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P. Schotanus SCIONIX Holland B.V. Dedicated Scintilllation Detectors scionix.nl

Recent developments in neutron detection with scintillators : some new data on liquid scintillators for neutron / gamma discrimination. P. Schotanus SCIONIX Holland B.V. Dedicated Scintilllation Detectors www.scionix.nl. IAEA neutron workshop March 21-24 2011. Detection of Neutrons :

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P. Schotanus SCIONIX Holland B.V. Dedicated Scintilllation Detectors scionix.nl

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  1. Recent developments in neutron detection with scintillators : some new data on liquid scintillators for neutron / gamma discrimination P. Schotanus SCIONIX Holland B.V. Dedicated Scintilllation Detectors www.scionix.nl IAEA neutron workshop March 21-24 2011

  2. Detection of Neutrons : - Physics (e.g. particle Physics, HEP) - Security (SNM e.g. Pu, U) - Health Physics (dosimetry, often non spectrometric) Neutron energy : Thermalised 0.025 eV – MeVs (fast neutrons > 50 keV) Interaction with Nucleus of absorber : A. Scattering or B. Nuclear reactions - Elastic scattering (protons) - Inelastic scattering + prompt gammas Nuclear Reactions e.g. : 10B(n α) 7Li , 3He(n p)3H, 6Li(n γ) 3H, 157Gd(n α) 158Gd IAEA neutron workshop March 21-24 2011

  3. Most common detection method for neutrons : • 3He- tubes ( pressurised) • BF3 counters • Most unproblematic detector is the He-3 tube : • Easy to operate (no special electronic needed) • gamma / neutron rejection > 106 • No serious safety issues • Large sizes possible ( meters long) World wide structural He-3 shortage  availability / cost problems Possible alternative : Detection of neutrons with scintillators : A. Thermal neutrons via nuclear reactions on Li, B or Gdin the material. B. Fast neutrons via elastic (recoil) scattering in proton containingmaterials (organic scintillators) IAEA neutron workshop March 21-24 2011

  4. Usually , neutrons associated with gammas, both will interact with most scintillators but : Neutron + 6Li  alpha + Triton (4.78 MeV total) (particles !)  Peak at > 4 MeV LiI(Eu) scintillator Neutron / gamma separation possible via Pulse height However LiI(Eu) 96 % enriched is a relative expensive material that cannot be made in large sizes. The 6LiI(Eu) alternative is an excellent solution for hand held instruments and dosimeters but is not an option for e.g. radiation portals IAEA neutron workshop March 21-24 2011

  5. Alternative 6-Li containing scintillators 6Li Loaded glass scintillator (Ce doped) - Also expensive - Low neutron peak location ( approx. 1.8 MeV) implying more problem with gamma rejection - no large lengths possible An alternative if gamma flux is low and of low energy and if time resolution is an issue ( scintillation is fast, 60 ns decay time) Neutron / gamma spectrum 6-Li glass New materials like CLYC:Ce adressed in other presentation. IAEA neutron workshop March 21-24 2011

  6. Boron Loaded scintillators : No inorganics known except for Li Borate glasses with low light output The other alternative : Liquid scintillators Some activated organics liquids show different pulse shape for neutrons and gammas. Liquid scintllillators known since 1960’s Most well known NE213( = EJ301 = BC501A) Xylene based. Real measured pulse shape difference EJ301 = NE213 EJ309 IAEA neutron workshop March 21-24 2011

  7. Different ways to do neutron/gamma separation with PSD • QDC with 20 ns and 1 microseconds gate (or different) • Converts signals to time spectrum using a double delay line amplifier and CFDs (time spectrum shows two peaks) • Digitize the signal using wave form analyzer (500 Mhz to 1 GHz flash ADC) Traditionally NIM electronics were used. For many real field applications outside Physics this is very unpractical • Other disadvantages of liquid scintillators : • Expand with increasing temperature (expansion volume is needed) • - e.g. EJ301 is a low flash point (flammable) toxic material (transport + safety issued) New developments : - High flash point non toxic liquids - PFGA based pulse digitising techniques IAEA neutron workshop March 21-24 2011

  8. These new liquids open up applications Neutron PSD almost just as good as for old fashioned liquids EJ301 EJ309 (NEW) EJ301 is a scintillating liquid equivalent to NE213EJ309 is a scintillating liquid especially designed for neutron BC501A specially designed for neutron / gamma discrimination It has a high flash point, low vapour pressure discrimination. and low toxidity. Properties : Light output (rel. to antracene) : 78 % 75 % Photon yield / MeV electrons : 12.000 11.500 Maximum of emission wavelength : 425 nm 424 nm Density: 0.874 g/ cc 0.964 g/ cc H:C ratio: 1.21 1.25 No. C atoms per cc : 4.0 1022 5.46 1022 No. H atoms per cc : 4.81022 4.37 1022 No. electrons per cc : 2.3 1023 3.17 1023 Flash point : 26 oC 144 oC Refractive index : 1.50 1.57 Decay time short component 3.2 ns approx. 3.5 ns Decay time long components 32.3, 270 ns ---- IAEA neutron workshop March 21-24 2011

  9. When Liquid scintillators are Boron doped, the neutron sensitivity is increased Boron peak at higher energies than with EJ301 (100 keV versus 60 keV) Gammas, fast neutrons and slow neutrons can be separated by PSD Boron doped EJ301 All these scatter plots measured by Swiderski et al. Soltan Institute fro Nuclear studies, Poland using the setup shown (analog technique). IAEA neutron workshop March 21-24 2011

  10. HOWEVER : For good gamma rejection the gamma interaction with the liquid imposes a problem. (important for detecting neutrons with High Gamma rate Low gamma rate Swiderski et al. NIM In press.

  11. It has been demonstrated that using electronic techniques the gamma pile-up can be reduced. With additional shielding the neutron / gamma separation can be improved from 3 . 104 to 5 . 106 This is still an order of magnitude + away from He-3 tubes but may be sufficient for some applications. ELECTRONIC DEVELOPMENTS Advanced pulse sampling techniques using FPGAs allow on line 250 kHz throughput neutron / Gamma separation Instruments presented elsewhere in the workshop like the Mixed Field Analyser developed by Hybrid instruments (UK) IAEA neutron workshop March 21-24 2011

  12. Design of Liquid cells for timing and PSD General : 1. Liquids expand (T) and expansion space has to be available ( 3-5 % for temp. range –20 – +50 oC) With properly designed windows and hardware,“bubblefree” cells are possible (all orientations). 2. Optical windows of UV glass or quartz provide best optical transmission. (Boron Free requirement sometimes). Readout : Classical readout with large PMTs is often NOT needed for good timing and/or PSD Readout with smaller PMTs provides good PSD

  13. Larger sizes cells of different geometries also provide a good neutron / gamma PSD SOLUTIONS THAT WORK IAEA neutron workshop March 21-24 2011

  14. HOWEVER : Neutron / Gamma PSD can be spoilt by a geometry where the light paths to the PMT are much different ! a 1:1 plug-in replacement of He-3 tubes by long liquid cells is not possible ! The above geometries do not show a good neutron / gamma discrimination via PSD. IAEA neutron workshop March 21-24 2011

  15. There is the widespread belief that for adequate PSD in liquid scintillators a fast PMT is needed. New data show the contrary FOM is the same for a “ fast” XP4512 as a “ slow” ETL 9390 (data by Swiderski et al.) Note that Fast PMTs suffer from a non-ideal photoelectron collection efficiency HOWEVER for some applications (neutron / Gamma coincidences) a optimum time resolution is needed ( < 1 ns FWHM) IAEA neutron workshop March 21-24 2011

  16. Time resolution at FWHM, Dt [ps] for Co-60 gamma rays Time resolution ( ps) [phe/MeV] XP4512-thr. 50keV 550 ± 20 1520 ± 50 ETL9390-thr. 50keV 1420 ± 40 3200 ± 100 XP4512 is obsolete, fast Hamamatsy R1250 is nowadays used Using a less fast PMT, a significant gain in photoelectron yield can be obtained at cost of the time resolution Fast 127 mm diameter PMTs are very costly and bulky. Suitable PMT should have a low time jitter though… IAEA neutron workshop March 21-24 2011

  17. AND : A proper design of the electronics (CW High V supply) allows to run liquid scintillators at high rates but there are limits, • Conclusions : • The current availability of high point non dangerous goods liquid scintillators opens up possibilities to use these detectors where was was prohibitive in the past. • The current availability of digital techniques allows the construction of novel instruments for neutron / gamma discrimination in mixed neutron / gamma fields • Liquid scintilltors can be a replacement for He-3 tubes in some applications • The size He-3 detectors are often used in ( 1 m long 50 mm diameter) is not a viable geometry for liquid scintillators where PSD is needed • For adequate neutron / Gamma PSD fast PMTs are not needed but to achive the best possible time resolution they are. Thanks to : Marek Moszynski and Lukasz Swiderksi (Soltan Institute for Nuclear studies, Otwock, Swierk, Poland) Chuck Hurlbut (ELJEN Technologies) IAEA neutron workshop March 21-24 2011

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