Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 51 (2018)

Two methods for invariant mass reconstruction
from events with multiple charged particles

Counts/10 keV

S. Koyama∗1,∗2 and H. Otsu∗2 for SAMURAI08 collaboration
Invariant mass spectroscopy is one of the techniques
to explore unbound states of nuclei. Relative energy (Er ) or the energy above threshold energy is reconstructed from four-momenta of decay particles. Experimental resolution is important since the level density above particle thresholds is higher than that below particle thresholds and unbound states have ﬁnitewidth. Another important aspect is acceptance. Thesetwo aspects often compete with each other.
The SAMURAI spectrometer is developed to investigate unbound states of nuclei. In the standard setup
with the SAMURAI magnet, we install four multi-wiredrift chambers, BDC1, BDC2, FDC1, and FDC2, tomeasure the four-momentum of a charged particle.1)
FDC1 is placed between the target and the SAMURAI magnet, while FDC2 is placed at the downstreamof the magnet. By using positions and angles measured by these detectors, we can deduce the direction
vector and the magnetic rigidity of a charged particle, which is converted to the four-momentum. For
invariant mass reconstruction from events with multiple charged particles such as α decay into 2 charged
particles of α + residue, positions of each charged particle have to be deduced with both FDC1 and FDC2.
The positions are separated at FDC2 for particles with
diﬀerent A/Z values, while the positions can be closeat FDC1. The cell size of FDC1 is 10 mm, and therequirement of deducing 2 positions with FDC1 canlimit the acceptance, especially for a small opening angle corresponding to a low Er . The four-momenta ofcharged particles can be deduced only from the reaction point on the target, positions and angles deduced
from FDC2, and the magnetic ﬁeld map of the SAMURAI magnet2) without FDC1, though the resolution of
the direction vector is worse than with FDC1. Therefore, we performed two diﬀerent methods to deduce
Er , without FDC1 and with FDC1. The former yields
a worse resolution but full acceptance, while the latterachieves a better resolution but with biased acceptancefor two particles, especially for a small spatial separation at the FDC1 location.
We analyzed the data of the SAMURAI08 experiment3) in which the α decay of 16 C∗ is investigated.
We used known unbound states of 12 B and 11 B to compare the two methods. Figure 1 shows Er spectra of
12B reconstructed from the 8 Li + α decay channel. Aclear peak is visible at Er = 2.75 MeV in both spectra, without FDC1 (black line) and with FDC1 (redline). The better resolution with FDC1 allows us to∗1∗2350
3002502001501005000123456Er (MeV)

Fig. 1. Er spectra for 12 B∗ → 8 Li + α. The black line
represents the Er reconstructed without FDC1, while
the red line represents that reconstructed with FDC1.

Counts/10 keV

ﬁnd another peak at Er = 0.90 MeV, while the corresponding peak is not so clear without FDC1. Figure 2
shows Er spectra of 11 B reconstructed from the 7 Li +α decay channel. 11 B has doublet unbound states at
Er = 0.52 and 0.61 MeV with negligible width. Thedoublet peaks are not well separated without FDC1,
while a dip between the doublet peaks can be seen
with FDC1. With √FDC1, the resolution (σ) of Er is
approximately 0.04 Er MeV, while the acceptance isapproximately 80% of that without FDC1.
700
600500400300200
100000.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

1Er (MeV)

Fig. 2. Same as Fig. 1 but for 11 B∗ → 7 Li + α.

In summary, two methods to reconstruct invariant
mass were evaluated. Both methods have advantages
and disadvantages. They should be used as per theintended application.
References
1) T. Kobayashi et al., Nucl. Instrum. Methods B
317 (2013).
2) H. Sato et al., IEEE Trans. Appl. Supercond. 23,4500308 (2013).
3) H. Otsu et al., RIKEN Accel. Prog. Rep. 47, highlight
page xx (2014).

Department of Physics, University of Tokyo
RIKEN Nishina Center- 33 -