A Source-Based Test-Bed for Fast-Neutron Irradiation

The "3He crisis" has resulted in a major effort worldwide to develop replacement neutron-detector technologies. Such technologies are in their infancy and need to be thoroughly tested before becoming mainstream. Neutron sources for controlled irradiation include accelerators, nuclear reactors and radioactive sources. Neutrons produced in nuclear reactors and at accelerators have a very high cost per neutron. In contrast, radioactive sources produce neutrons at a significantly lower cost per neu- tron and with a substantially lower cost of entry. A drawback associated with radioac- tive sources is the associated isotropic mixed neutron/gamma-ray field. This work in- troduces a cost-efficient test-bed for the production of 2-7 MeV neutrons based on an 241Am/9Be source, with the aim of lowering the barrier for precision neutron testing of detector technologies. Well-understood nuclear physics coincidence and time-of- flight measurement techniques are applied to unfold the mixed neutron/gamma-ray field and unambiguously identify the energies of the neutrons on an event-by-event basis

Overcoming High Energy Backgrounds at Pulsed Spallation Sources

Instrument backgrounds at neutron scattering facilities directly affect the quality and the efficiency of the scientific measurements that users perform. Part of the background at pulsed spallation neutron sources is caused by, and time-correlated with, the emission of high energy particles when the proton beam strikes the spallation target. This prompt pulse ultimately produces a signal, which can be highly problematic for a subset of instruments and measurements due to the time-correlated properties, and different to that from reactor sources. Measurements of this background have been made at both SNS (ORNL, Oak Ridge, TN, USA) and SINQ (PSI, Villigen, Switzerland). The background levels were generally found to be low compared to natural background. However, very low intensities of high-energy particles have been found to be detrimental to instrument performance in some conditions. Given that instrument performance is typically characterised by S/N, improvements in backgrounds can both improve instrument performance whilst at the same time delivering significant cost savings. A systematic holistic approach is suggested in this contribution to increase the effectiveness of this. Instrument performance should subsequently benefit.Comment: 12 pages, 8 figures. Proceedings of ICANS XXI (International Collaboration on Advanced Neutron Sources), Mito, Japan. 201

Neutron Position Sensitive Detectors for the ESS

The European Spallation Source (ESS) in Lund, Sweden will become the world's leading neutron source for the study of materials. The instruments are being selected from conceptual proposals submitted by groups from around Europe. These instruments present numerous challenges for detector technology in the absence of the availability of Helium-3, which is the default choice for detectors for instruments built until today and due to the extreme rates expected across the ESS instrument suite. Additionally a new generation of source requires a new generation of detector technologies to fully exploit the opportunities that this source provides. The detectors will be sourced from partners across Europe through numerous in-kind arrangements; a process that is somewhat novel for the neutron scattering community. This contribution presents briefly the current status of detectors for the ESS, and outlines the timeline to completion. For a conjectured instrument suite based upon instruments recommended for construction, a recently updated snapshot of the current expected detector requirements is presented. A strategy outline as to how these requirements might be tackled by novel detector developments is shown. In terms of future developments for the neutron community, synergies should be sought with other disciples, as recognized by various recent initiatives in Europe, in the context of the fundamentally multi-disciplinary nature of detectors. This strategy has at its basis the in-kind and collaborative partnerships necessary to be able to produce optimally performant detectors that allow the ESS instruments to be world-leading. This foresees and encourages a high level of collaboration and interdependence at its core, and rather than each group being all-rounders in every technology, the further development of centres of excellence across Europe for particular technologies and niches.Comment: 8 pages, 1 figure. Proceedings from the 23rd International Workshop on Vertex Detectors, 15-19 September 2014, Macha Lake, The Czech Republic. PoS(Vertex2014)02

Neutron Irradiation Techniques

The He-3 “crisis” has become a fact in the last decade and large scale neutron detectors based on He-3 technologies have become unaffordable. This new He-3 reality has resulted in a major effort worldwide to develop replacement neutron-detector technologies. Many of these technologies are in their infancy and need to be thoroughly tested before becoming mainstream, while others are approaching commercialization and need to be certified. Still others which are commercially available today need to be tested before being installed in, for example, the instruments at the European Spallation Source ERIC. Detector technologies in all three stages of development need neutrons for controlled irradiations. Sources of neutrons for controlled irradiations include accelerators, nuclear reactors and radioactive sources. Neutrons produced in nuclear reactors and at accelerators have a very high cost of entry and a very high cost per neutron. In contrast, radioactive sources produce neutrons with a significantly lower cost of entry and lower cost per neutron. A drawback associated with radioactive sources is the associated isotropic mixed neutron/gamma-ray field. The Source Testing Facility has been established at the Division of Nuclear Physics at Lund University to provide a source-based neutron irradiation facility to local academic users and industry. This work presents the development of a cost-efficient test bed for the production of 2-6 MeV neutrons, an integral part of the Source Testing Facility. The test bed is based on actinide/9Be sources and lowers the barrier for local groups for precision neutron testing of detector technologies and shielding studies. Well-understood nuclear physics coincidence and time-of-flight measurement techniques are applied to unfold the mixed neutron/gamma-ray field and unambiguously identify the energies of the neutrons on an event-by-event basis. The Source Testing Facility thus developed is then used in conjunction with Arktis Radiation Detectors of Zurich, Switzerland for benchmarking the response of next generation He-4 based neutron detectors against standard NE-213 liquid scintillator detectors. The response of these standard NE-213 liquid scintillator detectors is then carefully unfolded and various models of the response of the scintillator itself are tested. The outputs of different actinide/Be-9 sources are then precisely compared in an effort to identify preferred actinides. And lastly, a complementary facility for the tagging of neutrons from the spontaneous-fission source Cf-252 is developed as a first step towards providing users the capability to measure the absolute, fast-neutron detection efficiency of their devices

Detectors for the European Spallation Source

The European Spallation Source (ESS) in Lund, Sweden will become the world's leading neutron source for the study of materials by 2025. First neutrons will be produced in 2019. It will be a long pulse source, with an average beam power of 5 MW delivered to the target station. The pulse length will be 2.86 ms and the repetition rate 14 Hz. The ESS is presently in a design update phase, which ends in February 2013 with a Technical Design Report (TDR). Construction will subsequently start with the goal of bringing the first seven instruments into operation in 2019 at the same time as the source. The full baseline suite of 22 instruments will be brought online by 2025. These instruments present numerous challenges for detector technology in the absence of the availability of Helium-3, which is the default choice for detectors for instruments built until today. Additionally a new generation of source requires a new generation of detector technologies to fully exploit the opportunities that this source provides. This contribution presents briefly the current status of the ESS, and outlines the timeline to completion. The number of instruments and the framework for the decisions on which instruments should be built are shown. For a conjectured full instrument suite, which has been chosen for demonstration purposes for the TDR, a snapshot of the current expected detector requirements is presented. An outline as to how some of these requirements might be tackled is shown. Given that the delivery of the ESS TDR is only a few months away, this contribution reflects strongly the content of the TDR

Development of graphene-based ionizing radiation sensors

We present the first steps to develop radiation sensors based on the graphene field effect transistor technology.Such a sensor exploits the ambipolar behavior of graphene near its Dirac point and it is not dependent oncollecting charges, but it senses ionizing radiation trough the change in conductivity of the graphene layerinduced by changes of the electric field. We designed the layout of the sensors with the help of SentaursTCAD. We simulated static operations and the dynamic response to radiation and calculated the source–drain current through the graphene layer with a quasi-analytical model. The transistors were produced at theNational Enterprise for nanoScience and nanoTechnology by depositing high quality graphene on silicon chipsmanufactured by the Fondazione Bruno Kessler foundry. To reduce the high contact resistance between grapheneand aluminum contacts caused by oxidation of the aluminum surface, we used gold/chromium interfaces. Weinvestigated the sensors behavior by mean of electrical measurements, extracting the graphene properties, suchas mobility and doping. We observed modulation of the source–drain current, determined the Dirac point andfound the optimal voltage levels to be sensitive to pulsed IR laser light and훽-particles

Characterization of the radiation background at the Spallation Neutron Source

We present a survey of the radiation background at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory, TN, USA during routine daily operation. A broad range of detectors was used to characterize primarily the neutron and photon fields throughout the facility. These include a WENDI-2 extended range dosimeter, a thermoscientific NRD, an Arktis He-4 detector, and a standard Nal photon detector. The information gathered from the detectors was used to map out the neutron dose rates throughout the facility and also the neutron dose rate and flux profiles of several different beamlines. The survey provides detailed information useful for developing future shielding concepts at spallation neutron sources, such as the European Spallation Source (ESS), currently under construction in Lund, Sweden