Volume 61, Number 1, 2011¸ 1Z4, pp. 54∼56
New Physics: Sae Mulli (The Korean Physical Society), DOI: 10.3938/NPSM.61.054
Proton Beam Test for a CTOF System with a Counter Equipped Fine-mesh PMTs
E. Milman · V. Kuznetsov · W. Kim∗
Kyungpook National University, Daegu 702-701
(Received 18 November 2010 : revised 3 December 2010 : accepted 17 January 2011)
The basic requirement for the central time-of-flight (CTOF) system for the CLAS12 upgrade in Hall B of TJNAF (JLab) is a TOF resolution of 50 ps. This requirement dictates a need to test CTOF prototype counters under the above-mentioned terms or to extrapolate obtained results to these conditions. In this study, we will present the test results for a CTOF counter equipped with magnetic-resistant fine-mesh photomultipliers tube (PMTs) for the proton beam of the MC50 Cyclotron at the Korea Institute of Radiological and Medical Sciences (KIRAMS).
PACS numbers: 29.40.Mc
Keywords: Sintillator, Time of Flight Detectors, Fine-Mesh Photomultipliers, Proton Beam
I. Introduction
Electron scattering is the fundamental tool to deter- mine the structure of atoms, nuclei, and hadrons. The Continuous Electron Beam Accelerator Facility (CE- BAF) at Thomas Jefferson National Accelerator Facil- ity (JLab) is an electron accelerator with beam energy of 6 GeV at Newport News, Virginia, USA. The elec- tron beam upgrade to 12 GeV will provide a more ad- vanced combination of high beam intensity (luminosity) and high energy. These performances will increase lon- gitudinal momentum that will allow more detailed study of Generalized Parton Distributions (GPD). GPDs de- scribe the full complexity of the nucleon’s structure and dynamics including the 3-dimensional description of an- gular momentum distribution of quarks in proton.
One of the JLab detectors is CEBAF Large Accep- tance Spectrometer (CLAS) at Hall B. CLAS is used to study photo- and electro-induced nuclear and hadronic reactions by providing efficient detection of neutral and charged particles over a good fraction of the full solid angle. At the higher energies, new requirements on par- ticle identification make improvements in electron/pion separation, and particle timing necessary.
The goal in the CLAS12 upgrade program is to archive a timing resolution σT OF ≤ 50 ps, which allow more than 200 ps separation of pions from kaons up to 0.64 GeV and pions from protons up to 1.25 GeV.
A new generation of experiments at JLab sets rigorous requirements for CTOF system:
- timing resolution of CTOF detector σT OF ≤ 50 ps;
- operation in magnetic field (MF) up to 1.5 Tesla (inhomogeneity ∆B/B ≤ 5 × 10−4) ;
- rate at a luminosity of 1035 cm−2 c−1 will be 1 MHz per counter.
This study examined the CTOF prototype counter.
The goal is to provide the estimate for the expected time-of-flight resolution for minimum-ionizing particles (MIPs). A new method to test scintillation coun- ters using a 35 MeV well-collimated proton beam was developed. The time-of-flight resolution was investi- gated as a function of light output in the CTOF pro- totype counter. The estimate of the TOF resolution for minimum-ionizing particles (MIPs) was obtained by us- ing cosmic muons for the calibration of the light output and the GEANT based simulations. The method was implemented at the MC50 Cyclotron of Korea Institute of Radiological and Medical Sciences (KIRAMS).
-54-
Proton Beam Test for a CTOF System with a Counter Equipped Fine-mesh PMTs – E. Milman 1px -55-
Fig. 1. Design with fine-mesh PMTs
II. CTOF at CLAS12 design of KNU Group
The KNU Nuclear Physics Group (NPG) suggests so- lution for the CTOF design based on magnetic tolerant fine-mesh photomultipliers (FM-PMs). FM-PMT can operate in magnetic field up to 1.5 Tesla. It could be placed much closer to a scintillator bars in the region, where magnetic field is 0.3 − 0.8 Tesla. The light guides would be approximately 0.6 − 0.8 m long only. The CTOF assembly would be simpler, more reliable and less expensive. In this research KNU NPG concentrates on
- time-of-Flight (TOF) resolution with fine-mesh phototubes;
- the design of the light guides;
- evolution of the design and improvements;
III. GEANT4 simulation
For measurements of prototype counter with fine mesh PMTs we used a premium plastic scintillator Bicron BC- 408 manufactured by Saint-Gobain Ceramics & Plastics.
Plastic scintillators do not respond linearly to the ion- ization density. Very dense ionization columns emit less light than expected on the basis of dE/dx for minimum ionizing particles. A widely used semi-empirical model by Birks posits that recombination and quenching effects between the excited molecules reduce the light yield.
These effects are more pronounced the greater density of the excited molecules. Birks formula is dLdx = 1+kdE/dx
bdE/dx,
L(MeV)
0 5 10 15 20 25 30 35 40
E(MeV)
0 10 20 30 40 50 60
0.14 0.11
Saint-gobain Colibration
Fig. 2. GEANT4 simulation of Birks law for different Birks’ constant
where L is the scintillator light production, x is a coordi- nate along the particle track inside a scintillator volume and kb is Birks’ constant, which must be determined for each scintillator by measurement.
Due to Birks’ effect, the beam protons which stop in- side a detector produce less light per unit of deposited en- ergy than relativistic minimum-ionizing particles. Simu- lation of Birks’ effect is shown on Fig. 2.
IV. Measurement at MC50 cyclotron
The KNU group has built a CTOF prototype counter made of one 32 30 660 mm BC-408 scintillator bar and two acrylic light guides. The light guides were glued to the bar with BC-600 optical cement. One light guide, 58 cm long, was inclined 7◦ and viewed by R7761-70 PM and the other was inclined at 40◦ and viewed by R5924- 70 PM. The bar and the light guides were wrapped round with reflective tape and with black light isolation. The beam was collimated by a collimator made of 3 stacked together 7 mm thick steel plates. The plates had suc- cessively reducing holes of 5, 3 and 1 mm diameter in the first, second and third plates respectively. The block diagram of the electronic scheme is shown in Fig 4. The PM timing signals were formed by Ortec 935 constant- fraction discriminators (CFD). These discriminators pro- vided the best timing and the ultra-low time walk (typi- cally better than ±25 ps over the dynamic range of pulse heights 100 : 1). The PM times, t1and t2and their pulse heights, e1 and e2 , were digitized using LeCroy 2228B
-56- ôDzDGÓüto<Ært “DhÓüto”, Volume 61, Number 1, 2011¸ 1Z4
Fig. 3. Photo of the CTOF prototype counters with fine- mesh photomultipliers.
Fig. 4. Block diagram of electronics.
TDCs (LSB ≈ 47 ps) and LeCroy 2229B ADCs. The co- incidence of both PM signals was requested to trigger the acquisition and to generate the QDC gates. One photo- multiplier generated the TDC START while the delayed signals of both PMs generated TDC STOPs. The com- plete list of date acquisition system equipment is shown below:
- CAMAC crate LeCroy 1434 A,
- Quad to digital converter (QDC) is LeCroy 2249, - Time to digital converter (TDC) is LeCroy 2228A, - Constant fraction discriminator (CFD) is ORTEC
935,
- NIM (BIN) ORTEC 4001A, - Quad coincidence LeCroy 622, - Quad coincidence CAEN N455, - 4-fold logic LeCroy 365AL,
- 8-channel fast discriminator CAEN N413A, - Gate & Delay generator ORTEC 416A.
The beam energy was set to 35 MeV and the beam cur- rent was set to 0.15 nA. At this current, the count rate in the counter was ∼ 2 × 104 − 105 Hz. The gains of both PMs were equalized by adjusting high voltages.
V. Preliminary results and discussion
We present a measurement with the CTOF proto- type counter equipped with Hamamatsu R5924-70 and R7761-70 fine mesh photomultipliers. The preliminary estimation of the TOF resolution is 39 ps. It proves that detector equipped by FM-PM can operate in high MF environment for precise timing measurements. And the count rate for CTOF prototype counter is more 1 MHz.
Our future plans are analysis of the obtained data and a test of the CTOF prototype counter in magnetic field.
REFERENCES
[1] The Hall B 12 GeV Upgrade Preconceptual Design Report, http://www.jlab.org/div dept/physics divi -sion/pCDR public/Hall B/.
[2] CLAS12 Technical Design Report, http://www.jlab.
org/Hall-B/.
[3] J. B. Birks, Proc. Phys. Soc. A64, 874, (1951).
[4] J. B. Birks, Theory and Practice of Scintillation Counting (Pergamon Press, Oxford, 1964).
[5] M.Bonesini et al., Nucl. Instrum. Methods A572, 465, (2007).
[6] Saint-Gobain Crystals Comp. Booklet “Scintillation Products”, p.11.
