IEEE Transactions on Nuclear Science, Vol. 33, No. 1, February 1986

A LARGE AREA, FINE GRAINED NEUTRON HODOSCOPE

Paola MACCIOTTA, Simonetta MARCELLO, Alberto MASONI*, Giovanna PUDDU and Sergio SERCI

Dipartimento di Scienze Fisiche, Universita di Cagliari, 09100 Cagliari, Italy

Istituto Nazionale di Fisica Nucleare, Gruppo collegato di Cagliari, 09100 Cagliari, Italy

and

Tullio BRESSANI, Giuseppe DELLACASA, Mauro GALLIO and Alfredo MUSSOIstituto di Fisica Superiore dell'Universita di Torino, 10125 Torino, Italy

Istituto Nazionale di Fisica Nucleare, Sezione di Torino, 10125 Torino, Italy

and

Mauro MORANDIN and Cesare VOCIDipartimento di Fisica, Universita di Padova, 35100 Padova, Italy

Istituto Nazionale di Fisica Nucleare, Sezione di Padova, 35100 Padova, Italy

and

Bruno MINETTIDipartimento di Fisica, Politecnico di Torino, 10129

Istituto Nazionale di Fisica Nucleare, Sezione di Torino,

Abstract

Results of measurements of the performances of a

time-of-flight neutron hodoscope consisting 3 of 50slabs of plastic scintillator, 100 X 2 X 5 cm each,are described. The efficiency and the spatiallocalization properties along a slab were studiedwith a variable energy (2.4 to 16.4 MeV) neutronbeam. The hodoscope was finally used in theexperiment PS 178 at the LEAR facility at CERN.

The use of large area scintillator hodoscopesfor neutrons, giving the time-of-flight (TOF) and theimpact position, is becoming more and more frequentin experiments on intermediate energy physics. Wewere faced with this problem in designing an

apparatus dedicated to the study of antineutron (in)physics at the LEAR facility at CERN. The source ofn's is the p+p - ii+n charge exchange reaction on a

Torino, Italy10125 Torino, Italy

liquid hydrogen target. It was mandatory to measure

the neutron energy (by TOF) and2 position (over a

surface from 0.5 to 1.0 m following theexperiments), in order to measure cross-sections andto obtain a tagged n beam (see e.g. [1],[2] andreferences quoted therein).

The usual technique adopted to achieve the bestperformances is that of using scintillator slabs ofsuitable thickness and of measuring the impactposition along each element by means of the timedifference given by two fast photomultipliers (PMs)coupled at the two sides. For large area detectorsthe system is very expensive, since many PMs of goodquality and associated electronic channels are

needed. In our hodoscope, since we had to detect onlya neutron at a time over the whole surface, we

coupled the scintillator elements to the PMs at the

two sides following a coding system that allows a

considerable reduction in the number of PMs. This

a)

Fig. 1 - Pictorial view of the neutron hodoscope. It

consists of two blocks that can be arrangedeither subsequently each other (a) or

adiacently (b).

* Partially supported by a grant of Credito Industriale Sardo (C.I.S.)

0018-9499/86/0200-0374$01.00O1986 IEEE

374

method was already used by us [3] for a chargedparticle hodoscope. Before starting the constructionof the apparatus, a careful study on a prototype wasdone [4] .

The hodoscope consists of two blocks of 21 slabsof NE 110 plastic scintillator, 100 X 2 X 5 cm each.The two blocks can be arranged either adiacently eachone2 (in order to obtain a total detecting area of1 m , see Fig. 1 b) or2subsequently (for a totaldetecting area of 0.5 m but with nearly doubledefficiency, see Fig. 1 a). The slabs of each blockare coupled to the PMs (Philips XP 2020) by lightguides arranged following a coding system thatconsiders each of the 25 scintillators as an elementof a square matrix of rank 5. Then only 10 PMs areneeded for each block, 20 for the whole hodoscopeinstead of 100 as with the usual method of coupling.

We studied carefully the performances of thehodoscope with a variable energy (2.4 - 16.4 MeV)neutron beam at the CN Van der Graaf accelerator ofthe Laborato5y Nazionali di 3Legnaro &Padova). Thed + d - n + He and the d+ H n + He reactionswere used as source of neutrons, at differentbombarding energies and emission angles. Theassociated particle technique was employed in orderto obtain a well collimated neutron beam, and Fig. 2shows the experimental layout. The thin productiontarget and a cojlimaXed solid state detector AE forthe associated He ( He) particles were installed ina vacuum chamber, having a thin stainless steelwindow at oie side. The neutron beam spot was scannedwith a 1 cm stilbene scintillator. At 2 m, where theneutron hodoscope was located, the measureddimensions of the spot were 9 cm (horizontal) X 4 cm(vertical). The absolute normalization of the neutronbeam, as measured by the A E detector, was + 3%.

The electronics associated to the hodoscope wassimilar to that described in Ref. [3] and the dataacquisition system was a Lecroy 3500 C/M. Aparticular care was devoted to the control of thethresholds on the PM pulses, whose fluctuationsaffect drastically the detector efficiency t , inparticular at low neutron energies. By measuring thepulse height spectra of radioactive sources placed atthe center of each strip the threshold was set at 1.0MeV equivalent proton energy, and it was checked

deuteronbeam

detector

neutron hodoscope

Fig. 2 Experimental layout for the measurements

with a neutron beam.

regularly and carefully during the runs. The neutronhodoscope, in the subsequential configuration (Fig.la), was mounted on a movable platform and then wecould study the performances over the whole surface.It was very important to obtain this informationsince we may expect a variation of 6 over thesurface, due to the attenuation of the light alongthe scintillator slabs. To simulate completely therunning conditions of the main experiment at LEAR ascintillator, 0.5 cm thick and covering the wholeuseful area (anticounter), was installed upstream thehodoscope.

Fig. 3 shows the pattern of the hits in thehodoscope when a 6.6 MeV neutron beam is impinging atthe center. The solid hystogram represents the

STRIPSFig. 3 - Distribution of the neutrons detected by

the strips of the first block (solid line)and of the second one (dashed line).

distribution of the neutrons detected by the slabs ofthe first block and is fully consistent with thevertical profile of the beam, as measured by thestilbene beam scanner. The dashed hystogramrepresents the distribution of the neutrons detectedby the second block. We observe a decrease in themaximum of the peak, due to the attenuation of theneutron beam in the 5 cm thickness of the firstblock, and a broadening due to neutrons detectedafter a scattering in the first block. Fig. 4 showsthe spatial distribution of the neutrons detectedalong the slab nr. 13 (corresponding to the maximumof Fig. 3). The position was inferred by means of thetime difference At measured by the two oppositegroups of PMs is coincidence with the timing signalgiven by the E detector. As constant ofproportionality X/At we used 8.5 cm/ns, directlymeasured by displacing the hodoscope relatively to

the n beam and in close agreement with our previousmeasurements on the prototype [4]. The maincontribution to the width of the peak shown by Fig. 4is due to the beam size (9 cm in the horizontalplane). We may then infer an upper limit of + 3 cm

fwhm on the incertitude in the determination of theimpact point. Unfortunately we could not measure

directly the time resolution due to the bad timingproperties of the electronics associated \to the A E

detector (-2 ns). From the incertitude on thelocalization we may infer an upper limit of 700 psfwhm on the intrinsic time resolution of the

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376

SPATIAL LOCALIZATION (cm)

Fig. 4 - Spatial distribution of thedetected along the slab nr. 13.

neutrons

hodoscope. Both values are in agreement with thosefound in the study of the prototype [4].

Fig. 5 finally shows the variation of s,

measured in the central position, as a function ofthe neutron energy En. The dots represent e for thewhole hodoscope, the stars that of the first block.The error bars include the statistical andnormalization uncertainties. Our measurements are inclose agreement with the values reported by otherauthors for similar counters (see e.g. [5] andreferences quoted therein). A complete Monte Carlo

E

30 _

20 _

10 F

2.4 44 6.6 En (MeV)

Fig. 5 - Detection efficiency e as a function ofneutron energy E . The dots represent s

the whole hodoscope, the stars that offirst block only.

simulation program for the evaluation of theefficiency is in progress.

After the described measurements the hodoscopewas transported at the LEAR machine at CERN, where itwas used very successfully in all the data-takingruns of the experiment PS 178 [6]. We believe thatthe coding technique could be extended to severalexperiments, for the detection of both charged andneutral particles.

We are very grateful to Prof. G. Moschini andto Dr. R. Cherubini for the installation of theneutron beam at the Laboratori Nazionali di Legnaroand for their help in the setting up of theexperiment.

References

1 T. Bressani, E. Chiavassa, S. Costa, G.Dellacasa, N. De Marco, M. Gallio, F. lazzi, B.Minetti, M. Morandin, A. Musso, G.a Puddu, C.Sciolla, S. Serci, E. Vercellin and C. Voci,"Perspectives of antineutron physics" inFundamental Interactions in Low-Energy Systems,eds P. Dalpiaz, G. Fiorentini and G. Torelli(Plenum Publishing Corporation, 1985), pp.

329-335.

2 T. Bressani, B. Minetti and C. Voci, "Future ofantineutron physics at LEAR" to be published onthe Proceedings of the Third LEAR Workshop on

Physics in the ACOL Era with Low-Energy CooledAntiprotons, Tignes (France), 19-26 January1985 (ed. J. M. Richard).

3 T. Bressani, M. Caria, S. Serci, F. Iazzi, B.Minetti, E. Chiavassa, S. Costa, G. Dellacasa,N. De Marco, M. Gallio and A. Musso,"Performance of an economic time-of-flightscintillator hodoscope", Nucl Instr. Meth.,vol. 221, pp. 355-362, April 1984.

4 M. Caria, P. Macciotta, S. Marcello, G.a Puddu,S. Serci and T. Bressani, "On the limits of theperformatices of time-of-flight scintillatorhodoscopes saving the number ofphotomultipliers"', IEEE Trans. on Nucl.Science, vol. NS-32, No. 1, February 1985, pp.609-611.

5 M.T. Tran, L.H. Guex, J.C. Alder, C. Joseph, B.Vaucher, E. Winkelmann, W. Bayer, H. Hilscher,H. Schmitt, C. Zupancic, T. Bressani, E.Chiavassa, J. Favier, D. Schinzel and P. Truol,"Differential Cross Sections for radiativecapture of pions on hydrogen in the (1232)region", Nucl. Phys., vol. A324, pp. 301-334,July-August 1979.

6 T. Bressani, M. Caria, E. Chiavassa, S. Costa,G. Dellacasa, U. Dosselli, M. Gallio, F. Iazzi,M.P. Macciotta, B. Minetti, M. Morandin, A.Musso, G.a Puddu, S. Serci and C. Voci, "Anexperimental program vith a tagged antineutronbeam at LEAR", int. doc. CERN/PSCC/84-27,PSCC/P74, May 1984.

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