Developments using Fast-Timing Scintillation Detectors for Precision Nuclear Spectroscopy Paddy Regan 1,2 & Robert Shearman 1,2,a 1 Department of Physics, University of Surrey, UK 2 National Physical Laboratory, Teddington, UK Funded by STFC (UK) & UK National Measurement Office & a NNL-NDA student bursary.

OUTLINE EM Transition rates – they matter. Limits with HPGe for fast-timing; 10 ps <  < 1 ns LaBr 3 detectors, characteristics, limitations and applications Some mixed-array (ROSPHERE) examples: N=80 ; 188 W. The FATIMA array for FAIR; what to do in the meantime? FATIMA + EXILL (e.g., 100 Zr) ; FATIMA + EURICA (e.g., 104,6 Zr) Upcoming arrays including LaBr 3 (Ce) for fast-timing and application. GAMMASPHERE using a 252 Cf source. NANA for traceable standards.

Nuclear EM transition rates between excited states are fundamental in nuclear structure research. The extracted reduced matrix elements, B( L) give insights e.g., Single particle / shell model-like: ~ 1 Wu (NOT for E1s) Deformed / collective: faster lifetimes, ~10s to 1000s of Wu Show underlying symmetries and related selection rules such as K-isomerism: MUCH slower decay rates ~ 10 -3→9 Wu and slower).

Some quick revision on extracting (nuclear excited state) lifetimes… Assuming no background contribution, the experimentally measured, ‘delayed’ time distribution for a  t measurement is given by: P(t’-t 0 ) is the (Gaussian) prompt response function and , where  is the mean lifetime of the intermediate state. See e.g., Z. Bay, Phys. Rev. 77 (1950) p419; T.D. Newton, Phys. Rev. 78 (1950) p490; J.M.Regis et al., EPJ Web of Conf. 93 (2015) 

Deconvolution and lineshapes? If the instrument time response function R(t) is Gaussian of width , If the intermediate state decays with a mean lifetime , then The deconvolution integral for a single state lifetime is given by (ignoring the normalisation coefficients). 1-erf(x) is the complementary error function of x.

HPGe  t coincidences struggle to measure direct coincidence lifetimes much less than 1 ns. LaBr 3 (Ce) coincidences allow lifetimes to be determined down to the tens of picoseconds using centroid shift analysis of time difference distribution.

gamma-gamma-time ? some early lessons from GDD ….after going to the library for a bit….

S.J. Bell, P.H.Regan & R. Shearman., CTBT, Science and Technology Conference, Vienna 662 keV CeBr 3 5.4% LaBr 3 3.8% HPGe 0.3%

FATIMA for DESPEC FATIMA = FAst TIMing Array = State of the art array for precision measurements of nuclear structure in the most exotic and rare nuclei. 36 LaBr 3 (Ce) detectors. – Energy resolution better than 3% at 1 MeV. – Detection efficiency of ~ 5% Full-energy peak at 1 MeV. – Excellent timing qualities (sub 100 ps). Use to measure lifetimes of excited nuclear states & provide precision tests of nuclear structure, uses a fully-digitised Data Acquisition System (CAEN 1 GHz digitizers).

TRD for FATIMA for NUSTAR.

FATIMA detector module 1.5” x 2” LaBr 3 (Ce) crystal, coupled to a fast- timing PMT. Housed in aluminium can.

ROmanian array for SPectroscopy in HEavy ion Reactions – ROSPHERE at IFIN-HH Bucharest 14 HPGe detectors (AC) are used to detect coincident γ rays as ‘gates/selection’: – 7x HPGe 37 o – 4x HPGe 64 o – 3x HPGe 90 o 11 LaBr 3 (Ce:5%) detectors used for fast-timing measurements – 7x ø2”x2” and 4x ø1.5”x2” (UK) 37, 64 and 90 o w.r.t. the beam axis.

( h 11/2 ) -2 only N=80 Isotones isomer Primarily (  d 5/2 ) 2 Primarily (  g 7/2 ) 2 N = 80 isotones above Z = 50 display 10 + seniority isomers from coupling of ( h 11/2 ) level decays also usually ‘hindered’ e.g., in 136 Ba,T 1/2 = 3.1(1)ns. Thought to be due to change in configuration and seniority.

2 neutrons more than heaviest stable Tungsten (Z=74) isotope ( 186 W). Populate 188 W using 186 W( 7 Li,  p) 188 W ‘incomplete fusion’reaction. see e.g., G.D.Dracoulis et al., J. Phys. G 23 (1997) p Lifetime of the yrast 2 + state in 188 W

T 1/2 =0.87(12) ns Sum of time differences between 143-keV and any higher lying feeding transition (assumes negligible half-life for intermediate states).

Lifetimes and deformation in neutron-rich Zr nuclei.

100 Zr populated in 235 U(n,f) and 241 Pu(n,f) reactions at ILL, gammas with EXILL+FATIMA EXILL + FATIMA array consisting of: a) ) 8 x EXOGAM clovers (32 HPGe crystals total) + b) 16 x LaBr 3 (Ce) crystals

F. Browne, A.M.Bruce, T. Sumikama et al., EURICA + FATIMA, accepted Phys. Lett. B July 2015

Other planned combined arrays? AGATA (GANIL) + FATIMA. GAMMASPHERE + ANL; 252 Cf fission source experiment, Dec 2015-Jan 2016.

Nuclei produced in 252 Cf fission.

Existing Gammasphere half – opposite half withdrawn 1020 mm

Applications / Impact outside nuclear structure research.  coincidences help hugely with signal isolation. CTBTO for low-level tracers from weapons/fission releases. CTBT / reactor radionuclides with detectable rapid/prompt beta-delayed cascades include 99 Mo, 103 Ru, 110m Ag, 125 Sb, 131,133 I, 132 Te, 134,6 Cs, 140 Ba/ 140 La….and others. S.J.Bell et al., CTBT conf  t = 5 ns

NANA – National Nuclear Array A national ‘standards’ array of LaBr 3 (Ce) in the UK to provide radioactive source measurements which are traceable to the Bq. Gamma-ray detection both in  coincidence mode (and later) for use in beta- gamma Current design, 12 LaBr 3 (Ce) in close geometry,

EM transition rates – they matter. LaBr 3 (Ce) detectors have the right characteristics. A mixed HPGe+LaBr 3 (Ce) array can give the best of both worlds. The DESPEC - FATIMA array is ready for action at FAIR. In the meantime, exploitation at facilities for ‘exotic’ nuclear structure research: FATIMA + EXILL ( 100 Zr) ; FATIMA + EURICA ( 104,6 Zr); Instrumentation development has impact and applications outside of traditional nuclear structure research; CTBT/ Nuclear forensics / radiation (primary) standard evaluations for radiopharmaceuticals etc.

138 La, T 1/2 =1.02x10 11 years A.A.Sonzogni, NDS 98 (2003) La Ba ec (66%) Ce  - (34%) Coincidence requirements remove most problems associated with intrinsic radioactive background of LaBr 3 (Ce) detectors. Typical intrinsic activities are ~ 0.1 →1 Bq/cm 3. Coinc times usually in the ranges ~ 0.1 → few x00 ps for prompt  in a (rotational) cascade to ~ 10 ns → few x00  s for measureable cascades across isomers