about npl … the uk’s national standards laboratory world leading national measurement institute...
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About NPL …The UK’s national standards laboratory
• World leading National Measurement Institute
• 450+ specialists in Measurement Science
• State-of-the-art laboratory facilities
• The heart of the UK’s National Measurement System to support business and society
What do we do
Develop & disseminate UK’s measurement standards, ensure they are internationally accepted
Multidisciplinary R&D and technical services for public and private sector
Knowledge transfer and advice between industry, government and academia
Promotion of science and engineering
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
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 223Ra)
– Alpha-emitting radionuclides have great potential for treating diffuse tumours
– 223RaCl2 (Xofigo) is the first a-emitting drug to be approved by the FDA and it was licensed in the EC in November 2013.
– Xofigo is used to treat patients with castration-resistant prostate cancer and symptomatic bone metastases.
New Primary Standard of a-emitting radiopharmaceuticals.
Courtesy: John Keightley et al.,
Alpha decay can also leave daughter in excited states which can then decay by (characteristic) gamma emission.
Idea:Build an array of LaBr3 (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 LaBr3 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 LaBr3 detectors (1.5” x 2” cylinders in three rings of 12 detectors)
• Other detector arrays including LaBr3(Ce) detectors for nuclear spectroscopy studies. e.g.,:– ROSPHERE (IFIN-HH, Romania)– EURICA array with 18 LaBr3(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.
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 2012-16).
– 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.
Expected, E1/2 dependence of FWHM on gamma-ray energy.
T.Alharbi et al., Applied Radiation and Isotopes, 70, 1337 (2012)
138La, T1/2=1.02x1011 yearsA.A.Sonzogni, NDS 98 (2003) 515
5+ 138La
1435.8138Ba82
2+
0+
ec (66%)
0+
2+
138Ce80
788.7
b- (34%)
137Cs source gives (initial) testenergy 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 138Ba)789 keV + b-
In 138Ce
Ba x-rays from 137Cs & EC from 138La 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 -g g in a cascade) to tens of microseconds (for delayed or isomeric cascades).
Measurement of lifetimes of excited nuclear states?
• HpGe coincidences struggle to measure direct coincidence lifetimes much less than 1 ns.
• LaBr3(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.
Beta-delayed LaBr3(Ce) coincs from EURICA: Excited state lifetimes in (defrormed) 102Zr
(following 102Y 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 dets. @ 37o
– 4x HPGe dets. @ 64o
– 3x HPGe dets. @ 90o
• 11 LaBr3(Ce:5%) detectors– 7x ø2”x2” and 4x ø1.5”x2”
(Cylindrical) @ 37, 64 and 90o w.r.t. the beam axis.
(h11/2)-2 only
N=80 Isotones
0+
2+
4+
6+
8+
10+isomer
Primarily(d5/2)2
Primarily(g7/2)2
• N = 80 isotones above Z = 50 display 10+ seniority isomers from coupling of (h11/2)-2
• 6+ level decays also usually ‘hindered’ e.g., in 136Ba, T1/2 = 3.1(1)ns.
• Thought to be due to change in configuration and seniority.
N=80 Isotones• Neighbouring N=80 nuclei, 138Ce and 140Nd expected to show
similar 6+ → 4+ hindrance.
• Competing transitions to negative parity states
S.-J. Zhu et al. Chin.Phys.Lett. 16, 635 (1999)
T1/2 = 140(11)ps
Using “delayed” HPGe gate
Typical -g g 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 LaBr3(Ce) in close geometry,
Space for additional ‘gating’ detectors (HpGe, or Si beta-detector) to be added later,
Courtesy:R.ShearmanNDA / 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 LaBr3(Ce)Scintillation crystals, 1.5” diameter and 2.0” in length.
Digital time stamped DAQUsing CAEN 1 GHzDigitisers.