Research Report of Laboratory of Nuclear Seience, Tohoku University Vol. 40 (2007)

23(LNS Experiment : #2582)

Current Status of the Electro-Magnetic Calorimeter
SCISSORS III
T. Ishikawa, R. Hashimoto, J. Kasagi, S. Kuwasaki, K. Motiduki, K. Nawa,
Y. Okada, Y. Onodera, M. Sato, H. Shimizu, K. Suzuki, and H. Yamazaki
Laboratory of Nuclear Science, Tohoku University, Sendai, 982-0826

A new electro-magnetic calorimeter complex FOREST with a solid angle of about 4π in total is underconstruction. A forward calorimeter SCISSORS III, a part of FOREST, was constructed last year. We
have installed plastic scintillator hodoscopes in front of SCISSORS III. A beam test has been performedfor the forward detector assembly. The π 0 peak is clearly observed in the γγ invariant mass distribution.

§ 1. Plastic Scintillator Hodoscopes in Front of SCISSORS III
Nucleon resonances are experimentally studied via π 0 and η photo-production by using an electromagnetic (EM) calorimeter in the GeV-γ experimental hall [1] at Laboratory of Nuclear Science (LNS),Tohoku University. These neutral mesons are identiﬁed in γγ invariant mass distributions. A new EM
calorimeter complex called FOREST (Four-pi Omnidirectional Response Extended Spectrometer Trio)with a solid angle of about 4π in total is under construction [2] to reduce a fraction of undetected γ’s.
It consists of three calorimeters. A forward one made up of pure CsI crystals ‘SCISSORS III’ was constructed last year [3]. Since an EM calorimeter itself cannot distinguish whether an incident particle isneutral or charged, a thin plastic scintillator hodoscope is usually placed in front of it. We have designed
and constructed a set of plastic scintillator hodoscopes named Spiral-shaped Particle Identiﬁcation Detector for Elementary Reactions (SPIDER), which is capable of determining the incident position ofcharged particles precisely.
SPIDER consists of 3 layers, each of which is made up of 24 identical plastic scintillators. The side
curve of each plastic scintillator has a shape of spiral as depicted in Fig. 1. The spiral curve may be
represented asr = exp {b(θ + θ0 )} ,(1)in the polar coordinate system, where r is a radius measured in mm, and b is a curvature parameter
being chosen to be 1.1. The inside and outside radii r range from 57 to 425(1 − 0.15 sin 15◦ ) mm and to425(1 + 0.35 sin 15◦ ) mm, respectively. The phase difference in θ between adjacent scintillators is 15◦ .
Two layers in SPIDER have different conﬁgurations in placing 24 spiral scintillators in a plane.
One of them is a left-handed layer and the other is right-handed. A pair of these left-handed and righthanded layers can determine the position of charged particles going through SPIDER. The third layer isplaced so as not to have inefﬁcient area taking place at the boundary region of spiral in each scintillator.

24yinside

θ

15or

outsidexFig.1. Spiral-shaped scintillators. The side curve of each plastic scintillator is represented asr = exp {b(θ + θ0 )} in the polar coordinate system. The phase difference between adjacent scintillators is 15◦ .

Figure 2 shows the schematic view and photo of SPIDER. The spiral-shaped scintillator is connected toa metal packaged photo-multiplier tube Hamamatsu R8900U through a twisted-type light guide.

IncidentParticle

Responded Plastic Scintillators
Incident Position
Fig.2. Schematic view and photo of SPIDER. The left panel illustrates how to reconstruct theincident position of charged particles. The right one shows the mounted SPIDER in frontof SCISSORS III.

§ 2. Beam Test of SCISSORS III
A beam test of SCISSORS III with SPIDER was performed from 3rd to 5th Jul. in 2007. A dataacquisition system for SCISSORS III was essentially the same as that for SCISSORS II. The energy andtiming of each crystal were digitized by ADC and TDC modules in two TRISTAN/KEK Online (TKO)systems [4]. The digitized data were collected and stored with Super Memory Partner (SMP) modules

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in a VME system through Super Control Header (SCH) modules in the TKO systems. All the digitizeddata were ﬁnally accumulated in a personal computer (PC) through the VME-bus. Figure 3a) shows a
dataﬂow of the data acquisition system for SCISSORS III.
{rva)

1400}tl
{rvb)

1200

Counts1000jw|wj

800
600
400ztw

hkj {kj2000zjo hkj {kj050 100 150 200 250 300 350 400 450 500

γγ Invariant Mass (MeV)

Fig.3. a) Dataﬂow of the data acquisition system for SCISSORS III. The digitized data of energies and timings are collected from ADC’s and TDC’s in a TKO system through SCH and
SMP modules. b) The γγ invariant mass distribution measured with SCISSORS III. The
π 0 peak is clearly observed.

The trigger condition was described as([SigmaTagger i] ⊗ [NS3 ≥ 2]) ,(2)iwhere ⊗ means coincidence of signals. Channels of STB-Tagger II [5] were divided into 16 groups sothat the counting rate of each group should be the same, and an OR signal of each group is denoted bySigmaTagger i (i = 1 . . . 16). Crystals of SCISSORS III were divided into 10 groups; a signal of eachgroup was generated when an analogue sum signal of the group exceeded the threshold. The NS3 ≥ 2stands for the signal in which more than two groups generate signals. A coincidence signal between
SigmaTagger i and NS3 ≥ 2 signals is required for taking data. Data can be taken with a higher
intensity tagged photon beam in this condition compared with the condition:
[SigmaTagger i] ⊗ [NS3 ≥ 2].

(3)i

A neutral cluster was selected as a γ particle, and the incident position is reconstructed by an energy
weighted average of the position vectors of the modules which joins the cluster. The energy calibration
is made so that the π 0 peaks in the γγ invariant mass distribution should be π 0 mass, where the events
are selected by the condition that both the cluster energies are larger than 200 MeV. Figure 3b) shows
the γγ invariant mass distribution measured with SCISSORS III. The π 0 peak is clearly observed.

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References[1]

H. Shimizu et al., Research Report of LNS, Tohoku University 37 (2004) 13.

[2]

T. Ishikawa, LNS Experiment #2536 (2005).

[3]

T. Ishikawa et al., Research Report of LNS, Tohoku University 39 (2006) 35.

[4]

KEK Data Acquisition Development Working Group, KEK Report 85-10 (1985).

[5]

T. Nakabayashi et al., Research Report of LNS, Tohoku University 37 (2004) 17.