The Neutron Science Laboratory (NSL) is situated within the North Campus of the University of Michigan and houses various neutron and gamma-ray sources and a range of nuclear radiation detection equipment. The laboratory is an integral part of Nuclear Engineering and Radiological Sciences (NERS) department at the University of Michigan (U-M) and is managed by the Applied Nuclear Science Group. The laboratory was renovated in 2017 with a specific goal to accommodate the use of a new DT neutron generator (Thermo Fisher Scientific, Model P211) in various open-beam configurations. NSL provides convenient access to high-fidelity monoenergetic and broad-energy neutron sources for basic research, nuclear security and nonproliferation, and other experimental needs.
The NERS department previously managed and used the Ford Nuclear Reactor at the Phoenix Memorial Laboratory. The reactor provided an access to a neutron source for instructional and experimental use. In 2003, the Ford Nuclear Reactor was shut down and decommissioning operations began. A neutron source was still required for educational and research use. In 2004, the NERS Department purchased a DD neutron generator (Thermo Fisher Scientific, Model MP320, producing 2.45 MeV with a rate of 106 n/s).
A decision was also made to have a dedicated facility for a neutron source, and in 2006, the first deliveries of components for a D711 DT neutron generator from Thermo Electron Corporation (USA) were made to the department. This generator was rated to produce up to 2x1010 n/s at 14.1 MeV, a useful flux for nuclear activation studies. Final installation and operation of the D711 neutron generator commenced in 2007. The original housing for the neutron generator consisted of a prefabricated concrete “cave”. The cave was about 3 m in length and 1.5 m wide and had windows on two sides. The walls were about 35cm thick. By simulating the cave and surrounding room, it was determined that 2.5cm of polyethylene lining added to the inside and outside of the cave would help thermalize the neutrons. In addition, another 60-cm thick wall was built of concrete blocks around the entirety of the cave. The floor was also lined with 5cm of polyethylene to reduce scatter from the floor. This reduced the average dose on the surface of the cave below 10 mRem/hr. “Hotspots” still existed at the points closest to the source, but they were less than 50 mRem/hr, well within the 100 mRem/hr regulation.
In 2017, the D711 neutron generator was decommissioned and a new DT neutron generator (Thermo Fisher Scientific, Model P211, producing up to 108 n/s at 100Hz) was acquired with a goal to provide a pulsed neutron source with near-monoenergetic spectrum by operating the unit in open space, far from structural or shielding walls and floor. The NSL was redesigned and renovated to ensure that the dose rates around the laboratory stay within the permissible limits. The renovated NSL provides a convenient access to both monoenergetic (DD/DT) and broad-energy (252Cf) neutron sources to the faculty, staff, and students of University of Michigan. The laboratory also houses various radioisotope gamma sources, detectors, and supporting nuclear instrumentation.
Left: Schematic of the NSL at the University of Michigan. Right: Picture showing the renovated laboratory (2017-Present).
P211 DT Neutron Generator
The P211 DT Neutron Generator manufactured by Thermo Fisher Scientific, utilizes the 3H(d,n)4He fusion reactions to produce 14.1 MeV neutrons. A beam of deuterons is accelerated to ~80 keV and bombards a tritium-impregnated target in the generator assembly. The generator is cylindrical in shape and has a rated output of 108 n/s. Individual pulses contain 106 neutrons and can be produced at various repetition rates - from single shot to 100Hz. The P211 generator is further designed so that no neutrons are produced between pulses.
P211 Neutron Generator at the Neutron Science Laboratory
Regulatory Limits
Safety interlock station at the NSL. The NSL has four such stations having a keypad and a light tree.
The State of Michigan of Community Health (MDCH) is the regulatory body to which the design of the NSL must comply. MDCH limits Class AA installations public exposure to be less than 2 mrem/h, measure 5cm from any accessible external surface point, while the source is operating at maximum radiation output. Therefore, the lab was designed such that the dose rates are less than 2 mrem/h at any point outside of the experimental area with DT generator in operation (since it is the most intense source in the laboratory) The concrete walls were made 3ft thick according to the guidance from The National Council on Radiation Protection and Measurements Report No. 144, “Radiation Protection for Particle Accelerator Facilities,” and the MCNP simulations. The walls were made 11ft tall to permit experiments at various heights of the DT generator. After renovation of the laboratory and installation of safety interlocks, the P211 DT generator was first operated on 28 November 2017. The dose rates were experimentally determined outside of the laboratory walls and were below the permissible limit of 2 mrem/h. On that day, NSL was approved for operation by the U-M Radiation Safety Service.
Background radiation is a measure of the level of ionizing radiation present in the environment at a particular location which is not due to deliberate introduction of radiation sources.
A beta particle, also called beta ray or beta radiation, is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay. There are two forms of beta decay, β− decay and β+ decay, which produce electrons and positrons respectively.
A neutron source is any device that emits neutrons, irrespective of the mechanism used to produce the neutrons. Neutron sources are used in physics, engineering, medicine, nuclear weapons, petroleum exploration, biology, chemistry, and nuclear power.
Ionizingradiation consists of subatomic particles or electromagnetic waves that have sufficient energy to ionize atoms or molecules by detaching electrons from them. The particles generally travel at a speed that is greater than 1% of that of light, and the electromagnetic waves are on the high-energy portion of the electromagnetic spectrum.
In physics, optically stimulated luminescence (OSL) is a method for measuring doses from ionizing radiation. It is used in at least two applications:
Health physics, also referred to as the science of radiation protection, is the profession devoted to protecting people and their environment from potential radiation hazards, while making it possible to enjoy the beneficial uses of radiation. Health physicists normally require a four-year bachelor’s degree and qualifying experience that demonstrates a professional knowledge of the theory and application of radiation protection principles and closely related sciences. Health physicists principally work at facilities where radionuclides or other sources of ionizing radiation are used or produced; these include research, industry, education, medical facilities, nuclear power, military, environmental protection, enforcement of government regulations, and decontamination and decommissioning—the combination of education and experience for health physicists depends on the specific field in which the health physicist is engaged.
The roentgen equivalent man is a CGS unit of equivalent dose, effective dose, and committed dose, which are measures of the health effect of low levels of ionizing radiation on the human body.
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, or divergent chain reaction. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses, if they occur in an unprotected environment.
Aneutronic fusion is any form of fusion power in which very little of the energy released is carried by neutrons. While the lowest-threshold nuclear fusion reactions release up to 80% of their energy in the form of neutrons, aneutronic reactions release energy in the form of charged particles, typically protons or alpha particles. Successful aneutronic fusion would greatly reduce problems associated with neutron radiation such as damaging ionizing radiation, neutron activation, and requirements for biological shielding, remote handling and safety.
Neutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus decays immediately by emitting gamma rays, or particles such as beta particles, alpha particles, fission products, and neutrons. Thus, the process of neutron capture, even after any intermediate decay, often results in the formation of an unstable activation product. Such radioactive nuclei can exhibit half-lives ranging from small fractions of a second to many years.
In nuclear and particle physics, the concept of a neutron cross section is used to express the likelihood of interaction between an incident neutron and a target nucleus. In conjunction with the neutron flux, it enables the calculation of the reaction rate, for example to derive the thermal power of a nuclear power plant. The standard unit for measuring the cross section is the barn, which is equal to 10−28 m2 or 10−24 cm2. The larger the neutron cross section, the more likely a neutron will react with the nucleus.
There are 21 recognized isotopes of sodium (11Na), ranging from 18 Na to 39 Na and two isomers. 23 Na is the only stable isotope. It is considered a monoisotopic element and it has a standard atomic weight of 22.98976928(2). Sodium has two radioactive cosmogenic isotopes. With the exception of those two, all other isotopes have half-lives under a minute, most under a second. The shortest-lived is 18 Na, with a half-life of 1.3(4)×10−21 seconds.
This article compares the radioactivity release and decay from the Chernobyl disaster with various other events which involved a release of uncontrolled radioactivity.
Technetium-99m (99mTc) is a metastable nuclear isomer of technetium-99, symbolized as 99mTc, that is used in tens of millions of medical diagnostic procedures annually, making it the most commonly used medical radioisotope in the world.
The University of Florida Training Reactor (UFTR), commissioned in 1959, is a 100 kW modified Argonaut-type reactor at the University of Florida in Gainesville, Florida. The UFTR is a light water and graphite moderated, graphite reflected, light water cooled reactor designed and used primarily for training and nuclear research related activities. The UFTR is licensed by the Nuclear Regulatory Commission and is the only research reactor in Florida.
The Ford Nuclear Reactor was a facility at the University of Michigan in Ann Arbor dedicated to investigating the peaceful uses of nuclear power. It was a part of the Michigan Memorial Phoenix Project, a living memorial created to honor the casualties of World War II. The reactor operated from September 1957 until July 3, 2003. During its operation, the FNR was used to study medicine, cellular biology, chemistry, physics, mineralogy, archeology, anthropology, and nuclear science.
A neutron research facility is most commonly a big laboratory operating a large-scale neutron source that provides thermal neutrons to a suite of research instruments. The neutron source usually is a research reactor or a spallation source. In some cases, a smaller facility will provide high energy neutrons using existing neutron generator technologies.
The Pakistan Atomic Research Reactor or (PARR) are two nuclear research reactors and two other experimental neutron sources located in the PINSTECH Laboratory, Nilore, Islamabad, Pakistan.
Recognized effects of higher acute radiation doses are described in more detail in the article on radiation poisoning. Although the International System of Units (SI) defines the sievert (Sv) as the unit of radiation dose equivalent, chronic radiation levels and standards are still often given in units of millirems (mrem), where 1 mrem equals 1/1000 of a rem and 1 rem equals 0.01 Sv. Light radiation sickness begins at about 50–100 rad.
Neutron capture therapy (NCT) is a radio-therapeutic modality for treating locally invasive malignant tumors such as primary brain tumors, recurrent cancers of the head and neck region, and cutaneous and extracutaneous melanomas. It is a two-step procedure: first, the patient is injected with a tumor-localizing drug containing the non-radioactive isotope boron-10 (10B), which has a high propensity to capture low energy "thermal" neutrons. The neutron cross section of 10B is 1,000 times greater than that of the other elements, such as nitrogen, hydrogen, and oxygen, that are present in tissues. In the second step, the patient is radiated with epithermal neutrons, the sources of which in the past have been nuclear reactors and now are accelerators that produce higher energy epithermal neutrons. After losing energy as they penetrate tissue, the resultant low energy "thermal" neutrons are captured by the 10B atoms. The resulting fission reaction yields high-energy alpha particles that kill the cancer cells that have taken up sufficient quantities of 10B. All of the clinical experience to date with NCT has been with the non-radioactive isotope boron-10, and hence this radio-therapeutic modality is known as boron neutron capture therapy (BNCT). The use of another non-radioactive isotope, such as gadolinium (Gd), has been limited to experimental animal studies and has not been used clinically. BNCT has been evaluated clinically as an alternative to conventional radiation therapy for the treatment of malignant brain tumors such as glioblastomas, which presently are incurable, and more recently, locally advanced recurrent cancers of the head and neck region and, much less frequently, superficial melanomas primarily involving the skin and genital region.
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