Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, Japan

PASJ2015 WEP092

J-PARC 主リングにおける高残留線量と低ビーム損失信号のパズル
LARGE RESIDUAL RADIATION BUT SMALL BEAM LOSS SIGNAL AT J-PARC MR
外山毅 #, A), イーレンドン ブルース A), 久保木浩功 A), 花村幸篤 B), 佐藤洋一 A), 魚田雅彦 A), 白形政司 A)
Takeshi Toyama #, A), Bruce Yee-Rendon A), Hironori Kuboki A), K.Hanamura B)
Yoichi SATO A), Masahiko Uota A) and Masashi Shirakata A)A)
KEK / J-PARC, B)Mitsubishi Electric System & Service Co.,LtdAbstract
In the ARC-A (downstream of the injection insertion section / INS-A) of J-PARC MR, we measured large residual
radiations (a few mSv/h at ~four hours after beam stop) at the addresses 45 and 52, even though the beam loss monitor
signals are one order of magnitude smaller. Residual radiation should be kept less than ~1 mSv/h for maintenance.
Therefore the reason of the large discrepancy and the source of the beam loss is a big issue. The tentative understanding
is reported here.

1.

Introduction

We measured large residual radiations (a few mSv/h at~four hours after beam stop) at the addresses 45 and 52 in
the ARC-A (downstream of the injection insertion section/ INS-A) of J-PARC MR, even though the beam loss
monitor (BLM) signals are one order of magnitude smaller.
Residual radiation should be kept less than ~1 mSv/h formaintenance. Therefore the reason of the large discrepancy
and the source of the beam loss is a big issue. The measured
data with big discrepancy is presented, dose distribution
during machine operation with OSL dosimeters, BLM
detector response simulation with PHITS code, and aparticle tracking simulation with SAD are presented here.

2.

Large discrepancy between a beam loss
signal and a residual dose

Beam loss signals measured with the BLMs during FX(fast beam extraction) mode operation are plotted in Figure
1 with a bar chart. In the same plot residual radiations
measured on the beam ducts with the handy dosimeter [1]are plotted where blue dots are measured in Mar. 11, 2015
and red dots in Mar. 17, 2015. The BLM signals are divided
by 2000 to fit to the residual radiation. At addresses #45
and #52, discrepancies are large between the beam loss
signals and residual radiations. The configuration of theBLM [2] and magnets are shown in Figure 2. The beamruns from the left side to the right side of the photograph.
The BLM is set on the quadrupole magnet. The beam loss
points are considered as the key of these discrepancies.

3.

Doses measured with the OSL dosimeter

To clarify the dose distribution Optically Stimulated
Luminescence (OSL) dosimeters “nano dots” [3] are put on
the beam ducts. After one operation period of the machine
the OSL dosimeters are replaced. The collected dosimeters
are analyzed with the OSL dosimeter. The dose distribution
at addresses #23, #45 and #52 are depicted in Figure 3.aand 3.b. Figure 3.a and 3.b correspond to the results ofbeam-loss irradiation during Jun. 17 – Jun. 29, 2015 andJun. 29 – Jul. 1, 2015, respectively. The address #23 is afocusing position in the vertical direction and #45 and #52
are focusing in the horizontal direction.

10#4

Residual radiation
Residual radiation ΜSv h[mSv/h]
Measured in Mar. 11 & 17, 2015

8Beam Loss Monitor
Bending magnet6Quadrupole magnet

4#520050100
BLM150200

Figure 1: Beam loss signals during operation vs. residual
radiation measured with the BLMs.
___________________________________________# takeshi.toyama@kek.jp- 725 -Steering magnet
Figure 2: BLM setting on the quadrupole magnets.

Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, Japan

PASJ2015 WEP092

#23
#52

QuadSaturation regionof OSL dosimeter

Steering#45

Figure 3.a: Dose measured with OSL dosimeters.
Measurement was done Jun. 17 – Jun. 29, 2015.

Figure 4: Dose measured with OSL dosimeters.
Measurement was done Jun. 17 – Jun. 29, 2015.

#45

#23

#52

Figure 3.b: Dose measured with OSL dosimeters.
Measurement was done Jun. 29 – Jul. 1, 2015.

Figure 5: Dose measured with OSL dosimeters.
Measurement was done Jun. 17 – Jun. 29, 2015.

At the address #23, the beam size grows in the vertical,
especially at the exit of the bending magnet where thevertical aperture is the smallest, and then beam may be lost.
Secondary particles from the loss point go to the BLM
detector through the air, steering and quadrupole magnet.
On the other hand, at the #45 and #52, the beam size has
the maximum horizontally in the quadrupole magnet.
Therefore the loss may occur in the quadrupole magnet.
Secondary particles from the loss point go to the BLM
detector through the thick quadrupole magnet core. The
scattering angle seems nearly 90 degree, larger comparing
with the #23 case. The dose measured with the OSL seems
to agree with this picture.

4.

Address #23

Simulation with PHITS

Estimation of the BLM detector response / sensitivity isgoing on using PHITS simulation code [4]. The materialsof magnets and the BLMs are simplified as cylindricalshaped irons. Two cases of beam loss points, at theentrance of the steering magnet and at the entrance of
quadrupole magnet are simulated. The configuration isshown in Figure 4. In the input of the simulation the BLM
detectors are placed in every 1-meter.
The particle flux and deposited energy at each BLM areplotted in Figure 5 and 6, respectively. By examining
Figure 5, the flux of #45, #52 at the BLM detector position
seems much smaller than #23. Serial num. of region 3 isthe present BLM position in Figure 6. Others are theAddress #45 and 52
Figure 6: Deposited energy at each BLM. Num. 3 is thepresent BLM position.
Others are the depositedenergies if the BLMs are put at these positions.
deposited energies if the BLMs are put at these positions.
Comparing serial num. of region 3 for both configurations,the deposited energy of #45, #52 configuration is nearlyfour times smaller than #23. Even though more realistic
geometry may be necessary for more precise discussion,tendency of both configurations may be understood asabove.

- 726 -Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, JapanPASJ2015 WEP092(a) Vertical: Gaussian, horizontal: hollow distribution.

(a) Vertical: Gaussian, horizontal: hollow distribution.

(b) Horizontal: Gaussian, vertical: hollow distribution.
(b) Horizontal: Gaussian, vertical: hollow distribution.

Figure 7: Initial distributions for the beam particle
tracking.

5.

Beam particle tracking simulation

In order to clarify the source of the beam loss the beam
particles are tracked with the SAD code [5]. The scattering
at the jaw of the MR collimator A causes additional kickon the beam particles. Two cases:(1) The Horizontal emittance is set at 70 π and thevertical at 80 π mm mrad, (both correspond to 3σ). The
fraction of the beam, 1 %, at the horizontal fringe hits thejaw.
(2) The emittances are set as same as the above. The
fraction of the beam, 1 %, at the vertical fringe hits the jaw.
A particle receives a kick after pass the Collimator A: arandom kick and dispersion in energy. The random kick isaround (+/- 5mrad). The dispersion in energy is between 0
to 1 %. The initial phase space distributions are shown inFigures 7. The tracked trajectories are shown in Figure 8.
The results obtained so far shows no beam losses at theaddresses #45 and #52.

6.

Summary and prospect

Beam Loss signals by the BLMs are nearly 10 times
smaller than those expected by the residual radiations at theaddresses #45 and #52. Detailed measurements areunderway. Using BLM #23 (QD) as a reference, BLMs #45and #52 are examined. At #23 (QD), beam loss may haveoccurred at the downstream of the bending magnet, on theother hand at #45 (QF) and #52 (QF), it occurred at theFigure 8: Trajectories of beam particles.
Quad. Low sensitivity of the #45 and #52 BLM may due tothick Quad core made of iron and large scattering angle.
These processes attenuate the secondary particles flux fromthe beam collision point, which is proved qualitatively witha simulation using the PHITS code. Beam loss mechanism
is investigated using SAD code with 70 π mm mrad beam
particles hitting the jaw of the collimator A. No beam loss
is obtained so far and the simulation is in progress withadditional assumptions.
To observe and to evaluate the beam loss amount,adding BLMs with scintillator etc. with small secondary
particle attenuation is progressing. Calibration of present
BLMs with the DCCT may be another solution.

Acknowledgements
The author (B.Y-R.) gratefully acknowledges H.Haradaand Y.Kurimoto for their help in executing particle
tracking.

References[1] J-PARC radiation safety section, Internal reports, “第１種管理 区 域 線 量 当 量 率 測 定 記 録 ”. Using detector is theGeiger-Müller tube survey meter.
[2] T. Toyama , in these proceedings and references therein.
[3] http://www.nagase-landauer.co.jp[4] H.Iwase,K.Niita etal., http://phits.jaea.go.jp.
[5] Strategic Accelerator Design,
http://acc-physics.kek.jp/SAD/- 727 -