Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 48 (2015)

Radiation monitoring in the RIBF using ionization chamber
M. Nakamura,＊1 K. Yamada,＊1 A. Uchiyama,＊1 H. Okuno,＊1 and M. Kase＊1
In recent years, we have attempted to monitor radiation
due to beam loss in the RIBF by using self-made ionizationchambers (ICs)1),2),3) However, in the course of RIBF
operations, a part of 238U86+ ion beam accelerated at 345
MeV/nucleon struck the septum electrode of theelectrostatic deflection channel (EDC) of RRC and theseptum was damaged in December 2012. To avoid suchserious damages, the part of septum where the ion beams
can easily irradiate was cut off and molded to the“V-shaped” edge and many thermocouples (TCs) were set
at the surface of the septum. Before the septum wasdamaged by the irradiation of ion beams and the consequent
rise in temperature, these signals are input to BIS.
However, the response time of these TCs are estimated in
the order of few seconds. Furthermore, such results
showed that the septum was damaged only when a part ofthe 238U86+ beam hit the septum within 10-1 to 1 s. Hence,a faster signal for BIS with a response time of at least of theorder of 10-1 s is required. On the other hand, the responsetime of IC can be estimated to the order of 101 ms. The
response time of the whole BIS is about 1〜max. 25 ms.
Accordingly, we can obtain the response time to the orderof 10-1 s when the alarm signal from IC is used for BIS.
Hence, we investigated to introduce the alarm signal fromIC around the EDC of RRC into BIS in the case of 238U86+ion beam acceleration.
Usually, we input the alarm signal from IC near the EDCof SRC to BIS after the “calibration experiments”1,2). In
these experiments, the ion beams were attenuated from1/100 to 1/10 and irradiated to the EDC for a fairly shorttime and IC voltages were measured. From these results,
we can estimate the alarm levels of IC to BIS. However, it
can be very dangerous to irradiate heavy-ion beams to theEDC of RRC because of a sudden increase in thetemperature of the septum. Therefore, we can consider thealarm levels of IC from the signals of TCs set at the septum.
When the temperature difference between the TC set at theupper part of the septum with the TC set at the beam-exitside of the septum (defined as “the maximum value ofseptum temperature rising”) becomes 10℃, the alarm
signal is input to BIS.
Hence, we have compared themaximum value of septum temperature rising with thesignal of IC set near the EDC of RRC in the machine timeof the 238U86+ beam. The result is shown in fig. 1. The
data showed little dispersion and the calibration curve in fig.
1 can be drawn. From this curve, we can recognize thatthe voltage of IC became about 1.4 V when the maximum
temperature rising of EDC reached 10℃, shown as red
dotted line in fig. 1. Then we can decide the alarm level ofIC for BIS for the 238U86+ ion beam.
＊1

RIKEN Nishina Center

Fig. 1 Correlation of IC voltage and maximum value ofseptum temperature rising
We have input the alarm signal to BIS from April 28 toMay 12 and from October 16 to November 14, 2014 whenthe 238U86+ ion was accelerated at 345 MeV/nucleon. OnMay 5, the BIS by the alarm signal from IC acted andstopped the operations of RIBF. Fig. 2 shows the IC
signal from May 5, 0:00 to 9:00. At 5:45, the signal
suddenly rose up to 6.4 V and the alarm signal was sent toBIS. We could confirm that this signal reached BIS fasterthan the alarm signals from TCs set at EDC. After thissignal, any alarm from IC did not reach BIS in the machinetime of the 238U86+ beam in 2014. The cause of thisunusual signal shown in fig. 2 is unknown. In any case, asdescribed above, we could confirm that the faster alarm
signal to BIS can be input from the IC near the EDC ofRRC. Thus, we investigated to input the alarm signal fromIC in RRC to BIS each time heavy ions are accelerated,such as 238U86+.

Fig. 2 Signal from the IC near EDC of RRC
References
1) M. Nakamura et al.: RIKEN Accel. Prog. Rep. 44, 293 (2011)2) M. Nakamura et al.: RIKEN Accel. Prog. Rep. 45, 228 (2012)3) M. Nakamura et al.: RIKEN Accel. Prog. Rep. 47, 310 (2014)- 237 -完全版2014_本文.indd 237

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