II .MEASUREMENTSWITHLED I .INTRODUCTION E. Milman · H. Dho · J. Jang · W. Kim · A. Ni LEDTestofaCTOFCounterwithFine-meshPMsandOrdinaryPMs

Volume 62, Number 3, 2012¸   3 Z 4, pp. 240∼242

New Physics: Sae Mulli (The Korean Physical Society), DOI: 10.3938/NPSM.62.240

LED Test of a CTOF Counter with Fine-mesh PMs and Ordinary PMs

E. Milman · H. Dho · J. Jang · W. Kim ^∗ · A. Ni

Kyungpook National University, Daegu 702-701

(Received 12 December 2011 : revised 27 December 2011 : accepted 2 March 2012)

We have made a CTOF prototype counter equipped with fine-mesh PMs for the CLAS12 upgrade program. The requirements for the CTOF counter are to achieve a timing resolution of less than 50 ps and a count rate of 1 MHz in a high magnetic field of 1.5 T. The timing resolution of the counter has been measured with a CAEN SP5601 LED pulser. The timing properties of the counter were compared with those of conventional ultra-fast Hamamatsu R2083 PMs. The results demonstrate that the CTOF design with fine-mesh PMs meets the basic requirements for the CLAS12 detector.

PACS numbers: 29.40.Mc

Keywords: Scintillator, Time of Flight Detectors, Fine-Mesh Photomultipliers

I. INTRODUCTION

A Time-of-Flight (TOF) detector is placed in the cen- tral part of CEBAF Large Accep- tance Spectrometer (CLAS12) inside superconducting solenoid. The Central Time- of-Flight (CTOF) system of CLAS12 at Thomas Jefferson Accelerator Facility (JLab) is composed of one layer barrel of plastic scintillators read out by Photo- Multiplier (PMs) through light guides which deliver scin- tillation light to the regions outside the central solenoid where the magnetic field drops down.

A new generation of experiments at JLab sets rigorous requirements for CTOF system:

- timing resolution of CTOF detector σ _{T OF} ≤ 50 ps;

- operation in magnetic field (MF) up to 1.5 Tesla (inhomogeneity ∆B/B ≤ 5 × 10 ⁻⁴ ) ;

- rate at a luminosity of 10 ³⁵ cm ⁻² c ⁻¹ will be 1M hz per counter.

The Nuclear Physics Group of Kyungpook National University (NPG of KNU), suggests a solution based on magnetic-resistant fine-mesh photomultipliers (FM- PMs). FM-PM can operate in magnetic field up to 1.5

∗

E-mail: [email protected]

T. Thanks to light guides PMs could be placed at posi- tion, where magnetic field is 0.7–1 T. The light guides would be short and straight, 58 and 75 cm long only.

Our previous tests on the plastic scintillation counter and FM-PMTs with and without acrylic light guides es- timated a good capabilities of our design for CTOF de- tector requirements [1]. We have used method for eval- uation of the timing resolution of a scintillation counter with FM-PM(Hamamatsu R7761-70) and conventional PMT R2083 by proton beams irradiation method [2,3].

Also we have been tested the first prototype counterwith 1.5 and 2 inches of diameter fine-mesh PMs (R7761-70 and R5924-70) [4].

In this study we present results of measurements with realistic prototype counter, which employed LED illumi- nation methods to study the operation of scintillation counters.

II. MEASUREMENTS WITH LED

The purpose of LED pulser system is to provide means

for testing, commissioning, calibrating, and monitoring

the gains and timing of the counters. The CAEN SP5601

is a fast LED pulser and represents the ideal light source

for photomultipliers tests and characterization. The

SP5601 features tunable intensity, repetition rate and

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LED Test of a CTOF Counter with Fine-mesh PMs and Ordinary PMs – E. Milman 1 p x -241-

Fig. 1. (Color online) Design with fine-mesh PMTs

Fig. 2. (Color online) Setup for measurements with LED pulser

pulse duration. The key feature is pulse duration, typi- cal 5 ns, and well below the typical time development of the PM signals. The optical signal is routed to the cen- ter of counter through a fibre [5]. In the present work we used available violet LED with peak wavelength is 400 nm.

The advantage of the pulser is a possibility to adjust easily the light pulse intensity of LED by varying a power supply voltage. The pulser has been triggered by a pos- itive pulse of from a fast pulse generator. The optical fibre has been used to illuminate counter at the center of the scintillation bar with light spot of diameter ∼ 0.5 mm.

The prototype counter of BC-408 plastic scintillation bar of 3.0 × 3.2 × 66 cm ³ with attached photomultipliers has been tested to study gain and timing performances of PMTs. Photomultipliers of different types (R2083, R7761, and R5924) were attached to both sides of the counter. Such technique will make it possible to mea- sure the dependence of timing properties and gain on frequency and intensity of LED pulser. Block diagram of experimental setup is given in Fig. 3. Measurements were repeated 6 times with increasing of delays on the

Fig. 3. (Color online) Block diagram of electronics.

COMMON START of the counter (from 0 to 10 ns with step of 2 ns). Generally the frequency of LED pulser was set at 10 kHz with 5 ns width of the pulse.

III. RESULTS AND DISCUSSION

We can conclude that FM-PMT can operate with good timing resolution comparable to resolution of ultra-fast R2083 PMTs.

We can assume, that prototype scintillation counter of 3 × 3.2 × 66 cm ³ with light guides equipped with fine mesh PMTs gives almost same timing resolution than that with ultra fast R2083 PMs as shown in Fig. 4

Timing resolutions of the prototype counter with light

guides, equipped with two R2083, two R5924-70 FM-

PMT, and with combination of R7761-70 and R5924-70,

respectively are:

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Fig. 4. Typical spectra of the timing resolutions of the prototype scintillation counter BC-408 with light guides, viewed by a) conventional PMTs R2083 on both ends (top), b) FM-PMTs R5924-70 on both ends (center), c) FM-PMTs R5924-70 and R7761-70 (bottom) .

σ T OF R2083 = 40.09 ± 0.10 ps (1) σ T OF R5924−70 = 39.59 ± 0.10 ps (2) σ T OF R7761−70 = 50.04 ± 0.12 ps (3)

This number should be considered taking into account geometrical difference between output window of up- stream light guide and 1.5 inches FM-PMT R7761-70.

The output window of light guide 40% larger than FM- PMT photocathode.

σ T OF R7761−70R5924−70 = 40.95 ± 0.10 ps (4)

And finally we present ratios of timing resolutions for the prototype counter, which are:

σ T OF R5924−70

σ T OF R2083

= 0.99 ± 0.10 (5) σ T OF R7761−70

σ T OF R2083

= 1.25 ± 0.12 (6) σ T OF R7761−70R5924−70

σ T OF R2083

= 1.02 ± 0.10 (7)

This ratios mean that the TOF resolution of FM- PMTs would be equal to that of ultra-fast R2083 PMTs.

These results prove good timing performance and suit- ability of FM-PMTs R5924-70 and R7761-70 FM-PMTs for CTOF at CLAS12.

REFERENCES

[1] V. Kuznetsov et al., “Test Measurements of Proto- type Counters for CLAS12 Central Time-of-Flight System using 45 MeV protons”, CLAS-note 2009-016.

[2] V. Kuznetsov et al., Nucl. Inst. Meth. A 621, 184 (2010).

[3] V. Kuznetsov et al., J. Korean Phys. Soc. 55, 448 (2009).

[4] A. Ni et al., J. Inst. 5, (11):P11001 (2010).

[5] “SP5601 Led Driver for SIPM development kit.”

CAEN, 2009.

[6] Saint-Gobain Crystals Comp. Booklet “Scintillation

Products”, p. 11.