10

Research Report of Laboratory of Nuclear Seience, Tohoku University Vol. 40 (2007)(LNS Experiment : #2564)

Energy Calibration of STB-Tagger II by Using e+e− PairProduction
K. Suzuki, R. Hashimoto, T. Ishida, T. Ishikawa, J. Kasagi, S. Kuwasaki,
K. Mochizuki, K. Nawa, Y. Okada, Y. Onodera, H. Shimizu, and H. Yamazaki
Laboratory of Nuclear Science, Tohoku University, Sendai, 982-0826
The energy calibration of a bremsstrahlung tagging system STB-Tagger II was carried out by usinga dipole magnet RTAGX. The tagged photon energy was determined by a momentum analysis of e+ e−pairs converted from bremsstrahlung photons at two energies of STB circulating electrons. The relative
relation was obtained between the tagged photon energy for 920 MeV circulating electrons and that for1200 MeV electrons. In this report, We present the method and the result of the energy calibration.

§ 1. Introduction
A tagged photon beamline has been constructed in the GeV-γ experimental hall at Laboratory ofNuclear Science (LNS), Tohoku University [1]. We insert a radiator made of a carbon ﬁber, 11 μm in
diameter, into circulating electrons in the Stretcher-Booster (STB) ring to generate a high energy photon
beam. The electrons strike the radiator just upstream from the bending magnet BM5 of the STB ring,and produce bremsstrahlung photons which are used as a beam. Recoiled electrons are analyzed by
the BM5 and detected with a tagging system called STB Tagger II, which consists of 116 telescopes of
two-layer scintillating ﬁbers [2]. The energy Eγ of a produced photon is determined by the energy of therecoiled electron Ee and that of the circulating electron E0 as
Eγ = E0 − Eebecause the energy transferred to the nucleus is negligibly small. Thus Ee has to be measured precisely to provide a deﬁnite Eγ . However, there are some elements having slightly unknown factors in
the STB Tagger II. The map of the magnetic ﬂux in BM5 is not determined completely. The positionand direction of the recoiled electron emitting a bremsstrahlung photon are not known precisely at theradiator. And the exact position of STB Tagger II is not well known. Therefore the energy calibration isrequired for STB Tagger II. To determine Ee , Eγ has to be given conversely, which has so far been madewith two methods.
1. Missing mass MX analysis for the γ + p → π 0 + X process [3], and
2. Direct Measurement of Eγ with a lead glass Cherenkov counter [4].
In the former method, however, there is an uncertainty in the vertex point of the π 0 decay, and theenergy and position resolutions for incident photons are not good enough for this purpose. In the lattercase, the energy response of the lead glass Cherenkov counter is not precisely investigated. These two
methods did not work very well.

11The tagged photon energy Eγ can be obtained by measuring momenta of the e+ e− pair converted
from the incident photon. Eγ is given as a sum of the e+ and e− energies Ee+ and Ee− , namely
Eγ = Ee+ + Ee− .

(1)

We have performed meson photoproduction experiments with an electro-magnetic calorimeter SCISSORS II to study nucleon resonances, using a tagged photon beam. Two circulating-electron energiesof 920 and 1200 MeV have been selected to cover the incident photon energy from 600 to 1150 MeV.
We can make a consistency check for STB Tagger II by comparing the overlap energy region measured
under these different conditions of the electron energy. In this measurement, therefore, a dipole magnet
RTAGX [5] is operated at a ﬁxed current to analyze the momentum of the e+ and e− , regardless of theelectron energy.

§ 2. Experiment
We used the RTAGX magnet for the momentum analysis of e+ e− pairs. A 20 μm thick Au foil
was placed in front of RTAGX to produce e+ e− pairs from the tagged photons. Electrons/positrons
having different momenta were bent with different curvature by RTAGX , and were detected with two
scintillating ﬁber hodoscopes located behind RTAGX. Each scintillating ﬁber measures 3×3 mm2 in crosssection, and 16 ﬁbers are arranged in a hodoscope. One hodoscope was placed 1850 mm downstream fromthe center of RTAGX and at −46.6◦ with respect to the beam axis. The other hodoscope was movable soas to detect various energy electrons. Figure 1 shows the experimental setup for this energy calibration.

Rail +side
Movable Hodoscope

23.32㫦

Rail Position 709
31.38㫦

1018

Rail Position 1143

e−Fixed Hodoscope

902

Rail 723

e+481

46.63㫦

Yoke
1073

Pole
Coil

878

2400
400

Au 0.02 mm

Pb collimator
㱏-ray

Fig.1. Experimental setup for the energy calibration of STB-Tagger II. The e+ e− pairs are generated in the interaction of incident photons with a Au foil, and are detected by a ﬁxed
scintillating ﬁber hodoscope and a movable one.

12A triple coincidence signal from STB-Tagger II and two scintillating ﬁber hodoscopes was used toform a trigger for the data acquisition:(trigger) = (tagger) ⊗ (ﬁxed hodoscope) ⊗ (movable hodoscope).
We measured the energy of each tagged photon deﬁned with STB Tagger II using this trigger by detecting an e+ e− pair generated from the tagged photon. The experiment was performed for two different
electron energies of 920 and 1200 MeV.

§ 3. Data Analysis
A pair of e+ and e− were detected with the ﬁxed and movable hodoscopes, respectively. Energies ofe+ and e− were determined from the position of responding ﬁbers in these hodoscopes. Here, the ﬂight
path of e+ /e− with a given energy was calculated by utilizing the ﬁeld map of RTAGX, where the energy
loss of e+ /e− was taken into account in the Au foil and the air. The magnetic ﬂux of RTAGX is described
elsewhere [6, 7]. Figure 2 shows the e+ /e− energy as a function of the bending angle determined by aresponding ﬁber.
㪊㪇1200㪈㪇 㪋㪋㪇㩷㫅㫊㫋㩿㫄㫆㫍㪀㩷㪄㩷㫋㩿㪫㪸㪾㪾㪼㫉㪀㩷㩿㫅㫊㪀Energy (MeV)

1000
800
600400㪉㪇㪈㪇 㪊㪈㪇㪇㪈㪇 㪉㪄㪈㪇200㪈㪇㪄㪉㪇㪈㪉㩷㫅㫊0102030405060

70

Bending angle (degree)+−Fig.2. The e /e energy as a function of thebending angle.

㪄㪊㪇㪄㪊㪇㪄㪉㪇㪄㪈㪇㪈㪉㩷㫅㫊㪇㪈㪇㪉㪇㪊㪇㪈㫋㩿㪽㫀㫏㪀㩷㪄㩷㫋㩿㪫㪸㪾㪾㪼㫉㪀㩷㩿㫅㫊㪀Fig.3. Timing Correlation between the movable and the ﬁxed hodoscope.

Since the intensity of the bremsstrahlung photons was more than 10 MHz, there was a large fractionof accidental coincidence events. To estimate the number of true coincidence events, we made a twodimensional plot of the timing signals from the movable and ﬁxed hodoscopes. Figure 3 shows thetiming correlation between the movable and ﬁxed hodoscopes. The total area of the two-dimensional
plot is divided into 16 regions by broken lines, the center region is the prompt area, and the otherregions correspond to background events. The background events in the prompt area were subtractedwith the method of two-dimensional side band subtraction. This method is described elsewhere [8, 11].
The energy of a tagged photon can be obtained with a sum of the e+ and e− energies as expressedin Eq. (1). Figure 4 shows e+ e− energy sum distributions corresponding to tagger channel, 1, 70 and13-5

Yieldx 10-4

Tag 1x 10-4

Tag 70x 10

0.6

0.15

0.3

0.4

0.1

0.2

0.2

0.05

0.1

0008001000

1200

Tag 116800

1000

1200800

1000

1200

Ee- + Ee+ (MeV)
Fig.4. Energy sum distributions of e+ and e− converted from the tagged photons corresponding
to tagger channel, 1, 70 and 116. Each curve is a Gaussian function ﬁtted to the data.

116 after the background subtraction of accidental coincidence events. The centroid of the e+ e− energy
sum is determined by ﬁtting a Gaussian function to the data.

§ 4. Tagged Photon Energy
The energy of photons tagged with STB Tagger II was calibrated by measuring the energies ofe+ e− pairs. STB-Tagger II covers the tagged photon energy range from 579 to 890 MeV for 920 MeV
circulating electrons, and 752 to 1155 MeV for 1200 MeV electrons. Since the RTAGX coils carried aﬁxed current, the tagged photon energy for 920 MeV electrons was determined relatively to that for1200 MeV electrons. In Fig. 5, the measured photon energies are plotted as a function of the telescope
number in STB Tagger II.

1200
1100

1200 MeV mode
Energy (MeV)

1000
900
800920 MeV mode

700
600
5000

20406080100120

Telescope numberFig.5. Relation between the photon energy and the telescope number in STB Tagger II at two
circulating-electron energies of 920 and 1200 MeV.

14We made another calibration by measuring the threshold energy of the γ + p → η + p reaction.
This calibration identiﬁed the tagging telescope corresponding to the incident photon having the η production threshold energy. The threshold photon energy in the γ + p → η + p reaction can be calculated
precisely, that is Eγ = 707.25 MeV. We compared the η threshold energy with the tagged photon energy
corresponding to the η threshold point by the momentum analysis for e+ e− pairs. It turned out thatthe tagged photon energy determined by the momentum analysis was 0.63% larger than the thresholdenergy [9]. This difference was mainly caused by an overall normalization uncertainty in the magnetic
ﬁeld of RTAGX. Thus the normalization uncertainty was resolved [10]. The geometrical errors arise
from a position measurement uncertainty of the ﬁxed and movable hodoscopes, a setup uncertainty ofthe movable hodoscope location, and a position uncertainty of the incident photon beam [11]. The total
uncertainty of the tagged photon energy is 0.3%.

References[1]

H. Shimizu et al.: Research Report of Laboratory of Nuclear Science, 37 (2004) 13.

[2]

T. Nakabayashi et al.: Research Report of Laboratory of Nuclear Science, 37 (2004) 17.

[3]

S. Suzuki: Master’s thesis, Department of Physics, Tohoku University (2005).

[4]

T. Ishikawa: Internal GeV-γ analysis note No.15 (2006).

[5]

T. Ishikawa et al.: Research Report of Laboratory of Nuclear Science, 39 (2004) 39.

[6]

T. Ishikawa: Internal GeV-γ analysis note No.20 (2006).

[7]

T. Ishikawa: Internal GeV-γ analysis note No.34 (2006).

[8]

T. Nakabayashi: Internal GeV-γ analysis note No.18 (2006).

[9]

T. Ishikawa: Internal GeV-γ analysis note No.57 (2007).

[10] K. Suzuki: Internal GeV-γ analysis note No.59 (2007).
[11] K. Suzuki: Internal GeV-γ analysis note No.35 (2007).