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HPS - Aprime

Design and construction of a lead tungstate calorimeter

 From the DVCS experiment built in 2004, the whole mechanics and integration is changed for the HPS project in order to stack the 440 crystals in a horizontal way. Large crystals supports frames, cooling insulated black box stabilized at 0.1°C, and new electronics boards are designed in Orsay. We study also a particular flat vacuum chamber with thin walls in order to slide it in between the 2 halves of the calorimeter

An Electromagnetic Calorimeter for the HPS Experiment in the CLAS Detector

Principle of the experiment

The Heavy Photon Search HPS is studied at Jefferson Lab (JLAB -Newport News, VA, USA) using the CEBAF 6 GeV polarized electron beam and the CLAS detector . One of the aims of the experiment is the measurement of the beam spin asymmetry in the reaction ep->ep + ?. It requires the detection of the photons in a compact electromagnetic calorimeter with high energy and space resolutions. The responsibility of the R&D Detection group is the mechanical aspects of the design, fabrication of the support structure, thermal stabilisation and also the integration of the electronic and optical fiber equipment.
 
Calorimeter Definition : Physics and Technical Choice

Crystal assembly (CAD)


Simulated ? shower in the crystal

The calorimeter will be composed of 424 lead tungstate tapered crystals (13 × 16 × 160 mm). It is positioned behind CLAS detector. The tapered shape permits the crystals to focus on a particle downstream the beam. Different array configurations were discussed and physics simulations were performed by the R&D Detection group. GEANT 4 simulations helped in the choice of the support structure by showing the influence of the mechanics on the energy resolution. The readout is done by an Avalanche Photo Diode (Si APD S8664-55 from Hamamatsu), glued on the back of the crystal and connected to a preamplifier designed by the IPN Orsay electronics department (SEP). Each crystal is wrapped in VM2000 multilayer polymer. The calorimeter is composed of 2 rectangular parts. A flat vacuum chamber is specially designed to enable the beam to go in between.
 
Mechanical Studies : the Support of Fragile Crystals

A layer of wrapped scintillator crystals

For the crystal supports, where a global piling up has to be avoided, we use aluminium frames stacked one upon each other. Each frame is loaded with a row of crystals on two stainless steel foils (120 µm thick) stretched between the frames sides.
 
Thermal cooling stabilized at 0.1 °C

Thermal testing

In order to keep their performances, the PbWO4 crystals and the APDs must operate at a temperature stabilized with a precision of 0.1 °C. The thermal power to evacuate is 120 W. The calorimeter is equipped with copper thermal screens cooled with serpentines in which flows the water coolant at 17.6 °C. The insulation is made with a 12 mm thick styrene foam covered with an aluminized mylar foil. A chiller supplies the cooling circuit with a flow of 2 liters/min. The temperature measurements are performed with 20 Pt100 sensors recorded with a National Instruments data acquisition system. The thermal stabilization was measured to be around 0.05 °C for an external drop of 0.5 °C.
 
Design a flat vacuum chamber

FEA analysis of the vacuum chamber

Made of aluminium, a NIDA sheet is inserted inside to take the load pressure and avoid big deformations and excessive stress on the welds. FEA analysis was performed to design and guarantee the integrity of the structure under vacuum.
 
Assembly at the Jefferson Laboratory

Assembly on site

Delicate positioning of the crystals in their support frames

The assembly of the calorimeter in USA was performed with Russian and American teams and took about one month. The crystals were first installed on their support frames. During the crystals stacking, each row of preamplifiers was tested. The calorimeter was installed in front of a solenoid magnet and connected to the CLAS detector set-up. Its position has been controlled and fixed by the survey department with a laser tracker.
 
Experiment on the CEBAF beam
This detector ran successfully at the CEBAF beam in 2012. This experiment was a test run to show the feasibility of the future research experiment for the heavy photon search.
 

Contact : Bernard Génolini

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IPN

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

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