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Realising the Bq -The principle of coincidence counting dates back to 1924 in visual observation of scintillations on a screen (Geiger) -Rutherford later wrote: ‘…by an ingenious treatment of the observations they were able to obtain the true number of scintillations.’ NPL’s coincidence counting system in 1954

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Idea: Use gamma-ray coincidence array for radioactive source decay measurement for: a) Standardisation and use as a traceable primary radiation standard. b) Use for nuclear assay and identification of which radionuclides are present and in what amounts (activity). c) Use for nuclear structure / decay data measurements to improve nuclear decay and energy level scheme knowledge based for specific radionuclides (e.g 223 Ra)

– Alpha-emitting radionuclides have great potential for treating diffuse tumours – 223 RaCl 2 (Xofigo) is the first  - emitting drug to be approved by the FDA and it was licensed in the EC in November – Xofigo is used to treat patients with castration-resistant prostate cancer and symptomatic bone metastases. New Primary Standard of  -emitting radiopharmaceuticals. Courtesy: John Keightley et al.,

Alpha decay can also leave daughter in excited states which can then decay by (characteristic) gamma emission.

Can use (E , E  ) ‘prompt’ coincidences to select unambiguous decay paths

Idea: Build an array of LaBr 3 (Ce) detectors for best combination of 1) Energy resolution ; 2) ‘Fast-timing’ coincidences; 3) Detection efficiency; …and cost.

Use experience developed by use of other (recent) arrays of LaBr 3 detectors for (nuclear) gamma-ray spectroscopy/spectrometry FATIMA (‘Fast Timing Array) – STFC funded large array for decay spectroscopy of radionuclide decays with unusual proton to neutron ratios (nuclear structure physics). – 36 LaBr 3 detectors (1.5” x 2” cylinders in three rings of 12 detectors) Other detector arrays including LaBr 3 (Ce) detectors for nuclear spectroscopy studies. e.g.,: – ROSPHERE (IFIN-HH, Romania) – EURICA array with 18 LaBr 3 (Ce) at RIKEN, Japan – EXILL-FATIMA (Nuclear structure studies of prompt fission fragments at ILL-Grenoble).

FATIMA detector module 1.5” x 2” LaBr3(Ce) crystal, coupled to a fast- timing PMT. Housed in aluminium can.

R9779 PMTs from Hamamatsu

The Future: FATIMA for DESPEC FATIMA = FAst TIMing Array = State of the art gamma-ray detection array for precision measurements of nuclear structure in the most exotic and rare nuclei. Part of the ~ £8M STFC NUSTAR project grant (runs ). – Good energy resolution (better than 3% at 1 MeV). – Good detection efficiency (between than 5% Full-energy peak at 1 MeV). – Excellent timing qualities (approaching 100 picoseconds). Use to measure lifetimes of excited nuclear states & provide precision tests of theories of nuclear structure, uses a fully-digitised Data Acquisition System. Collaboration with NPL (Radioactivity group) through NMO project on ‘Nuclear Data’ (Judge, Jerome, Regan et al.,) on parallel development of NPL-based array for use in traceable radioactive standards and traceability to the Bq.

New (gamma) detection devices: LaBr 3 detectors for gamma-spec.

Expected, E 1/2 dependence of FWHM on gamma-ray energy. T.Alharbi et al., Applied Radiation and Isotopes, 70, 1337 (2012)

138 La, T 1/2 =1.02x10 11 years A.A.Sonzogni, NDS 98 (2003) La Ba ec (66%) Ce  - (34%)

137 Cs source gives (initial) test energy resolution of ~3.5% at 662 keV. Note presence of internal radioactivity in detector. PMT HV range ~1300 V 1436 keV EC (2 + → 0 + in 138 Ba) 789 keV +  - In 138 Ce Ba x-rays from 137 Cs & EC from 138 La decay

Coincidence requirements (either gamma-gamma or beta-gated gamma coincidences) remove most problems associated with intrinsic radioactive background of LaBr 3 (Ce) detectors. Typical activities are the order of 1 Bq/cc. Typical coincidence requirements for true coincidences are usually between picoseconds (for prompt  in a cascade) to tens of microseconds (for delayed or isomeric cascades).

Tests with 152 Eu source (measure lifetime of I  = keV level in 152 Sm)

Measurement of lifetimes of excited nuclear states? HpGe 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. Accurate measurements of nuclear excited state lifetimes are of interest for nuclear structure. The inform on nuclear shapes, underlying nuclear spectroscopic configurations and geometrical symmetries of nuclear charge distributions.

Fast-timing Techniques Gaussian-exponential convolution to account for timing resolution

Annual Review of Nuclear Science (1968) 18 p

T 1/2 = 1.4ns Tests with 152 Eu source to measure lifetime of I  = keV level in 152 Sm.

EURICA at RIBF, Japan, 18 LaBr3(Ce) detectors plus 12 x 7 element HpGe Cluster detectors.

Beta-delayed LaBr 3 (Ce) coincs from EURICA: Excited state lifetimes in (defrormed) 102Zr (following 102 Y decay) F.Browne, A.M.Bruce et al., Acta Physica Polonica B (2015)

The ROSPHERE Gamma-ray Spectrometer array (at IFIN-HH Bucharest) 14 HPGe detectors (AC) are used to detect coincident γ rays: – 7x HPGe 37 o – 4x HPGe 64 o – 3x HPGe 90 o 11 LaBr 3 (Ce:5%) detectors – 7x ø2”x2” and 4x ø1.5”x2” 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.

N=80 Isotones Neighbouring N=80 nuclei, 138 Ce and 140 Nd expected to show similar 6 + → 4 + hindrance. Competing transitions to negative parity states

130 Te( 12 C,4n) MeV

T.Alharbi et al., Phys. Rev. C87 (2013) Ce 80

S.-J. Zhu et al. Chin.Phys.Lett. 16, 635 (1999) T 1/2 = 140(11)ps Using “delayed” HPGe gate Typical  analysis, massive increase in signal to noise by coincidence requirement

NANA: The NAtional Nuclear Array Compact, high-efficiency, good granularity, acceptable resolution gamma-ray spectrometer array. Gamma-ray detection both in g-g coincidence mode (and later) for use in beta-gamma Current design, 12 LaBr 3 (Ce) in close geometry, Space for additional ‘gating’ detectors (HpGe, or Si beta-detector) to be added later,

Courtesy: R.Shearman NDA / NNL funded PhD Student at NPL & U. Surrey Conceptual design, for NANA. Initial design: 12 detectors in fixed geometry with source in central position. Cylindrical LaBr 3 (Ce) Scintillation crystals, 1.5” diameter and 2.0” in length. Digital time stamped DAQ Using CAEN 1 GHz Digitisers.

Acknowledgements Zsolt Podolyák, Peter Mason, Thamer Alharbi, Christopher Townsley (Surrey) Steven Judge, Robert Shearman (NPL) Alison Bruce, Oliver Roberts (Brighton) Nicu Marginean et al., (Bucharest) Funding for detectors and DAQ from STFC UK and UK NMO (for NANA funding).