Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

Eﬀect of stray ﬁeld of the SAMURAI spectrometeron the neutron detector array WINDS
Y. Kubota∗1,∗2 and M. Sasano∗2
The (p, n) reaction has been used as a powerful probe
to study nuclear isovector responses such as Gamow–Teller transitions, as extensively done in the region ofstable nuclei1) . The extension of such studies to unstable nuclei can realized by combining the neutron
detector array WINDS2) with the SAMURAI spectrometer3) for measuring high-intensity radioactive ionbeams at the RIKEN RIBF.
From an experimental point of view, however, there
is concern that the stray ﬁeld of the SAMURAI magnet may deteriorate the gain of photomultiplier tubes(PMTs) of the WINDS bars. It is well known that theeﬀect of the stray ﬁeld is maximized when the directionof the magnetic ﬁeld is parallel to that of the PMT. In
this experiment, we examined this eﬀect with several
magnetic settings and also tested the restoration ofgain with additional magnetic shielding on PMTs.
One of the WINDS bars was vertically placed near
the entrance of the SAMURAI spectrometer. Thus,
the PMTs (Hamamatsu H7195) attached at both endsof the bar are also aligned vertically. The directionof the stray ﬁeld can also be considered more or lessvertical, and the strength is large around the PMTs.
Therefore, this setup provides the most severe gaindeterioration conditions. In the present work, threesettings — (i) without any additional shielding, (ii)with one-fold shielding, and (iii) with two-fold shielding — were tested, as shown in Fig. 1. The magnetic
ﬁeld settings were 1.6, 2.2, 2.9, and 3.0 T. For thesesettings, the stray ﬁeld at the location of the PMT wasmeasured to be 0.5, 0.7, 4.5, and 6.0 mT, respectively.
The left panel of Fig. 2 shows the light output spectra of 137 Cs for 2.2 T. The relative gain was calibrated
with the Compton edge of the 661.7-keV γ-ray emitted from 137 Cs. For each spectrum, we assumed theposition corresponding to 70% of the maximum heightas the Compton edge. The relative gain of the PMT
decreased to 28% at 2.2 T when no additional shield-Fig. 1. Schematic view of the additional shielding consisting of 2-mm-thick iron (SUY-1). We use only inner(a) as a one-fold, and both (a) and (b) as a two-foldshielding.
∗1∗2Center for Nuclear Study, University of Tokyo
RIKEN Nishina Centering was used. With the addition of one-fold shielding,
the relative gain was restored up to 97%. It was also
conﬁrmed that a relative gain of 92% or more can beachieved up to 2.9 T by using two-fold shielding, ifnecessary. However, at 3.0 T, a relative gain of 59%
was obtained even with two-fold shielding. This is because of the drastic increase of the stray ﬁeld due tothe saturation eﬀect of the iron yoke of the SAMURAImagnet around 3 T4) .
The right panel of Fig. 2 shows the light output spectra of 241 Am. Gamma rays of 60 keV from 241 Am produce almost same light output as 100-keV neutrons.
The spectrum was strongly distorted when the magnetic ﬁeld was changed from 2.9 to 3.0 T because of
the same reason as stated above. Below 2.2 T, the effect of the distortion can be made negligibly small withat least one-fold shielding.
In summary, the additional one-fold or, at most,
two-fold magnetic shielding is suﬃcient to restore thegain for the SAMURAI magnet settings up to 2.9 T,
but at 3.0 T, shielding of more than three-fold or
thicker will be necessary. Thus, the setting of 2.9 T
is practically much better than that of 3.0 T for thesimilar setup with PMTs, if there is no signiﬁcant difference between 2.9 and 3.0 T in the performance of
the SAMURAI spectrometer.
We acknowledge H. Sato for his support with theoperation of the SAMURAI magnet.
#Events#Events

x0.97
x0.28

QDC channel

QDC channel

Fig. 2. Left panel shows light output spectra of 137 Cs
without additional shielding (purple) and with one-foldshielding (green) at 2.2 T. Black line shows that at
0.0 T. Each vertical line shows the position of each
Compton edge. Right panel shows light output spectra of 241 Am with two-fold shielding at 0.0 T (blue),
2.9 T (green), and 3.0 T (red).

References
1) M. Sasano et al., Phys. Rev. C 85, 061301 (2012).
2) K. Yako et al., RIKEN Accel. Prog. Rep. 45, 137 (2012).
3) T. Kobayashi et al., Nucl. Instr. Meth. B 317, 294(2013).
4) H. Sato et al., IEEE Trans. Appl. Supercond. 23,4500308 (2013).

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