Using LaBr3(Ce) detectors for precision lifetime measurements of

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Using LaBr3(Ce) detectors for precision lifetime measurements
of excited states in ‘interesting’ nuclei*
P. H. Regan
Department of Physics, University of Surrey, Guildford, GU2 7XH, UK &
National Physical Laboratory, Teddington, Middlesex, TW11 OLW, UK
E-mail: [email protected]
www.surrey.ac.uk
Precision measurements of electromagnetic transition rates provide accurate inputs into
nuclear data evaluations and are also used to test and validate predictions of state of the
art nuclear structure models. Measurements of transition rates can be used to ascertain or
rule out multipolarity assignments for the measured EM decay, thereby providing spinand parity-difference information for states between which the EM transition takes place.
This conference paper reports on a measurements of electromagnetic transition rates
between excited nuclear states using coincidence 'fast-timing' gamma-ray spectroscopy
with cerium-doped, lanthanum-tribromide (LaBr3(Ce)) detectors. Examples of recent
precision measurements using a combined LaBr3-HpGe array based at the tandem
accelerator, Bucharest, Romania include studies around the N=20 and N=82 shell
closures using stable-beam induced fusion-evaporation reactions; and the evolution of
nuclear deformation around in neutron-rich Hf, W and Os nuclei using 7Li-induced lightion transfer reactions. This paper also presents the ongoing development of a new multidetector LaBr3(Ce) array for future studies of exotic nuclei produced at the upcoming
Facility for Anti-Proton and Ion Research (FAIR) as part of the NUSTAR–DESPEC
project, and reports on the pre-NUSTAR implementations of detectors from this array to
study electromagnetic transition rates in neutron-rich fission fragments at ILL-Grenoble,
France and RIBF at RIKEN, Japan.
Keywords: Gamma-ray spectroscopy; nuclear structure physics; electromagnetic
transition rates; LaBr3(Ce) scintillation detectors.
1. Introduction
1.1. Measurements of decay rates from nuclear excited states.
The measurement of electromagnetic transition rates from nuclear excited states
gives direct information on the underlying structural makeup of the initial and
final wave functions. Mean lifetimes of excited nuclear states, which are the
*
This work is supported by the STFC (UK) and UK National Measurement Office.
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inverse of the transition rate, can range from femtoseconds [1,2] to more than
1015 years [3] depending on the energy of the transitions and the wave function
characteristics, including spins and parities of the initial and final states. The
electromagnetic transition rate, which corresponds to the probability of decay
per unit time, can be written as [4].
1
8
1
ħ 2
1 ‼
ħ
: →
Thus, measurement of the lifetime of decay for an electromagnetic decay of a
known decay energy gives direct information on the reduced matrix element
which links the initial and final quantum states via that particular EM multipole
operator.
A range of measurements techniques can be applied to the determination of
accurate lifetime evaluations for excited nuclear states [1,2] depending on the
time regime involved. For transitions in the tens of nanosecond regime and
higher, the (fast) electronic timing method has been well established [5,6].
2. LaBr3(Ce) detector dimensions, characteristics and performance for
use in current multi-detector arrays.
The idealized gamma-ray spectrometric detectors would have a combination of
excellent full-energy peak resolution, together with good full-energy peak
detection efficiency and excellent timing characteristics. No current detector
material is superior in each of these categories, with the energy resolution
associated with multi-detector arrays of hyper-pure germanium detectors [7–9]
and the timing capabilities for gamma-ray detection from BaF2 scintillation
detectors [6,10,11] representing the contemporary limits in these areas for
standard nuclear spectroscopic measurements.
The recent developments in the use of novel halide scintillation materials,
and in particular, cerium-doped lanthanum tri-bromide (LaBr3(Ce)) has enabled
a material with reasonable full-energy peak resolution and excellent timing
characteristics to be used in a range of nuclear spectroscopy measurements.
Among the first measurements which utilized LaBr3(Ce) detectors for subnanosecond timing measurements of discrete states, were studies of exotic
neutron-rich exotic species by Mach and collaborators following beta decay
using the Advanced Time-Delayed (t) method for fast-beta-gamma
coincidence measurements [12].
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Following studies with  delayed sources, a number of studies of
lifetimes of excited states from nuclei produced via ‘in-beam’ light-ion-induced
fusion-evaporation and light-ion transfer reactions have been reported, in
particular a following the ongoing development of the mixed, combined HpGeLaBr3 spectrometer array ROSPHERE, based at the IFIN-HH laboratory,
Bucharest [13-18].
An array of LaBr3(Ce) detectors, ultimately for use with as part of the
DESPEC (“DEcay SPECtroscopy”) collaboration within the NUSTAR collaboration at the future FAIR facility is currently under development [19,20] with an
initial design of 32 detectors surrounding the final focal plane of the Super
Fragment Separator at FAIR. For this initial design, 32 cylindrical LaBr3(Ce)
detectors, each of length 2″ and diameter 1.5″ and coupled to a Hamamatsu
R9779 fast-timing photo-multiplier tube, have been purchased via the UK
Science and Technology Facilities Council (STFC) NUSTAR grant. Figure 1
shows a photograph of three such detector units.
Fig. 1. Three 1.5″ diameter by 2″ long LaBr3(Ce) detectors.
These detectors have been characterized using a range of source
measurements at the radiation laboratories at the University of Surrey, with
values for timing FWHM down to 210 ps measured for gamma-ray energies of
1332 keV using a 60Co coincidence source [20], and typical full-energy peak
resolutions of ~3% for 662 keV [19,20]. Figure 2 shows a typical source
spectrum obtained using one of the LaBr3(Ce) detectors for a 137Cs source.
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In addition to the expected full-energy peak associated with the 661.7 keV
line associated with the decay of the metastable I=11/2- spin/parity state in
137
Ba populated via the 137Cs - decay, and the barium K X-rays between 31 and
38 keV emitted following the competing internal conversion branch of the 11/2state internal decay [21], the spectrum also clearly exhibits lines associated with
the internal radioactivity from the decay of 138La which is present in the detector
material itself. This primordial radionuclide makes up approximately 0.09% of
naturally occurring lanthanum and has a radioactive half-life of 1.0×1011 years,
decaying by both electron capture (65.6%) and – decay (34.4%) to the yrast
spin/parity 2+ states in 138Ba (at 788.7 keV) and 138Ce (1435.8) respectively [22].
The detector material also exhibits internal radioactivity associated with the
natural decay chain of 227Ac, which has a 22 year half-life and occurs in nature
as a member of the 235U (actinium) natural decay series [23]. Chemically
speaking, actinium is similar to lanthanum and it is present in trace amounts in
all LaBr3(Ce) detectors. Actinium-227 decays to stable 207Pb via five discrete
alpha decays which are also measured in the internal detector response, albeit
with a quenched energy compared to direct measurements of external gammarays of the same energy.
Fig. 2. Source spectrum of a single LaBr3(Ce) detector with a 137Cs source, Note the presence of the
internal radioactivity associated with direct decays to the lowest lying 2+ states in 138Ce and 138Ba
from the internal 138La radioactivity.
227
Figure 3 shows the effect of the internal radioactivity from the 138La and the
Ac decay series on the internal response of the LaBr3(Ce) detector itself. Also
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shown for reference is the measured gamma-ray emissions from one of the
LaBr3(Ce) detector in a passively shielded HpGe detection system within the
Environmental Radiation Laboratories at the University of Surrey [24]. As for
signals measured which arise from the LaBr3(Ce) internal radioactivity sources
in external detectors, only the discrete gamma rays from the decay of the excited
states populated in the 138La are observed. The internal radioactivity associated
with such detectors is of the order of ~1 Bq/cm3 and while this gives rise to an
easily measureable background signal (which can also be used as internal
calibration signal), in the presence of stringent coincidence conditions with other
correlated signals from radioactive decay and/or gamma rays which occur within
10 ns or so of the signal of interest, this level of background activity does not
affect most spectroscopic studies.
The timing responses associated with spectroscopy-grade LaBr3(Ce)
detectors used in coincidence mode has been established down to the ~10 ps
level by Regis and collaborators [25,26] using the mirror symmetric centroid
shift method. Here, off-line software corrections for the full-energy peak
dependent, prompt timing response for each detector are applied using empirical
information from a well-defined ‘prompt’ coincident source, such as 152Eu.
Regis and collaborators have demonstrated that, that if sufficient coincident
statistics can be measured from a point-like, localized source, discrete level
lifetimes at the tens of picoseconds levels are measureable using LaBr3(Ce)
coincident fast-timing.
3. Examples of coincidence experiments using LaBr3(Ce) fast-timing
detectors at IFIN-Bucharest
A number of experiments have taken place using a coincidence array consisting
of LaBr3(Ce) and HpGe detectors at the IFIN-HH laboratory in Bucharest,
Romania. These have included studies of magnetic quadrupole (M2) singleparticle transitions approaching the N=20 magic number [15,18], measurements
of transitions rates between competing shell-model configurations in the N=80
(2 neutron-hole) system 136Ba [16] and measurements of the first excited state in
the neutron-rich shape-transitional system 188W [17].
An example of the spectral quality is from these experiments is shown in
Fig. 4 below from the 18O+18O fusion evaporation reaction used to study decays
from excited states in 34P via the pn-evaporation channel [15,18]. The channel
corresponding to the nucleus of interest represented a relatively small fraction of
the total fusion cross section in this experiment, estimated to be a few tens of
millibarns from a total fusion cross section approaching a barn.
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Fig. 3. (Top:) Internal radioactivity as measured in a passively shielded 2″ × 1.5″ diameter
LaBr3(Ce) detector for 12 hours. Note the presence of signals arising from both 138La decay and
members of the 227Ac decay chain. (Bottom:) LaBr3 detector spectrum from raw detector placed in
passive graded lead shield showing the internal radioactivity from the 138La decays.
Figure 4 shows the total projection of the LaBr3(Ce)–LaBr3(Ce) energy
coincidence matrix from this experiment, and identifies where in this spectrum
the full energy peaks corresponding to the 429 and 1048 keV transitions of
interest in 34P reside. The presence of such discrete peaks is not obvious in the
total projection, but once random and Compton background subtracted energy
coincidence gates were applied to the matrix, clear coincidence peaks associated
with previously-identified discrete energy transitions for the cascade built on the
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spin/parity 1+ ground state of 34P are clear. By placing two-dimensional fullenergy gating conditions on these transitions, the measured time-difference
distribution between pairs of transitions can be determined, with the mean shift
of this distribution away from the prompt coincident response, corresponding
the mean-lifetime of the intermediate state between the feeding and decay
transition.
Fig. 4. (Left:) Total projection of LaBr3-LaBr3 gamma-ray energy coincidence matrix from the
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O+18O spectrum reaction from Refs. [15,18]. (Right:) A background-subtracted coincidence gate
on the 429 keV transition in 34P from the LaBr3-LaBr3 matrix clearly identifies the mutually
coincident transitions at 1048 and 1876 keV.
Fig. 5. Background-subtracted time-difference spectra between 429 and 1048 keV transitions (top)
and prompt decay between the 429 and 1876 keV transitions (bottom) in the decay of 34P. The
lifetime of the lowest-lying spin/parity 4– state of 2.0(2) ns is clearly evident [15,18].
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Figure 5 shows the effect of gating on a LaBr3(Ce)–LaBr3(Ce) energy-timedifference 3-D coincidence matrix, across states with different lifetimes, highlighting the precision available for in-beam studies with LaBr3 detectors. The
presence of the T1/2 = 2.0(2) ns lifetime for the spin/parity 4- state in 34P is
evident from these data, and compares with the prompt signal distribution
associated with the lifetime of the yrast 2+ state in the same nucleus which has a
much shorter lifetime, in the few ps regime [15]. This was used to establish an
almost pure M2 decay for this state and experimentally verify the tentative
negative parity assignment for this state, corresponding to a particle excitation
into the f7/2 intruder orbitals across the N=20 shell closure.
4. Other nuclear spectroscopy arrays incorporating LaBr3(Ce)
Detectors
The thirty-two 1.5″ diameter and 2″ long LaBr3(Ce) detectors which have been
purchased by the UK Fast-Timing Project within the FATIMA collaboration,
have already been incorporated into a number of other combined HpGeLaBr3(Ce) detection systems and used in structural studies of exotic nuclei.
4.1. LaBr3(Ce) used in coincidence with EURICA at RIKEN
Eighteen of the LaBr3(Ce) detectors used in conjunction with former RISING
stopped beam germanium detector array. Figure 6 shows a CAD design drawing
of a section of this combined array. Known as EURICA, this array consists of
twelve seven-element germanium cluster detectors and is positioned at the focal
plane of the Big RIPS fragment separator at the RIKEN laboratory, Japan
[27,28]. EURICA has been used to measure a range of neutron-rich nuclei
following production via projectile fission. For example, fast-timing
coincidences between the LaBr3(Ce) detectors in this array and a fast – particle
decay signal at the focal plane have allowed the determination of the lifetimes
(and associated transition quadrupole moments, in a range of neutron-rich
zirconium isotopes following the – decay of their yttrium parent nuclei [29].
4.2. EXILL with FATIMA at Grenoble
Sixteen LaBr3 detectors (8 from the STFC funded group and 8 more from the
IFK Koln and TU Darmstadt groups) were used together with the EXOGAM
HpGe clover detectors for experiments using thermal neutrons at the ILLGrenoble facility in 2013. These experiments included in-beam spectroscopy of
prompt fission fragments produced following cold-neutron-induced fission on a
235
U target placed in the centre of the array, including 235U(n,f) [30,31].
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Fig. 6. CAD drawing of partial EURICA gamma-ray array, showing 12 of the 18 LaBr3 detectors
used in its configuration.
Fig. 7. Photograph of the target position of the EXILL+FATIMA setup at ILL-Grenoble. This array
combined the HpGe clover detectors in the EXOGAM array with 16 LaBr3 detectors for fast-timing
measurements of prompt fission fragments.
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5. Summary and Conclusions
Arrays for nuclear spectroscopic measurement which include the capability for
coincidence spectroscopy between discrete gamma-ray transitions using halide
scintillation detectors are rapidly becoming mainstream tools in nuclear
structure physics research worldwide. The ability of LaBr3(Ce) detectors to give
suitably fast timing information coupled with acceptable energy resolution
makes them an ideal tool for discrete nuclear spectroscopy studies. This is
particularly true in cases where (a) the spectral level-density is not too high, for
example following tagged β-decays of exotic nuclei produced at radioactive-ion
beam facilities and/or (b) when used in coincidence with a complementary array
of hyper-pure germanium detectors which can be used as a channel/decay path
selection device to isolate the particular scintillator coincidences across a
nuclear level of particular interest. Other related, (more) ‘background free’
detector materials, such as CeBr3 are also of interest in this area of research [32]
which can also be transitioned from fundamental research studies, to practical
measurements of discrete radionuclide releases for health physics, nuclear power
and nuclear waste management purposes [23].
Acknowledgments
The author is grateful for ongoing discussions and contributions from many
colleagues within the UK and wider FATIMA fast-timing collaborations,
including Alison Bruce, Zsolt Podolyák, Christopher Townsley, Peter Mason,
Thamer Alharbi, Frank Browne, Nicu Marginean (and the entire nuclear
spectroscopy group at IFIN-Bucharest), Luis Fraile, Gary Simpson, Aurelien
Blanc, Gilles De France, Jean-Marc Regis, Steven Judge and Jan Jolie. The
author is supported by funding grants from the STFC (UK) and the UK National
Measurement Office.
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