

(Number ofmeasurementsto obtain totalspectrum)M

Z11)

[s- ']

ZZ)

[s-1Typical Counter telescope [6] 6,5 0 .5 70 10 8 -20 1 0 .0066 0 .0066Livermore Quadrupole 5-10 3 0.3 2.5X10 11 2 8 0 .039 0.00016

telescope [7] 2.5 6 0 .432 0 .00172Geel Telescope [2] 30 5 500 10 9 10-30 1 0 .22 0 .22Vienna Telescope [3] 50 100 24 10 9 13 1 0 .076 3 ) 0 .076 31

Ohio quadrupole telescope [8] 8 4 7 .5 1 .5 x 10 9 3 8 0 .0072 0 .0047Brown University telescope [4] 7 .4 2 .4 24 10 9 11 1 0 .0837 0 .0837Ohio TOF telescope 8 4 6.2 1 .5 x 10 9 2.5 1 0 .047 0 .031

U

Wcó 7 1

Channel Number

M. Ahmad et al / Time-of-flight telescope

Fig. 6. a) Sample-in and sample-out spectra for 58Ni(n,xa) at9.4 MeV and 120° . The energy calibration is 30 keV/channel.b) Background subtracted spectrum for the above energy andangle for SsN1(n, xa).

spectrometers are for all counters together . Spectrome-ters such as those of Maxon et al . and the present modelcould yield higher counting rates if an array of two tofour such telescopes were in use. Such an expansion of aquadrupole system is probably precluded by expenseand by the geometrical size of each spectrometer. Anarray of two or more time-of-flight spectrometers wouldproduce a counting rate roughly comparable to theVienna or Geel spectrometers.

Maxon et al . [41 have emphasized that the length oftime needed for a measurement of given statistical accu-

353

racy depends not only on the efficiency of the spectrom-eter but also on the ratio of sample- in to background

counts . For this reason, it is difficult to compare theeffectiveness of various spectrometers with just the val-ues of Z7 and ZZ listed in the table. More informationon the relative background is needed in order to de-termine which spectrometer is more appropriate for agiven situation. Also, the two spectrometers (Geel andVienna) with the largest counting rates have measuredonly alpha spectra to date ; it is not clear whether aseparate run would be required to obtain proton spec-tra.

4. Summary

A time-of-flight telescope has been developed fordetecting charged particles produced in neutron-in-duced reactions. In contrast to most counter telescope,the flight path is long enough to allow the energy of theparticle to be deduced from the time-of-flight as well asfrom the pulse height in the E detector . The E and 4Edetectors are separated by about one meter to reducebackground caused by charged particles produced in theE counter by neutrons. We have measured charged-par-ticle spectra consisting of protons, deuterons, tritonsand alpha particles with energies between 1 and 10 MeV

and have been able to achieve mass identification . Thepresent version of the telescope has a counting ratewhich is much larger than that obtained with quadru-pole spectrometers.

References

H. Vonach, Proc . 2nd Int. Symp . on Neutron inducedreactions, Smolenice (June 25-29 1979) (Slovak Academyof Sciences, Bratislava, 1980) p. 59.

[2] A. Paulsen, H. Liskien, F. Arnotte and R. Widera, Nucl .Sei. Eng. 78 (1981) 377.C. Derndorfer, R. Fischer, P. Hille, G. Stengl and H.Vonach, Nucl . Instr. and Meth . 187 (1981) 423.

[4] D.R. Maxon, D.C. Palmer and J.R . Bading, Nucl . Instr.and Meth . 142 (1977) 479.

[5] M. Livingston and H.A. Bethe, Rev. Mod. Phys. 9 (1937)245.

[6] M. Brendle, M. Morike, G. Staudt and G. Steidle, Nucl.Instr. and Meth . 81 (1970) 141.

[71 K.R. Alvar, H.H. Barschall, R.R. Borchers, S.M. Grimesand R.C . Haight, Nucl . Instr. and Meth . 148 (1978) 303.

[8] V. Kulkarni, J. Rapaport, R.W . Finlay, G. Randers-Pehr-son and R.D . Koshel, Nucl . Phys. A367 (1981) 157.

O

I0

I100

L'' .- . _J~ "li, l

200 300

Channel Number

`J

400

L.LIJ'-- - -

500

38[-(b)

29mmc 20tU

LJ -]_

N 11

c rZJ

óU 2

0

I I I I " _J0 100 200 300 400 500

35

s-N,(n,xa)E n = 9 .4 MeV

28B Lab = 120'

m Totalm 21- Background

L IrU

I 14