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Study of the time resolution of a plastic scintillator assembly for the central neutron detector of CLAS12


The CLAS12 detector

The CLAS12 detector is the upgrade of the previous existing CLAS, at the Jefferson Laboratory, and it is designed to satisfy the basic requirement for the study of nucleons and nuclei after the Continuum Electron Beam Accelerator Facility (CEBAF) energy upgrade at 12 GeV. The primary interest of the experiments using the CLAS12 detector will be the study of the proton and neutron internal dynamics structure, which will be accomplished via the measurements of specific processes involving the exchange of a photon (i.e. the Deeply Virtual Compton Scattering, DVCS). With the upgrade at 12 GeV, more than 80% of the neutron produced in the basic process of neutron DVCS will have an angle bigger than 40deg, and thus in the region covered by the central detector. For this reason it is crucial to have a dedicated neutron detector in the central part of the CLAS12 apparatus. The strict constrains, defined by the limited space available and the very strong magnetic field present in the region, make the design of this detector quite challenging. Performed simulations show that to ensure good neutron identification in the momentum range between 0.2 and 1 GeV the detector should provide a time resolution better than 150 ps.

The central neutron detector (CND) design at IPNO

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Fig.1 The Central Neutron Detector (CND) for CLAS12

The IPNO has proposed a solution based on the use of three layers of scintillator barrels coupled to photomultipliers tube by means of long light guides, which allow placing the PMTs in a region with reduced magnetic field. Furthermore a “U turn” light guide is used to optically connect two extremities of neighbor barrels’ thus allowing the possibility to collect the light all at the same side of the scintillators. The final detector should then count of 144 PMTs, scintillator barrels and long light guides and 72 “U turn” light guide, arranged in three layers, as shown in fig.1.
Test on the CND prototype
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Fig.2 The scintillator barrels optically connected at one side by means of a semicircular "U turn" light guide

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Fig.3 The single layer of the CLAS12 central neutron detector prototype

The R&D Detection group at IPNO studied several solutions for the choice of the PMTs, the “U turn” light guides, the scintillators, and the reflector material for the CND design with the aim to optimize the detector time resolution performances. As a matter of fact the time resolution property of the apparatus is the key factor that leads to the identification of the neutrons from the gamma background. A single element of the detector, realized by means of two PMTS and two scintillators, optically connected at one edge by means of a semicircular light guide, fig.2, has been realized and tested at the IPNO with cosmic rays, fig3. The trigger of the system was defined by the coincidence of two small scintillators placed above and below one scintillator barrel of the couple under test. In this way when a cosmic ray passed through the scintillators, it produced a coincidence signal in the trigger and a scintillation event in the CND prototype. The position of the scintillation event produced in the long barrel was then defined by the position of the trigger. The light produced in the scintillation event propagated along the barrel, and then, in turn, on the light guides and finally was collected on the PMTs ; in particular PMT Dir was detecting the direct light produced in the barrel, while the other photomultiplier, PMT Ind, was detecting the indirect light, reflected from the “U turn” light guide. For each scintillation event we measured the light collected at the PMTs photocathode and the intervals of time needed for the light to reach them, in order to evaluate the time resolution. The trigger detectors were mounted on a support that allowed a rigid translation along the barrel, and so the time resolution and the charge collection of the detector were measured for five different positions of the trigger. The performed tests permitted to choose the PMT R10533, the scintillator EJ-200 and the aluminum foil as reflector material for the final design of the CND. With this choice for the detector we measured a time resolution between 114 and 155 ps for PMT Dir and between 205 and 280 ps for PMT Ind.
The “three-layer” prototype
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Fig.4 Charge and time spectra acquired with the "three layer" prototype

When all the different elements of the detector were chosen we built a “three-layer” prototype, which is actually one block over 24 of the final detector. It was made up with 6 EJ200 scintillator bars, 700 mm long, coupled to 6 PMTs R10533 by means of six 1500mm long light guides. The bars were optically coupled two-by-two by means of three semicircular light guides, and all was wrapped in Al foil and black tape for light tightness. The prototype was firstly tested layer by layer with the use of the external trigger, to compare with previous results, and then it was characterized in terms of time response and light collection using cosmic rays with the trigger defined by the triple coincidence, fig. 4. For the “three-layer” prototype we measured a Time-of-Flight resolution equal to 140ps.

Detector Construction

Fig.5 The CND mechanical structure

The construction phase of the Central Neutron Detector started on March 2014 at the Institut de Physique Nucléaire d’Orsay. It takes around one week of work to assemble each block ; particular care is needed to glue pairs of scintillator bars to the U-turn light guide and to wrap the block with Al foil and black tape respecting the clearance. After the assembling, one more week is needed to fully characterize the block using cosmic rays : around 2.5 million of triple coincidence events are collected in order to assure enough statistics. From the raw charge and time data, acquired for each assembled block, we can estimate the light collection, the effective light velocity in the scintillator, the light attenuation and the time resolution. Furthermore, at IPNO, we built a mechanical prototype to reproduce the magnet geometry in order to test the mounting of the structure and the positioning of the blocks. To this aim, once each block has been characterized, it is positioned in the structure, as shown in fig. 5, in order to test the stability and strength of the final CND assembly.

  • CLAS12 assembly
  • CLAS12 central detector

Contact : G. Hull, J. Bettane

Publications and reports :

Links : Clas12 at Jefferson Lab



Institut de Physique Nucléaire Orsay - 15 rue Georges CLEMENCEAU - 91406 ORSAY (FRANCE)
UMR 8608 - CNRS/IN2P3

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