PhD: Neutron Irradiation Techniques

The 3He “crisis” has become a fact in the last decade and large scale neutron detectors based on 3He technologies have become unaffordable. is new 3He 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. e 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. is 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. e 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. e Source Testing Facility thus developed is then used in conjunction with Arktis Radiation Detectors of Zurich, Switzerland for benchmarking the response of next generation 4He based neutron detectors against standard NE-213 liquid scintillator detectors. e 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. e outputs of different actinide/9Be 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 252Cf is developed as a first step towards providing users the capability to measure the absolute, fast-neutron detection efficiency of their devices.

In late 2016, Lund is on the verge of becoming the center of accelerator-based material science in Europe. e MAX IV Laboratory, an electron synchrotron, was inaugurated in June 2016 with a scientific program to commence shortly and the European Spallation Source ERIC (ESS), a spallation neutron source, is presently under construction and anticipated to be fully operational by 2022. These facilities will put Lund firmly on the map as the location to conduct world-class research in advanced material science and engineering. And while both facilities share many scientific objectives, they provide access to very different probes of matter.
The soft photons produced at the MAX IV Laboratory preferentially interact with atomic electrons. In contrast, as neutrons are electrically uncharged, they tend to interact with the atomic nucleus. This makes neutrons highly penetrating particles and allows them to deeply probe samples. Further, neutrons have a magnetic moment. They can therefore be used to test magnetic properties of materials. As scattering cross sections are highly isotope dependent, neutrons are excellent probes for the identification of the isotopic composition of a sample. These properties make neutrons an ideal complementary probe to photons, which scale in their interaction strength by atomic number.
When constructed, ESS will be the neutron source with the highest neutron brilliance in the world. is makes ESS a technically challenging design with many firsts for the neutron-scattering community. With respect to neutron detection, two major challenges must be overcome to make ESS a success. First, the detectors of many instruments are designed to perform at never before seen neutron count rates and resolutions. Second, the lack of commercially available 3He has resulted in large- scale neutron detectors based on 3He technologies no longer being affordable. is new 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, being commercially available today, need to be tested before being installed in instruments.
To test detector technologies in all three stages of development, neutron sources for controlled irradiation are needed. Such sources of neutrons 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 at a significantly lower cost-per-neutron and with a substantially lower cost-of-entry. e Source Testing Facility (STF) has been established in Lund to provide a source-based irradiation facility to academic users and industry.
In this work, the development of a fast-neutron test bed, the core component of the STF, is presented. The neutron-irradiation techniques subsequently discussed are based on well-understood nuclear-physics methods. In particular, the “neutron tagging” technique is employed to measure neutron energy by time-of-flight. Time-of- flight is simply the elapsed time between neutron emission from a source and neutron detection at a well-defined distance from the source. is technique is used to establish energy-dependent neutron-response functions of detectors. To improve the resolution of the measurements, particle identification methods such as pulse-height and pulse-shape discrimination are employed. is is possible because the signals produced by many detectors differ in amplitude or shape for different incident radiations.
This thesis presents an overview of the results of a successful, inaugural collaboration between the Division of Nuclear Physics at Lund University and the Detector Group of the European Spallation Source ERIC.