Particle identification for neutron rich nuclei ⁶³⁶⁵Cr from knockout reactions

 Nuclear Science and Technology, Vol.8, No. 4 (2018), pp. 20-25 
 Particle identification for neutron rich nuclei 63,65Cr 
 from knockout reactions 
 N. D. Ton1, L. X. Chung1, A. Corsi2, A. Gillibert2, P. D. Khue1, B. D. Linh1, and A. Obertelli2,3 
 1Institute for Nuclear Science and Technology, P.O. Box 5T-160, Nghia Do, Hanoi, Vietnam 
 2Institute of Research into the Fundamental Laws of the Universe (IRFU), CEA Saclay, France 
 3Institut für Kernphysik, Technische Universität Darmstadt, 64289 Darmstadt, Germany 
 Email: nguyenducton92@gmail.com 
 (Received 16 November 2018, accepted 28 December 2018) 
 Abstract: This paper presents the identification for 63,65Cr isotopes as the products of knockout 
 reactions for the first time, measured at RIKEN, Japan, within the framework of the “Shell Evolution 
 And Search for Two-plus energies At RIBF” (RIBF- Radioactive Isotope Beam Factory) project, in 
 short SEASTAR. Based on the Bρ-ΔE-ToF method, these nuclei were well separated. The results will 
 be used in the spectroscopic study on 63,65Cr contributing data to the structural study around the N=40 
 “island of inversion”. 
 Keywords: SEASTAR, BigRIPS, ZeroDegree,neutron - rich nuclei of Cr, PID Correction. 
 I. INTRODUCTION 55,57,59Cr in Refs. [8-11], and 61,63Cr in Ref. 
 [12]. Heavier odd Cr isotopes were not studied 
 The neutron-rich nuclei near N = 40 yet due to the difficulty in the Radioactive 
have attracted much attention due to the Isotope Beam (RIB) production because of 
 68
magicity of 28Ni40. This nucleus has high their very low productive cross sections and 
 +
energy of the first excited 2 1 state [1] and a short lifetimes. As aforementioned, more data 
 + +
low B(E2; 2 1-> 0 1) rate [2] which indicate a on odd-A neutron rich nuclei are requested to 
subshell closure at N=40. However, this 
 verify the f7/2-νf5/2 isospin interaction in the 
subshell closure is very fragile if protons are 
 N=40 “island of inversion”. 
removed from 68Ni. The collectivity in even-
even Fe and Cr isotopes were conducted as the In the SEASTAR project [13], thanks to 
 + the combination of advanced BigRIPS, 
decrease in energies of the 2 1 states [3, 4] and 
 ZeroDegree facilities [14] and MINOS [15], 
the increase in B(E2) values [5, 6] along these 
 DALI2 [16] devices, many new neutron-rich 
isotopic chains. In the odd-A Cr and Fe, as 
 isotopes were produced and their gamma 
well as the odd-odd Co and Mn isotopes, the 
 spectroscopies were detected. The particle 
onset of the collectivity leading to the prolate 
 identification (PID) method and the correction 
deformation has been attributed to the proton 
 procedure for some exotic nuclei with Z>24 
( f7/2)-neutron (νf5/2) attractive interaction [7]. from SEASTAR experimental data were 
The N=40 shell gap is narrower resulting in the discussed in Ref. [17]. However, the 
high probability of neutron excitation out of pf correcting parameters are not universal over 
shell [7]. Up to now, the experimental the whole data. In this paper, the 
 63,65
information on the neutron-rich odd-A Cr identification for Cr24 is reported. Firstly, 
isotopes is scarce. The available data are the PID procedure described in Ref. [17] was 
 ©2018 Vietnam Atomic Energy Society and Vietnam Atomic Energy Institute 
 NGUYEN DUC TON et al. 
applied. One can see in Fig. 1 that the Cr24 was done in addition to enhance the PID 
isotopes are not aligned in atomic number Z. quality and, thus, for the spectroscopic study 
Therefore, this correction for the Cr isotopes later on. 
 Fig. 1. The correlation between atomic number Z versus the ratio of mass and charge A/Q of particles at 
ZeroDegree. The Cr isotopic crowds are marked by a rectangle, in particular by circles for 63,65Cr. This figure 
 is taken from Ref. [17]. 
Fig. 2. Experimental setup. The labels Dn and Fn indicate the positions of dipole magnet and foci, respectively. 
 BigRIPS is from F1-F8. ZeroDegree is from F9-F11. PPACs and MUSICs were used for tracking and 
 identifying purpose. The inset is a sketch of the main detectors MINOS and DALI2 with an illustration for 
 66 65
 Mn(p,2p) Cr. Zv is the vertex point. More details are explained in text. 
 II. EXPERIMENTAL SETUP AND PID primary target at the F0 focal plane of the 
 METHOD BigRIPS separator. 
 The experimental setup is described in The secondary beam was created as the 
Fig. 2. A 238U primary beam with the mean products of fragmentation reactions, 
intensity of 12 pnA was accelerated up to 345 transported to the user location at the F8 focal 
MeV/u energy by the Superconducting Ring plane and interacted with MINOS LH2 active 
Cyclotron (SRC) before impinging on a 9Be target [15] to induce knockout reactions. The 
 21 
 PARTICLE IDENTIFICATION FOR NEUTRON RICH NUCLEI 63,65Cr 
target was a cylindrical cell (see the inset on was measured by MUlti-Sampling Ionization 
Fig. 2). Its effective thickness and length were Chambers (MUSIC) [17]. This method is 
735(8) mg/cm3 and 102(1) mm [18], called Bρ–ΔE–ToF method. More details are 
respectively. The secondary beam’s energy at discussed in Refs. [14, 17]. 
the entrance of the MINOS target was about 
260 MeV/u. Prompt gamma de-excitation from III. PID CORRECTION AND RESULT 
residues was detected by the DALI2 NaI 
 During the SEASTAR experiments, 
crystals [16] surrounding the target. Measuring 
 the PID correction at BigRIPS was carried 
gamma-ray energies emitted from particles of 
 out by the BigRIPS experts. At ZeroDegree, 
interest were the aiming information of the 
 the correction needed to be performed 
SEASTAR experiments. according to the particles of interest. For 
 The PIDs for the secondary beam at 63,65Cr identification, the A/Q values were 
BigRIPS and residue at ZeroDegree were corrected following the procedure in Ref. 
performed via the correlation between A/Q and [17]. The argument is that the dependence 
Z. These quantities were determined according of A/Q on position (X) and angle (A) 
to time-of-flight (ToF), particle trajectory’s measured by the PPACs should be removed 
radius (ρ), and energy loss (ΔE) using below because this is an intrinsic value of a 
equations: particle. As the results, the A/Q vs. X (A) 
 plot should be a straight-like line. The A/Q 
 L 63
c correction of Cr was made by adding(1) 
 ToF
 higher order dependences on X and A as 
 Bc follow: 
 AQ (2) 
  mu -5 -8
 A/Qcorrect = A/Q + 35x10 . F9X – 18x10 . 
 2 -9 3 -4 -5
 42 2 (F9X) + 6x10 . (F9X) + 7x10 . F9A – 2x10 . 
 dE 4 eZ 2mve 22
 E Nz ln ln(1  ) , 2 -4 -6 2
 2 (F9A) +2x10 . F11X + 16x10 . (F11X) – 
 dx me v I
 5x10-7. (F11X)3 + 10-4. F11A + 10-5. (F11A)2 
(3) 
 (4) 
 where, B and L are magnetic field and 
the flight-path length, respectively. υ is particle Where, F9(11)X and F9(11)A denote the 
 position and angle, respectively, measured at 
velocity, β = υ/c, γ=1/√ , c is the light 
 the F9(11) focal plane. A comparison between 
velocity. m = 931.494 (MeV) is the atomic 
 u uncorrected (upper panels) and corrected 
mass unit. m and e are the electron mass and 
 e (lower panels) A/Q vs. X(A) correlation is 
the elementary charge. N, z and I are the 
 shown in Fig. 3. Because the statistic for 65Cr is 
atomic density, atomic number and mean 
 low (see the crowds at A/Q 2.71 in Fig. 3). 
excitation potential of the material, 
 The PID quality was not significantly 
respectively. The ToF was measured by two 
 improved by the correction. Therefore, the 
thin plastic scintillators installed at L distance 
 above correction for 63Cr was also used for the 
apart [17]. The magnetic rigidity (Bρ) was 
 case of 65Cr. 
conducted by using positions and angles 
measured by position-sensitive Parallel Plate As mentioned before, the Z-correction 
Avalanche Counters (PPAC) [17]. Finally, ΔE is necessary because it is not aligned for all 
 22 
 NGUYEN DUC TON et al. 
Cr isotopes as shown in Fig. 1. In principle, The result, Zcorrect, is presented in the right 
Z is an intrinsic value of nuclei. Therefore, panel of Fig. 4 with the centroid of 23.994(4) for 
it is not dependent on the measured velocity Cr isotopes improving from that of 23.862(5) 
divided by the speed of light of these without correction shown in the left panel. The 
particles (β). Similarly to the correction for PID quality is estimated by the resolution of the 
 Z distribution which is the projection of these 
A/Q, higher order dependences on β was 
 plots on the Z axis. The resolution in  after the 
introduced to remove the dependence as: 
 correction is 0.14, much better than that of 0.19 
Zcorrect= 0.87Z + 6.43β – 0.52 (5) without correction. 
 Fig. 3. The dependence of A/Q versus position (X) and angle (A) at F9 and F11. The upper panels are for 
 uncorrected A/Q. The lower panels are for the corrected ones. The correction was performed to select 63Cr 
 events marked by the rectangles. 
 Fig.4. Correlation between atomic number Z and the velocity divided by the speed of light 
 β before (left) and after (right) the correction. 
 23 
 PARTICLE IDENTIFICATION FOR NEUTRON RICH NUCLEI 63,65Cr 
 Taking into account both A/Q- and Z- resolution () is 0.114 and 0.118 for 63,65Cr, 
corrections in Eqs. (4) and (5), the PID plot is respectively. Before the correction these values 
displayed in Fig. 5a. The projection of this plot are 0.180 for 63Cr and 0.156 for 65Cr (obtained 
on the absciss is shown in Fig. 5b. The A/Q from Fig. 1). 
 Fig. 5. Particle identification via Z vs. A/Q for 63,65Cr is marked by the circles in panel a). Panel b) is the 
 projection of panel a) on the absciss. 
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