Nuclear Science and Technology - Volume 9, Number 3, September 2019

still controversial. The first study 
LE XUAN CHUNG et al. 
49 
observed the 367 keV transition [9]. This level 
was thought to be isomeric state 9/2
+
 decaying 
to the ground state 5/2
-
 via a M2 transition, see 
Figure 1.a. Afterwards, M. Sawicka et al. 
reported that beside 367 keV they also 
observed 387 keV transition from 
67
Fe isomer 
[10]. The 387 keV level was concluded 
isomeric and the 367 keV was in the cascade of 
this isomer when it decays to the ground state. 
The branching ratio (I ) of these two 
transitions was I(387)/I(367)=0.11(2) [10]. 
The summary of the study in Ref. [10] is 
presented in Figure 1.b. Recently, an important 
conclusion was reported by J.M. Daugas et al. 
in Ref. [11]. Where, the 367 and 387 keV 
prompt transitions of 
67
Fe were observed in the 
-decay of 67Mn. It meant that the 387 keV 
level is not isomeric. Moreover, the ratio of 
these transitions were determined to be 
I(387)/I(367)=0.77(26), different from the 
above value of 0.11(2) obtained in Ref. [10]. 
This led to the conclusion that the initial 
isomers of the 367 and 387 keV transitions 
may be different. No information concerning 
the direct feeding of the isomeric states was 
extracted by the 
67
Mn -decay experiment in 
Ref. [11]. Together with the studies in Refs. [9-
10], the isomeric levels were proposed to be 
above 387 keV, which decays via highly 
converted transition or  transition of too low 
energy to be observed. Therefore, the isomeric 
levels were proposed to be less than 420 keV. 
The explanation for the measurement in Ref. 
[11] is shown in Figure 1.c. According to the 
calculation [11], the 2 possibly isomeric states 
are 5/2
+
 and 7/2
+
. The ground state is 1/2
-
obtained from Ref. [13]. In addition to the 
gamma spectrum, the half-life of the isomeric 
state which decays to the 367 keV level was 
determined with large discrepancy, 43(30) µs 
in Ref. [9] and 75(21) µs in Ref. [10]. 
In this paper, a study of the above 
mentioned isomers of 
67
Fe is reported. First, we 
discuss the delayed-gamma-ray energy 
spectrum. Afterwards, we discuss the half-life 
of the isomeric state based on the time-
dependence of the observed events. The 
experiment was performed within the 
framework of the “Shell Evolution And Search 
for Two-plus energies At RIBF” (RIBF- 
Radioactive Isotope Beam Factory) project 
[14], in short SEASTAR. 
II. EXPERIMENTAL METHOD 
A 
238
U primary beam with the mean 
intensity of 12 pnA was accelerated up to 345 
MeV/u energy by the Superconducting Ring 
Cyclotron (SRC). Afterwards, it bombarded a 
9
Be primary target at the F0 focal plane of the 
BigRIPS [15] separator to produce the 
secondarily cocktail beam. The secondary 
beam was transported to the user location at the 
F8 focal plane and interacted with MINOS [16] 
LH2 active target. Prompt gamma de-excitation 
from residues was detected by the DALI2 [17] 
NaI crystals surrounding the MINOS target. 
Measuring prompt gamma-ray energies was 
the main purpose of the SEASTAR 
experiments. The experimental setup for this 
purpose is shown in Figure 2, and described in 
details in Refs. [18-19]. 
For the delayed-gamma study, an 
additional detector setup, EURICA (Euroball-
RIKEN Cluster Array) [20], was located at the 
end of the experimental setup described in 
Figure 2, at the F11 focal point. This gamma-
ray detector array consists of 84 high-purity 
germanium crystals (HPGe) subdivided into 12 
7-crystal clusters distributed in three different 
rings at 51
o
 (five clusters), 90
o
 (two clusters), 
and 129
o
 (five clusters) relative to the beam 
axis at a nominal distance of 22 cm from the 
center. The energy resolution of the HPGe 
crystal detector was better than 3 keV at Eγ=1.3 
MeV with a photo-peak efficiency of about 
STUDY ON THE ISOMERIC DECAY OF NEUTRON-RICH ISOTOPE 
67
FE 
50 
15% for Eγ= 662 keV [20]. The beam was 
stopped in a thick-aluminium plate centered in 
the arrays. A picture of the stopper surrounded 
by the EURICA clusters is shown in Figure 3. 
Fig. 2. Experimental setup for prompt-gamma detection in SEASTAR experiments. The label Fn indicates 
the position of foci. 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 
68
Fe(p, pn)
67
Fe. Zv is the vertex point. EURICA was located at F11 for the decay study. 
Fig. 3. Illustration of EURICA detector with a thick-aluminium-plate stopper at the center. 
III. DATA ANALYSIS AND RESULTS 
For the present isomeric study, the 
identification for the implanted 
67
Fe ions in 
the aluminium stopper was considered. This 
required the particle identification (PID) 
from the ZeroDegree spectrometer [15], in 
other words the PID for outgoing particles 
from the MINOS target. EURICA detected 
the gammas emitted from implanted ions. 
The independent ZeroDegree and EURICA 
LE XUAN CHUNG et al. 
51 
data was merged according to their time 
stamp with an additional in-beam trigger 
from DALI2 for separation of the data into 
different reaction channels, or analyzed 
independently for high-statistics total 
isomer-yield. 
Due to the inclusion of BigRIPS data via 
the DALI2 data stream, it was possible to 
identify the relative ratio of 
67
Fe isomeric-
decay intensity from the different channels 
[21]. The channel PID has been discussed in 
details in Ref. [18-19]. As an example, the 
68
Fe(p, pn)
67
Fe identification is shown in 
Figure 4. 
Fig. 4. Particle identification via atomic charge (Z) versus mass-to-charge ratio (A/Q). The marked crowns are 
identified for 
68,67
Fe at BigRIPS and ZeroDegree, respectively, for 
68
Fe(p,pn)
67
Fe channel. 
Fig. 5. Delayed gamma energy spectra of 
67
Fe from (p,2p) and (p,pn) channels detected by EURICA. 
STUDY ON THE ISOMERIC DECAY OF NEUTRON-RICH ISOTOPE 
67
FE 
52 
The delayed gamma energy spectra of 
67
Fe from (p,2p) and (p,pn) channels are 
presented in Figure 5. In both cases, the 
gamma of 367 keV are clearly observed. 
For higher statistics, the trigger 
without DALI2 gamma detection was used. 
In this case, only the ZeroDegree trigger was 
consider to identify implanted 
67
Fe ions into 
aluminium thick-plate stopper, see Figure 3. 
The total isomeric spectrum is presented in 
Figure 6. Two lines are observed at 367 and 
387 keV. The relative ratio I(387)/I(367) is 
determined to be equal to 0.126(3) in 
agreement with the value 0.11(2) reported in 
Ref. [10]. This might be from the fact that 
the implanted 
67
Fe ions in the present study 
and Ref. [10] were produced by knockout 
reactions of an approximately 250 MeV/u 
cocktail beam on a proton target and by 
fragmentation of the 60 MeV/u 
86
Kr beam on 
nat
Ni target, respectively. As the result, the 
67
Fe isomers were fed by these similar 
mechanisms that was not the case of the 
67
Mn -decay experiment in Ref. [11]. 
Fig. 6. The total isomer-yield gamma energy spectrum of 
67
Fe detected by EURICA. 
From Figure 5 and 6, it is seen that the 
387 keV line is visible only in the total 
isomer-yield gamma spectrum. This is 
explained that either the particular (p, 2p) 
and (p, pn) reactions do not feed the isomer 
which decays to the 387 level or the statistic 
is not enough. 
The decay curve was built by gating on 
367 keV in the EURICA HPGe array and 
plotting the time-difference between the HPGe 
and the final BigRIPS scintillator, see Figure 7. 
This curve was fitted with an exponential 
function to get the half-life of the decay. From 
this we obtained a half-life of T1/2=150(10) µs. 
Compared to the previous result, the current 
value is about twice the most recently reported 
value of 75(21) µs [10]. This discrepancy could 
be related to the time range in the current 
experiment, up to 100 µs, while the range was 
only 45 µs in Ref. [10]. For a long half-life, 
this leads to a bias of the fitting results for too 
short time-ranges. Moreover, our statistics are 
much higher than previous work [10] which 
also influences the fitting results. 
LE XUAN CHUNG et al. 
53 
Fig. 7. Decay curve of the isomer via 367 keV state in 
67
Fe. The points with error bar are experimental 
data. The solid line is the fitting exponential curve. 
IV. CONCLUSIONS 
In this paper, the study on the 
isomeric decay of neutron-rich isotope 
67
Fe 
is reported. The gamma-delayed energy 
spectra of this isotope were recorded from 
68
Co(p, 2p)
67
Fe and 
68
Fe(p, pn)
67
Fe channels 
as well as 
238
U fission. The spectra obtained 
from these first 2 channels clearly show the 
peak at 367 keV. While the total isomer-
yield spectrum clearly presents 2 lines at 
367 and 378 keV. The invisibility of 378 
keV line was explained either the (p, 2p) 
and (p, pn) channels did not feed these 
isomer which decays to ground state via 387 
keV gamma emission or the statistics is not 
enough. The half-life time of the isomer 
which decays to the 367 keV level was 
measured to be equal to 150(10) µs, 
significantly longer than previously 
measurements. 
This work is partly supported by 
VINATOM via the Grant No. 
ÐTCB.09/17/VKHKTHN. 
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FE 
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