A New Total Absorption Spectrometer for the Study of
Neutron Deficient Nuclei in the A 70 Region
E. Poirier1 and the ISOLDE Collaboration
1 Institut
de Recherches Subatomiques, IN2 P3 -CNRS, F-67037 Strasbourg Cedex 2, France
Abstract. Far from the line of stability, β -decay studies are often a primary, if not the sole, source of information on nuclear
structure. The measured β -strength distribution for a given decay can be used to verify the accuracy of our theoretical
description of the parent nucleus ground state and the states populated in the daughter nucleus. Total absorption spectrometers
based on large NaI crystals are well suited tools to determine the β -strength distribution over the whole QEC decay window.
The newly built spectrometer TAgS, dedicated to such studies, will be presented and its performance and possibilities
discussed in the light of experiments performed at the CERN/ISOLDE mass separator.
INTRODUCTION
For many years, β -decay experiments proved to be a
powerful tool to study nuclear systems and the investigation of unstable nuclei came to the forefront of nuclear physics research. β -decay measurements continue
to provide a wealth of information about fundamental aspects of the nuclear medium and the weak interaction,
such experiments being the first, if not the only, to be
performed to learn about the nuclear structure of new isotopes.
The Gamow-Teller strength distribution B(GT) carries fundamental information about nuclear structure.
However, its experimental determination is not straightforward. If germanium detectors are used, their limited
high energy detection efficiency, combined with the β strength fragmentation at high excitation energy, leads
to large systematic errors in both the total B(GT) and
the B(GT) distribution. An alternative method is to use
the total absorption technique to extract the complete
strength distribution [1]. This method is based on measuring the total energy released in the γ -decay of each
level populated in the β -decay of the parent nucleus. Recent experiments of this type have been carried out with
a total absorption spectrometer at the GSI online mass
separator [2, 3].
The N=Z, A 70 region of the nuclear chart is of particular interest in regards of the nuclear matter properties
especially because of the rapid changes in nuclear shapes
in this region. Close to the proton drip-line, theoretical calculations predict that the Gamow-Teller strength
will be concentrated at high excitation energy in the
daughter nucleus, but will still be accessible through β decay studies, and clear differences in the Gamow-Teller
strength distributions are expected depending on the parent ground state deformation [4, 5].
In order to obtain reliable information on the complete
β -strength, and therefore make valuable comparisons between experiment and theory, total absorption spectrometry together with high resolution measurements is necessary [6]. In the present paper we report on a new total absorption spectrometer specifically designed for the
study of very short-lived nuclei, and presently installed
at the CERN/ISOLDE mass separator.
THE TAGS SPECTROMETER
The new TAgS spectrometer built by a MadridStrasbourg-Surrey-Valencia collaboration consists of
a large, cylindrical NaI(Tl) crystal (38 cm diameter,
38 cm length) with a 7.5 cm hole drilled perpendicularly
to the symmetric axis. The detector, called Lucrecia and
manufactured by Saint Gobain Crystals and Detectors,
is viewed by eight 5 inch photomultiplier tubes monitored by light emitting diodes. A photograph of part of
the spectrometer is presented in Fig. 1.
The whole experimental setup is specifically designed
to study very short-lived nuclei. Exotic nuclei are collected in the center of the crystal and are implanted on a
50 µ m thick aluminum tape that can be moved in order to
limit the build-up of daughter activity. The tape transport
system, functioning under vacuum, routinely operates at
speed of 1 m/s and possibly at higher speeds, up to 2 m/s.
Excellent energy resolution is achieved. Under experimental conditions, resolutions of 7.1% and 5.4% were
measured at 662 keV and 1332 keV, respectively. Experimental total and photopeak efficiencies of 91% and
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
317
spectrum in the energy region beyond the QEC -value
before subtraction (see Fig. 2). Second and higher order
pileup contributions are negligible ( 0.25%) if one
limits the counting rate to 10 kHz or below.
counts
63%, respectively, were measured at 1.33 MeV with a
calibrated 60 Co source. The overall good total detection
efficiency (85% from 500 keV to 8 MeV) and energy
resolution, make the TAgS spectrometer one of the most
powerful total absorption spectrometer ever.
(a)
10 6
10 5
10 4
10 3
10 2
10
FIGURE 1. Photograph of the Lucrecia crystal.
1
In order to disentangle the β and EC-components of
the decay process, coincidences with X- and γ -rays as
well as β particles can also be recorded using ancillary
detectors located close to the collection point. Low energy γ -rays are detected using a germanium telescope
that consists of a 1 cm planar detector backed by a 5 cm
thick coaxial detector. A 2 mm thick plastic (NE102) detector located in front of the germanium telescope is used
to detect the β -particles. Recently a new plastic counter
was designed with particular care taken to minimize the
entrance and exit windows in order to reduce the β energy threshold and the X-ray absorption.
To reduce the contribution of the background due
to the activity (neutrons and gammas) in the experimental hall, the spectrometer and its ancillary detectors
are placed inside a 19 cm thick shielding made of a
polyethylene-lead-copper-aluminum sandwich. The typical background activity measured with the shielding
closed amounts to about 1.0 kHz, mainly coming from
the decay of 40 K present inside the crystal.
β DECAY OF 74 KR AND 76 SR
Analysis Method
To extract an accurate value of the β -strength over
the full energy range, experimental raw spectra must
be corrected for different effects of distortion, the main
one being the pulse pileup. This effect occurs when
two pulses overlap and are interpreted by the analyzing
system as corresponding to a real event. Though analytic
solutions exist to calculate the pileup contribution for
quadratic or gaussian pulse shapes, we use a numerical
pulse pileup correction that requires the knowledge of the
true pulse shape. Details on the correction algorithm and
method can be found in Ref [7]. Finally, the calculated
pileup contribution is normalized to the experimental
318
(b)
10 6
10 5
10 4
10 3
10 2
10
1
0
100
200
300
400
500
Energy (ch)
FIGURE 2. (a) Raw spectra measured for the 76 Sr decay
(solid line) and the daughter (dashed line) and background
(dotted line) activities. The daughter activity spectrum is background subtracted. The intense peak around channel 430 corresponds to the LED monitoring signal. (b) Experimental spectrum (solid line) corrected for the daughter and background
activities. The hatches shows the pileup contribution to be subtracted (see text).
To obtain the β -decay contribution of the parent free
of distortion from the raw spectra, one must in addition
consider and subtract the background and daughter activity contributions as shown in the upper panel of Fig. 2.
The corrected energy spectra, d (counts/ch), can be related to the level feeding distribution f using the equation [8]
d R b f (1)
where R (b) is the response function matrix of the spectrometer which depends on the emitted radiation and γ
branching ratios (b) in the daughter nucleus as well as on
the characteristics of the spectrometer. If one considers
that the response to a particle with a given energy does
not depend on the response to other particles, then the
matrix R can be constructed from the individual response
distributions.
The Monte-Carlo simulation package GEANT 4 [9] is
used to calculate the TAgS response function. The experimental apparatus geometry is carefully modelled and
the light production process in the NaI crystal is included
as described in Ref. [8]. Results are compared to experimental data obtained for different radioactive calibration
sources (22 24 Na, 60 Co, and 88 Y). The response matrix
R is then computed for radiations with energies ranging
from 20 keV to 8 MeV (photons), and from 20 keV to
Qβ (positrons or electrons). Finally, in order to obtain
the feeding distribution f, Eq. (1) is inverted using the
expectation-maximization method based on the Bayes
theorem [10, 11].
B(GT) / 100 keV
Results
0.25
(a)
0.2
0.15
0.1
0.05
0
0.25
(b)
0.2
B(GT) distributions are in good agreement. Our new results allow to extract the β -strength over the full QEC window. Mainly due to the uncertainty on the QEC -value
(3140 60 keV), we present our results up to 3 MeV.
The calculated distributions in Fig. 3 are given in units
of g2A 4π and are scaled by a factor 0.6 to account for the
quenching of the strength experimentally seen in charge
exchange reactions and in β -decay measurements.
One observes that neither of the two calculated distributions can reproduce the experimental B(GT)-values
over the full range of excitation energy. This result is
consistent with calculations [15] where the 74 Kr ground
state is best described as a mixture of prolate and oblate
shapes. A preliminary estimate of the configuration mixing can be obtained looking at the accumulated GT
strength. Figure 4 shows the experimental accumulated
strength compared to the result of Hartree-Fock calculations that uses the SG2 Skyrme type interaction [12, 13].
Excellent agreement is observed over a wide energy
range for a oblate / prolate mixing ratio of 1.5, while neither the prolate nor the oblate solution reproduces the
data. This result is consistent with previous in-beam experiments [16]. The total Gamow-Teller strength to states
with excitation energy below 3 MeV is measured to be
0.67(3). The discrepancy between the experimental results and the calculations at low energy comes from the
high β -feeding of the 306 keV level in 74 Br observed
in the previous experiment [14] and in this work. Further theoretical developments would have therefore to be
done to investigate this disagreement.
In the case of the 76 Sr decay, due to the importance of
the internal conversion process, reliable information on
the β -strength function is not easily accessible through
ΣB(GT) (g A/4π)
0.15
2
0.1
0.05
0
0
500 1000 1500 2000 2500 3000
0.8
0.7
0.6
0.5
0.4
0.3
74
Excitation energy in Br (keV)
0.2
FIGURE 3. Experimental Gamow-Teller strength distribution measured for 74 Kr compared to Hartree-Fock calculations [12, 13] assuming an oblate (a) or a prolate (b) shape for
the 74 Kr ground state.
0.1
0
500 1000 1500 2000 2500 3000
E (keV)
Figure 3 presents the experimental B(GT) strength distribution obtained for the decay of 74 Kr (solid circles)
and compares it to Hartree-Fock calculations performed
by P. Sarriguren et al. [12, 13] assuming a pure oblate
(β0 =-0.15) or a pure prolate (β0 =0.39) shape for the
74 Kr ground state. Compared to the previous determination of the β -strength done up to 0.978 MeV [14], both
319
FIGURE 4. Accumulated GT strength in 74 Kr as a function
of the excitation energy of the daughter nucleus. The solid
line corresponds to a 0.6/0.4 mixing of the oblate and prolate
solutions of HF calculations using the SG2 interaction. The
dashed and dotted lines correspond to the oblate and prolate
solutions, respectively.
Σ B(GT) (gA2/4π)
the EC-component. Figure 5 shows the experimental accumulated B(GT) strength in 76 Sr as a function of the
excitation energy in 76 Rb obtained from the analysis of
the β -component. The data are compared to HartreeFock calculations performed assuming a pure oblate
(β0 =-0.11) or a pure prolate(β0=0.42) shape for the 76 Sr
ground state [12].
Acknowledgments
The authors would like to thank their colleagues of the
IS370 collaboration, engineers and technicians, from the
IReS-Strasbourg and CLRC-Daresbury, whose collaboration has been so valuable during the development of
the TAgS project and during the first experiment. The
collaboration is especially grateful to M. de Saint Simon
from the CSNSM-Orsay for providing us with the beam
transport calculations for the new ISOLDE RC3 beamline. This work has been partly supported by the European Large Scale Facility program under the contract
AEN99-1046-CO2-01/02.
2
1.75
of the B(GT) distribution over two thirds of the QEC window. The trend observed indicates a prolate ground
state deformation for 76 Sr. A more detailed analysis is in
progress.
PRELIMINARY
1.5
1.25
1
0.75
0.5
0.25
0
0 1000 2000 3000 4000 5000 6000
Excitation energy (keV)
REFERENCES
FIGURE 5. Preliminary accumulated GT strength in 76 Sr as
a function of the excitation energy of the daughter nucleus. Experimental data correspond to the lone β -decay of 76 Sr. The
dashed and solid lines correspond respectively to the oblate and
prolate solutions of HF calculations using the SG2 interaction.
Experimental data are shown for energies up to
4.2 MeV. This energy limit is due to the selection of
the β -component, the absorption of the positrons in the
plastic in front of the β -counter and to the uncertainty on
the QEC -value (6090 300 keV). Therefore, in this presentation of our preliminary results we restrict the analysis to the low energies. The complete set of data (β
and EC-decays) is currently being analyzed. For excitation energies below 4.2 MeV, the total Gamow-Teller
strength is found to be 1.35(16) and the data are in agreement with the prolate solution of the Hartree-Fock calculations. This partial analysis supports the results of previous β -decay and β -delayed proton emission measurements in this N=Z nucleus [17, 18].
CONCLUSIONS
A new TAgS spectrometer has recently been successfully
used to study the neutron-deficient Kr and Sr isotopes at
ISOLDE/CERN. We have measured the B(GT) strength
distribution of 74 Kr over most of the QEC -window and
obtained the first experimental signature for shape coexistence in the ground state of this nucleus through β decay studies. We have also studied the β -decay of the
N=Z 76 Sr nucleus. Our preliminary analysis based only
on the β -component allows the study of the behaviour
320
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