6Research Report of Laboratory of Nuclear Seience, Tohoku University Vol. 40 (2007)(LNS Experiment : #2563)

Electron/Positron Test Beamline
T. Ishikawa1 , R. Hashimoto1 , T. Ishida1 , J. Kasagi1 , S. Kuwasaki1 , K. Maeda2 ,K. Mochizuki1 , K. Nawa1 , Y. Okada1 , Y. Onodera1 , H. Shimizu1 , K. Suzuki1 ,and H. Yamazaki1
1

Laboratory of Nuclear Science, Tohoku University, Sendai, 982-0826

2

Department of Physics, Tohoku University, Sendai, Miyagi 980-8578

An electron/positron beamline for testing detectors has been constructed by utilizing a charge
sweeping magnet RTAGX, which can analyze the momentum of electrons and positrons at a ﬁxed bending angle. The measured energy, proﬁle, and intensity of the beam are reported together with a simulation result.

§ 1. Electron/Positron Beamline for Testing Detectors
A dipole electromagnet RTAGX was installed two years ago [1] in the GeV-γ experimental hall,where meson production experiments have been conducted with an incident γ beam [2]. Besides sweeping out undesirable charged particles in the γ beam, the RTAGX can provide momentum-analyzed electrons or positrons at a ﬁxed bending angle. We have desired a beamline where performance tests canbe made for many kinds of detectors. Thus an electron/positron beamline has been constructed in theGeV-γ experimental hall for testing detectors. The beamline consists of a converter, the RTAGX magnet,
and lead apertures as depicted in Fig.1. A Au foil, one of the converters, with a thickness of 20 μm is

φ1073
100
φ105

30.00

qAu foil Pb Aperture

2445100878

RTAGXPb Aperture
Positron/Electron BeamFig.1. Schematic view of the electron/positron beamline for detector test. The beamline comprises a converter, a dipole electromagnet RTAGX, and entrance and exit lead apertures.
Electrons/positrons passing through the lead apertures are used as a beam.

7
placed 878 mm upstream of the pole center of the RTAGX. There is a 100 mm thick lead aperture justbehind the converter. Another 100 mm thick lead aperture with a diameter of 20 mm is placed 2445 mm
downstream at −30◦ with respect to the axis of the incident γ beam. A vacuum chamber and a vacuum
pipe were installed between two lead apertures on 23rd Apr. in 2007.
The incident γ beam coming into the GeV-γ experimental hall irradiates the converter in front of theRTAGX. Some photons in the beam are converted into electron-positron pairs. The generated electronsand positrons are bent by the RTAGX according to their momenta. Then semi-monochromatic electronsor positrons passing through the lead apertures are used as a beam.

§ 2. Estimated Energy and Resolution
The energy and resolution of the beam for a given RTAGX current are estimated by a simulationcode based on GEANT3 with a corresponding magnetic ﬁeld map [1, 3]. Figure 2 shows the estimated90080070060050040030020010003Energy Resolution σE/E (%)

Energy E (MeV)beam energy as a function of the RTAGX current and the energy resolution versus the beam energy.

2.5
21.5
10.5

0100200 300 400
Current I (A)50000200 400 600
Energy E (MeV)

800

Fig.2. Estimated energy and resolution of the electron/positron beam with a simulation basedon GEANT3. The left panel shows the mean energy as a function of RTAGX current. Theright panel shows the energy resolution as a function of the beam energy. The circle andsquare markers show the data points when the beam
 travels in the air and in the vacuum,respectively. Data points are ﬁtted with the form P1 E −2 + P2 E −1 + P3 + P4 E.

The electron/positron beam energy is almost proportional to the RTAGX current. The value of theenergy in MeV corresponds roughly to twice of that of the current in ampere. The energy resolution as afunction of the energy is described asσE(125.9 ± 13.5)(7.49 ± 0.26) × 105−+ (0.977 ± 0.028) − (3.9 ± 0.2) × 10−4 E(1)(E) =2EEEwhen electrons or positrons travel in the air, while that is described as(8.6 ± 78.1)(1.08 ± 0.15) × 105
σE−+ (0.695 ± 0.149) − (2.1 ± 1.1) × 10−4 E (2)
(E) =
EE
E2
when they travel in the vacuum. The numerical data are given in Ref. [4].

8

§ 3. Measured Beam Energy and Intensity
The beam energy was measured with a lead glass Čerenkov counter. The beam proﬁle was also measured with a beam proﬁle monitor (BPM). The BPM consists of two layers of scintillating ﬁber (SciFi)
hodoscopes, each of which is made up with 16 SciFi modules. The ﬁber measures 3 × 3 mm2 in crosssection. The upstream and downstream layers determine x and y positions from responded ﬁbers, respectively.
Figure 3 shows the measured energy distributions, and the beam proﬁle for the RTAGX currentof 230 A. The measured energy is consistent with an estimated one, although the energy spread of thebeam is not detectable because of poor energy-resolution of the lead glass counter. The size of the beam
spot is about 50 mm in diameter at the place about 7 m from the center of the RTAGX.

a)02058 A
210 A
215 A
521 A
026 A
933 A
401 A
470 A
5AFigure 4 shows the beam intensity as a function of the RTAGX current.

b)

800

20700y (mm)Counts1510600

5500

0400-5-10300-15200-200200

400600800 1000 1200

Lead Glass ADC

100-20-10010x (mm)20#Positrons / #Tagger 116 Hits (x 10 -3)

Fig.3. a) Measured beam energy distribution for several RTAGX currents. The unit of energy
is arbitrary (ADC channel). b) Measured beam proﬁle for the RTAGX current of 230 A.
Both data were measured before the vacuum chamber had been installed.

6543210050

100 150 200 250 300 350 400 450 500
RTAGX Current (A)Fig.4. Intensity of the positron beam as a function of the RTAGX current with a converter of 20μm thick Au foil. The intensity is normalized by the counting rate of the 116th channelof the STB-Tagger II.

9Since the beam intensity depends on the circulating electron current in the STretcher Booster ring,
it is normalized by the counting rate of the 116th channel of the STB-Tagger II [5]. More intense beam
can be obtained with a thicker converter. So far, the maximum intensity of the positron beam is about
700 kcps with an 8 mm thick Cu converter.

References[1]

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

[2]

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

[3]

T. Ishikawa: Internal GeV-γ analysis notes Nos. 20 and 34 (2006).

[4]

T. Ishikawa: Internal GeV-γ analysis note No. 43 (2007).

[5]

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