Orsay - October 2007IPNO-RDD-Jean Peyré 1

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Test Protocole for Crystals

with Gamma Sources

Jean PeyréOct 2007

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General requirements

Each prototype has to be tested in real condition, i.e.:• The crystal has to be close to the one described in

slide#31. One of the main challenge is to be able to collect correctly all emitted

light. So results have to be given with such crystal associated with a detector that does not exceed the shadow of the crystal

2. For example, crystal of exit face 10x10mm measured with photodiode of active surface 10x10mm but of greater package as well as Crystal of exit face 40x20mm neasured with PMT of diameter 60mm is not acceptable)

• The prototype study has to include the technology feasability of the choosen photodetector, i.e.:1. For systems that require cooling or temperature stability, the

demonstrator will have to include a “cold” technology similar to the one that will be used in final calorimeter

2. For systems that have a sensibility to magnet field, study and tests with Magnet Field must be conducted

The tests can be steered in 2 steps:1. Test of a single Crystal2. Test of an array of 18 Crystals or a sufficient number to study the

board effects (mechanics, cooling, magnetic field, multihit detection in several crystals, influence of thickness of matter between Crystals,…………)

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Size of Crystal

Size of crystal should be close to the one designed for Calorimeter, i.e. detection length of about 110mm , polar angle close to 2 degres, azimuthal angle of 5.625 degres. The crystal can be longer in order to create a light guide adapted to the photodetector (APD, Pin diode or PMt)

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Gamma source

The use of 137Cs and 60Co sources is of great interest.1. Peak at 662 keV of 137Cs source is a reference that you ofen

find in articles2. Peak at 1.17 MeV of 60Cs source is close to the peak of the goal

that has been assigned to those detectors: 5% resolution FWHM at 1 MeV

Measurements for 3 peaks at 662keV, 1.17MeV &

1.33MeV137Cs peak0,662 MeV

60Co peaks 1.17 MeV

1.33 MeV

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Crystal

Gamma source

The gamma sources must be collimated. A good compromise is to use a lead collimator (example 5cm thick) with a hole of 4 mm. The distance between the lead collimator and the crystal has to be smaller than 4 cm. In this way, the “stain” made by the source will be approximitly of 7-14 mm depending of the source material repartition in front of the hole. This detail is of great importance, because the homogeneïty along the crystal can be artificially better than reality if the source sprays a too great surface for a given experience.

Source

Photodetector

Lead collimator

Movement of collimator along the Crystal

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Front-End Electronics

1. Use of Charge PreAmplifier + shaper with peaking time of 3 μs

2. Feedback Capacitor Cf adapted to the gain of photodetector

3. Rf x Cf >> shaping time

4. Rf >> 1 MΩ to minimize noise

5. PreAmplifier Rise Time T=Cd/wc*Cf (be careful for LaBr3+APD)

6. Be aware that Front-End Electronics is never saturated and always adapted in gain to emitted light

Cd

Cf & Rf feedback capacitor and Resistor

Cd capacitor of Detector

wc Gain Bandwidth product

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Test of one Crystal

Following previous recommendations, the source will move along X and Z axis. For each point, resolution and quantity of collected light with be noted. A plot for both Resolution and Collected light will be drawn

Crystal

Photo detector

xz

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Test of an array of Crystals

The measurements will be made following previous recommendations. The source will interact first with a “central” crystal to be able to study the response of the whole array of crystals. This test will be valid only if a unique crystal meets the requirement of 5% resolution FWHM at 1 MeV Gammas.

Collimated source

Photo detectors

x