Simultaneous neutron-neutron proton-neutron and proton-proton interferometry measurements

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<ul><li><p>Nuclear Instruments and Methods m Physics Research A 335 (1993) 156-164North-Holland</p><p>Simultaneous neutron-neutron proton-neutronand proton-proton interferometry measurements</p><p>R. Ghetti a, L. Carln a, M. Crongvist b, B. Jakobsson a, F. Merchez ', B . NornD . Rebreyend d , M . Rydehell b , O. Skeppstedt ' and L. WesterbergDepartment of Cosmic and Subatomic Physics, Lund University, Lund, Sweden</p><p>b Department of Physics, Chalmers Institute of Technology, Gothenburg, Sweden` The Scedberg Laboratory, Uppsala, Sweden"Institute des Sciences Nucleatres, Grenoble, France</p><p>Received 11 May 1993</p><p>This paper describes a technique to perform simultaneous neutron-neutron, proton-neutron and proton-proton nuclearmterferometry measurements . Experimental arrangements for intermediate energy heavy ion interferometry experiments arepresented and their limitations are investigated . The construction of correlation functions, particularly with respect to normaliza-tion and background corrections is discussed. Some new results on correlation functions from the reaction 30 A MeV Ar+ 12Care shown and possibilities to improve the interferometry technique are discussed.</p><p>1. Introduction</p><p>When light particles are emitted in close proximityin space and time, their wave functions of relativemotion are modified by final state interactions andquantum statistical symmetries. By measuring two-par-ticle correlation functions at small relative momenta itshould be possible to obtain information about thespace-time characteristics of the emitting source [1-9].</p><p>Several effects in nuclear reactions lead to correla-tions between emitted particles, but many of them, likemomentum conservation and quasi-elastic scatteringcreate mainly large angle correlations [10,11] . Threeeffects are important for small angle nucleon-nucleoncorrelations [1] : the short range attractive nuclear in-teraction, which creates a positive correlation, the longrange Coulomb interaction and the Pauli exclusionprinciple for identical particles with the opposite ef-fect . At high energies, when the emission is fast, thecorrelation is dominated by the final state interactions(nuclear and Coulomb) . In the proton-proton (pp)case a positive correlation peak appears at relativemomentum q = I p t _P2112 = 20 MeV/c due to theattractive s-wave nuclear interaction disturbed by thelong range repulsive Coulomb interaction which cre-ates an anticorrelation for q = 0 . For the proton-neu-tron (pn) system Coulomb effects are absent and onemay expect a pronounced correlation at q = 0 MeV/cdue to nuclear attraction . The mean field Coulombinteraction with the proton may however obscure this</p><p>0168-9002/93/$06 .00 1993 - Elsevier Science Publishers B.V . All rights reserved</p><p>NUCLEARINSTRUMENTS&amp; METHODSIN PHYSICSRESEARCH</p><p>Section A</p><p>a</p><p>picture and deplete the nuclear interaction peak [12] .Neutron-neutron (nn) correlations should insteadprobe the pure nuclear final state interaction .</p><p>At lower energies where true compound nuclei arecreated, the time difference between the emission oftwo nucleons becomes large compared to the scatteringlength ; the effects of the final state interactions be-come negligible and the pure quantum statistical inter-ference can be observed [9,13] .</p><p>At intermediate energies it is difficult to asses therelative importance of spatial and time dependenceand there are therefore strong motivations to measuresimultaneously pp, pn and nn correlations . This paperdescribes a technique to perform such measurements .</p><p>2. Experimental interferometry technique</p><p>The interferometry technique requires measure-ments of the relative momentum (q) between twoparticles (fig. 1) . The relative momentum cannot bemeasured directly but must instead be determined fromsimultaneous measurements (correlations) of p t andp2 . In order to obtain high precision in these measure-ments the detectors must provide good particle identi-fication, good energy- and angular resolution.</p><p>The threshold for the relative momentum is deter-mined by the energy threshold of the detected particlesand by the smallest angle between neighbouring detec-tors . It is important to push this threshold to the lowest</p></li><li><p>R. Ghettc et al. / Simultaneous nn, pn and pp measurements</p><p>Fig . 1 . (a) Schematic illustration of two-nucleons correlationin which a nucleon (rl,p1) is detected by detector 1 simulta-neously with the detection of a nucleon (r2, p2) by detector 2.(b) Relative angle (B) and relative momentum (q) between</p><p>two particles .</p><p>possible value since the final state interaction signatureof the correlation function appears at very small valuesof q, particularly for nn and pn interactions.A wide dynamical range and solid angle coverage</p><p>are also desirable characteristics of the detector systemsince large energies (giving high q values) are impor-tant for the normalization of the measured correlationfunction and large solid angle for obtaining good statis-tics . Energy and angular determination should be asprecise as possible in order to minimize the errors in q .</p><p>Fig. 2 shows the setup used in our simultaneousmeasurement of pp, pn and nn correlation functions . A</p><p>compact Csl array (EMRIC) [14,15] was used for pro-ton detection (section 2.1), a spacious array of largearea liquid scintillators for neutron detection (section2.2) and a combination of the two for pn interferome-try . The CA array was placed 60 cm from the targetcentered at a laboratory angle of 45. Five hexagonalneutron scintillators were placed 3.5 m from the targetbehind the holes created by removing five Csl crystalsin the proton array, to allow for pn correlation mea-surement . Three of the hexagons were positioned inthe horizontal plane and two above and below thecentral detector at azimuthal angles of 8. The dis-tance between neighbouring centers was 50 cm . Sixcylindrical neutron detectors were placed at other an-gles in the horizontal plane with a distance of 73 cmbetween their centers. This setup allowed relative mo-mentum (q) thresholds of 4 MeV/c for pp and pncorrelations and 5 MeV/c for nn correlations.</p><p>The experiment was performed with a 30 A MeV40Ar beam, from the SARA coupled cyclotrons, bom-barding 3 mg/cm2 thick "Au, 12C and CH2 targets.The beam frequency was 12 MHz and one burst out oftwo was suppressed . This gave a time interval of 166 nsbetween the bursts which allowed registration of neu-</p><p>Cslt-is Hi-5</p><p>157</p><p>Fig . 2. The experimental setup . The upper part is a top view. CS'1-16 is the Csl array. H 1 5 are the 5 hexagonal liquid scintillatorswith plastic veto detectors (S) in front. They are positioned behind holes created in the Csl array (as shown in the front view in the</p><p>lower part of fig . 2) . C1_6 are 6 cylindrical liquid scintillators with a lead absorber in front (Pb) .</p></li><li><p>15 8</p><p>trons with threshold energy of 3 MeV. The intensitywas stabilized around 10 nA and the beam energyresolution (DE/E) was of the order of 2-3 x 10-3 .</p><p>2.1 . Proton detection</p><p>EMRIC is an array of 25 CsI(TI) scintillators thatcan be used in conjunction with a multiwire propor-tional chamber (MWPC) for a precise position deter-mination . The 25 independent modules fit in a spheri-cal arrangement at 60 em from the target . The activesurface is 4 x 4 cmz and the thickness 10 cm, whichallows detection up to 200 MeV protons. The CsIcrystals are coupled to XP2012 phototubes .</p><p>Particles with charge from I to 3, are easily identi-fied with pulse shape analysis [16,17] . Fig. 3a is anexample of the charge and mass resolution that isobtained when plotting the correlation between theslow and fast components of the pulse integrated dur-ing time gates of 400 ns and 4 ws separated by 1 .7 p,s(section 2.3 fig . 5b).</p><p>The CsI array allowed us to measure relative anglesin the range 3.8 (angular separation between centersof adjacent modules) to 16.6, since no MWPC wasused in this particular experiment . Each crystal sub-tended a solid angle of ~z 4.4 msr corresponding to aresolution of the relative angle of OB fihm = l .l .</p><p>For a more precise position determination the useof two MWPC planes covering the total CsI area isneeded. The spacing between the wires is 1 mm whichallows a precision of AO = 0.1 (at 60 cm from thetarget) . If a MWPC is used the minimum relative angleis then determined by the frame around the CsI detec-tors ; this is 2 mm wide resulting in a AO of 0.2 .</p><p>The energy threshold is low due to the fact that CsIcrystals are not hygroscopic and can be used in air with</p><p>R. Ghetto et al. / Simultaneous nn, pn and pp measurements</p><p>a thin shield of 10 Win carbon and 10 win Mylar.Outside a standard vacuum chamber (with a 50 winsteel window) the proton energy threshold was 8 MeV.</p><p>The relation between the light response of the Csl'sand the particle energy must be established for eachindividual crystal . Two different techniques can beused ; preceeding silicon detectors or time of flight(TOE) measurements . The first technique [18] makesuse of two totally depleted surface barrier detectorswhich are temporary located in front of the CsI'sduring a calibration run. The light particles are identi-fied in the DE-E correlation plot . The response of theAE detector is determined from particles having theminimum energy required to cross both detectors (fromthe semiempirical range-energy relation). Once the cal-ibration of the silicon detector is performed, it isstraightforward to determine the relation between thepulse height in the CsI detectors and the energy de-posited in the AE detector . This gives the CsI responsefunction .</p><p>In the TOF technique [14,15], a copper plate stopsthe beam well before the reaction chamber. Lightparticles (p, d, t, a, Li) from the reactions are emittedin a wide range of energies and their TOF is measuredwith two thin plastic scintillators separated by a flightpath of ~ 5 m. These particles are then detected in theCsI detectors and the correlation between the CsIsignal and the TOF provides particle identification andenergy determination (with a resolution of 1-2% in therange 25-100 A MeV) .</p><p>The energy resolution of the crystals, estimated byusing protons and a beams of well defined energy, is ofthe order of 2-3% for protons in the range 15-170MeV [14] . Gainshift problems in the pulse-shape dis-criminators caused by high counting rates, temperaturevariations, ac and do input interference signals etc. [19]may deteriorate the energy resolution to 5-10%.</p><p>Fig . 3 . (a) Charged particle identification from one Csl(TI) crystal obtained via pulse shape discrimination technique . (b) Neutronand y separation in one neutron detector</p></li><li><p>2.2. Neutron detection</p><p>Liquid scintillators are the most commonly usedneutron detectors in interferometry experiments . Inour experiment stainless steel containers (with wallthickness of 2 mm and coated inside with white reflec-tor paint) filled with Bicron BC-501 organic liquidscintillator were used . The thickness is 15 .6 cm and thediameter is 30 .5 cm for the cylindrical detectors and16 .7 cm (effective cylinder diameter) for the hexago-nally shaped modules (see fig. 2) . The glass window ofthe scintillators was optically coupled to a 12 .7 cmdiameter XP2041 PM tube .</p><p>Neutron-gamma separation (fig . 3b) was obtainedvia pulse shape analysis [19] for a wide range of ener-gies (3-200 MeV) .</p><p>Charged particles were rejected by using thin plasticveto-scintillator detectors or Pb absorbers placed infront of the neutron detectors .</p><p>The energy is determined from a measurement ofthe flight time of the neutron from the target to thedetector . The emission time of the neutron is taken tobe that of the accelerator RF start signal plus a con-stant offset determined from the known flight time of-y-rays hitting the detector . The total time resolution,determined from the width of the y peak, is 3 ns . Thisuncertainty is due partly to the time jitter between theRF signal and the actual interaction (i .e . the durationof the individual beam pulse) and partly to the timingresolution of the detectors .</p><p>Our neutron detectors allow energy thresholds aslow as 25 keV equivalent electron energy (ee) corre-sponding to = 0.3 MeV neutrons . In this experimentthe threshold was set at 1 MeV ee by using the Comp-ton edge of 'oCo, corresponding to a neutron energy ofabout 3 MeV [20] .</p><p>2.3 . Electronics and data acquisition</p><p>The triggering and digitizing electronics is shown infig . 4. For neutrons three data words were recorded foreach hit detector corresponding to time, full energyand tail of the energy signal . For protons slow and fastcomponents of the energy signal were recorded . Theanode signals from the PM tubes were split into threeparts using impedance-matched splitters. One of thesignals was fed into the constant fraction discrimina-tors (CFD) while the other two were fed into individu-ally-gated charge integrating analog to digital QDCs[21] . To ensure the proper timing for neutron detec-tors, the QDCs were gated by a signal generated fromthe overlap of the master trigger and the CFD signalfrom the detector . For neutron detectors, the gate forthe full signal started 20 ns before and the tail gate 50ns after the leading edge of the analog signal (fig . 5a).Both gates were open for 300 Its . For CsI detectors,</p><p>R. Ghetti et al. / Simultaneous nn, pn andpp measurements</p><p>11 units 16 units</p><p>159</p><p>Veto</p><p>Splitter</p><p>CFD</p><p>X11</p><p>MLU</p><p>X 1s</p><p>Master Triggei</p><p>ANDStrobe</p><p>FO</p><p>Strobe</p><p>stlp</p><p>PU</p><p>PtSteStirt p</p><p>TDCGte`J</p><p>GateQDC</p><p>QDC</p><p>BC-501</p><p>CFD Splitter</p><p>QDC</p><p>Inhibit</p><p>RF</p><p>CAMAC</p><p>Data Acq.</p><p>Fig . 4 . Schematic picture of the electronics setup where sealers,rate dividers etc . have been left out. The following abbrevia-tions have been used in fig . 4 : CFD = constant fraction dis-criminator . QDC = charge-to-digital converter. TDC= time-to-digital converter. MLU= multiplicity and logic unit . FO =logic fan out. GG = gate generator . AND = logic AND. PU =</p><p>patter unit .</p><p>slow and fast components of the pulse were integratedduring time gates of 400 ns and 4 Ws respectively,separated by 1.7 ws (fig . Sb).</p><p>The multiplicity outputs from the multiplicity unitwere added to give the total number of hit detectors .For coincidence data the system was triggered by amultiplicity of 2 or more . The overlap time was set toabout 150 ns to allow coincidences between particleswith different flight times. Singles data were also takenby triggering the system with multiplicity 1 signals,scaled down by a factor 100 with rate dividers (notindicated in fig . 4).</p><p>The time to digital converters (TDC) were startedby a signal that is not correlated with the time of thenuclear interaction. To allow the reconstruction of theproper time of flight, the time between the triggersignal and a signal from the cyclotron radio frequencyfield was measured for each event. The TDCs werecalibrated with cable delays, while the zero time wasdetermined from the position of the gamma peak inthe time of flight spectrum .</p></li><li><p>160</p><p>delayedprompt</p><p>-260 50</p><p>210 350</p><p>U~- i</p><p>fast slowR</p><p>0 0.4</p><p>1.7h</p><p>a)t( ns)</p><p>b)</p><p>Fig . 5 . (a) Charge integration of the neutron signal during two300 ns time gates....</p></li></ul>