RIKEN Accel. Prog. Rep. 48 (2015)

Ⅱ-1. Nuclear Physics

Identification of fission fragments from tracks measured by the FDC1S. Reichert,∗1,∗2 M. Sako,∗2 M. Sasano,∗2 D. Mücher,∗1,∗2 A. Andreyev,∗3 T. Aumann,∗4 H. Baba,∗2M. Böhmer,∗1 M. Dozono,∗5 N. Fukuda,∗2 R. Gernhäuser,∗1 W. F. Henning,∗6 K. Hirose,∗3∗2
N. Inabe, D. Kameda,∗2 N. Kobayashi,∗2 T. Kobayashi,∗7 Y. Kondo,∗8 T. Kubo,∗2 Y. Kubota,∗2,∗5 R. Lang,∗1L. Maier,∗1 Y. Matsuda,∗9 S. Mitsuoka,∗4 T. Motobayashi,∗2 T. Nakamura,∗8 N. Nakatsuka,∗9 S. Nishimura,∗2I. Nishinaka,∗3 K. Nishio,∗3 R. Orlandi,∗3 H. Otsu,∗2 V. Panin,∗2 S. Sakaguchi,∗2 H. Sato,∗2 Y. Shimizu,∗2L. Stuhl,∗2 T. Sumikama,∗2 H. Suzuki,∗2 H. Takeda,∗2 Y. Togano,∗2 T. Uesaka,∗2 J. Yasuda,∗2 K. Yoneda,∗2and J. Zenihiro∗2b=a1 + a2tanθ1 + tanθ2(1)where θ1/2 are the angles of the two fission fragments
1 and 2 in respect to the beam axis and a1/2 are definedas their most probable intersection point with the firstlayer of the FDC1.
The results for two 1/2 h runs with each with the∗1∗2∗3∗4∗5∗6∗7∗8∗9

Technical University Munich
RIKEN Nishina Center
ASRC JAEA
Technical University Darmstadt
Center for Nuclear Studies, University of Tokyo
ANLTohoku University
Tokyo Institute of Technology
Kyoto University

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hLayervsXges60

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0Distance to the beam axis [mm]Distance to the beam axis [mm]300200-200

2468101214# of Layer of the FDC 1-3000(a)2468101214# of Layer of the FDC 1(b)Fig. 1. Online plot of typical tracks observed in the FDC16000

Entries 113279

5000
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# of counts

filled target or with the empty one are shown in Fig.
2(a) and (b). In both cases the maximum of the verybroad peak is in good agreement with the real distanceto the target of 2215 mm. The sharp peak at b ≈4600 mm also appears in both spectra but it is not yet
understood. The peak in the case of the empty run
can be considered as interactions of the beam with thetarget foil. The main and important difference is thenumber of events which drops down from over 100.000
to ∼5000 entries removing the liquid hydrogen fromthe target.
# of counts

The purpose of the experiment NP1306-SAMURAI14
is to characterize parameters of fission of neutron richheavy nuclei, like the fission barrier heights and thefragment distributions. As a first step, we performedin April 2014 a 24-hour beamtime experiment using238
U as primary beam at 250 MeV/u and an intensityof 5×104 pps. A liquid hydrogen target in the setup ofNP1306-SAMURAI17 was used as a secondary target.
In this report we present a method to identify fission fragments by the forward drift chamber 1 (FDC1)
which is positioned upstream in front of the SAMURAI spectrometer. In the beam direction, the FDC1
has 14 layers. For each layer the positions of the particles passing the active volume are determined. The
most likely trajectory is defined as the composition of
the available positions in all layers which has the smallest χ2 . When two fission fragments are incident, there
are two possible positions for all the layers and theirtracks are given by the two smallest χ2 values.
In Fig. 1 we show examples of trajectories throughthe 14 layers in beam axis of the FDC1. In Fig. 1(a)the number of possible positions of the particles in eachlayer is one. Hence the track corresponds to a beamlike particle, showing no fission reaction is induced.
The situation in Fig. 1(b) is totally different where
each layer has two entries and two tracks with a large
gap can be reconstructed. Such events are interpreted
as two particles crossing the FDC1 at the same time.
For these cases the distance b between their intersection points and the first layer of the FDC1 in beam
direction is obtained by3000
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Distance b to the FDC 1 [mm]Distance b to the FDC 1 [mm]

(a)

(b)

Fig. 2. Intersection point of two tracks of the FDC1 in
respect to the beam axis. (a) is the result where thetarget was mounted, in (b) there was no liquid hydrogen
between the target foils.

This method seems promising for a fast and reliable way to identify fission stemming from reactions
inside the target. However the uncertainty in the reconstructed vertex position b is very large, σ ∼ 600mm. In the future this might be improved, e.g., by including the position of the incoming beam by the beamdrift chambers. A precise beam position and incident
angle is also needed for determining the A/Q value ofthe fission fragments in the downstream part of theSAMURAI spectrometer.

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