exotic nuclei structure and reaction noyaux exotiques ... - IPN - IN2P3

First fast-timing timing experiment at ALTO: nuclear-state half-life
life
measurements in 137,139 Cs and 138 Ba.
IPNO Participation: B. Roussière and the DA-ALTO group
Collaboration : CSNSM-Orsay, Tandar CNEA-Buenos Aires, INRNE BAS-Sofia
Le dispositif de ‘’fast-timing’’ développé pour mesurer, auprès d’ALTO, les vies moyennes des états excités
des noyaux dans la gamme de quelques dizaines de ps à quelques ns a été testé en ligne. Les mesures
ont été réalisées sur les niveaux excités de 137,139 Cs et 138 -
Ba obtenus lors de la décroissance des
noyaux 137-139 Xe. Les noyaux de Xénon riches en neutrons étaient produits par la photo-fission de 238 U induite
en bombardant une cible épaisse de carbure d’uranium par un faisceau d’électrons (E = 50 MeV, I ≤
500 nA). Les résultats obtenus sur la masse 138 ont permis, puisque les vies moyennes dans 138 Ba sont
bien connues, de caractériser très précisément le dispositif expérimental et de valider les techniques d’analyse.
Les vies moyennes des cinq premiers états excités de 139 Cs ainsi que celle du premier état excité de
137 Cs ont été mesurées pour la première fois.
Introduction
Nuclear-state half-lives, giving access to transition
probabilities, provide direct insight into the structure
of the nucleus and offer one of the most stringent
test of the nuclear models. The fast-timing
technique is well adapted to measure half-lives in
the sub-nanosecond range. Developed at first with
BaF 2 crystals [1], this method knows nowadays a
revival of interest due to the advent of LaBr 3 scintillators
[2] that have good spectroscopy properties
(high scintillation light yield, fast emission and the
best energy resolution among all scintillators). In
the frame of our lanthanide nuclei study program at
ALTO [3], we have developed a fast-timing experimental
set-up and optimized, by off-line tests with
radioactive sources, its efficiency for measuring
half-lives in the [100 ps - some ns] range by the
slope method [4]. To test on-line this fast-timing
system, we have performed measurements in
137,139 Cs and 138 Ba nuclei obtained from -decay of
137,138,139 Xe that are well produced at PARRNe,
either using the TANDEM deuteron beam or the
low-intensity ALTO electron-beam.
Experimental procedure
137-139 Xe were produced by photo-fission in a thick
uranium carbide target using a 100 to 500 nA, 50
MeV electron beam. The target, composed of 122
UC x disks of 14 mm diameter and 1 mm thick, was
associated with a MK5 hot plasma ion source. The
ions were extracted under 30 kV, mass-separated
and collected on a mylar-aluminium tape. The collected
radioactive sources were then transported in
front of the detection device consisting of one Ge
detector and three scintillators (one PilotU plastic
for detection, one BaF 2 and one LaBr 3 for detection,
all three with 25.4 mm diameter and 3 mm,
30 mm and 10 mm length respectively).
The yield of 139 Xe was measured to be 2×10 5 ions/
s. The measurement cycles were chosen in order
to optimize the counting rates: for A = 139 t collection =
t counting = 40 s, for A = 137 t collection = t counting = 4 m
and for A = 138 t collection = 7.5 m and t counting = 30 m.
Experimental results
In 138 Ba, except the level located at 1898.7 keV
(T 1/2 = 2.164(11) ns), the excited states most fed
–
by the decay of 138g Cs have very short half-lives
[5]. Therefore, in view of the time properties of the
detectors used, the transitions de-exciting these
levels can be considered as prompt transitions and
the 138 mass provides an ideal candidate for calibration
purpose. Thus measurements on mass 138
allowed us i) to validate our experimental setup for
the determination of half-lives in the ns range
since, for the level located at 1898.7 keV, we have
measured T 1/2 = 2.11 (5) ns, in excellent agreement
with the value reported in the literature; and
ii) to precisely characterize our fast-timing set-up,
in particular by the determination of the promptcurve
properties (position and width). Figure 1
shows the displacement of the center of gravity of
the prompt curve as a function of the -ray energy
when time measurement is performed between the
and LaBr 3 or BaF 2 detectors or between the
LaBr 3 and BaF 2 detectors; one can note in this
case the excellent agreement between the values
observed experimentally (red dots) and those extrapolated
from - measurements (yellow grid),
which points out the consistency of measurements
Figure 1
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