1 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. 2 inverse of the transition rate, can range from femtoseconds [1,2] to more than 1015 years  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 . 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 . 3 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 , 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. 4 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 , 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 . 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 . 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 5 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 . 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  and measurements of the first excited state in the neutron-rich shape-transitional system 188W . 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. 6 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 7 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 18 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]. 8 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 . 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 . 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]. 9 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. 10 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. 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