Neutron Fundamentals
Why Neutrons?
A neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of elements consist of protons and neutrons. The number of protons in a nucleus is the atomic number and defines the element. The number of neutrons is the neutron number and determines the isotope of the element. For example, the abundant carbon-12 isotope has 6 protons and 6 neutrons, while the very rare radioactive carbon-14 isotope has 6 protons and 8 neutrons.
While bound neutrons in stable nuclei are stable, free neutrons are unstable; decaying in just under 15 minutes. Free neutron beams are obtained from neutron sources by nuclear fission or nuclear fusion.
Until the introduction of Adelphi generators, access to intense neutron sources meant that researchers must travel to specialist neutron facilities like research reactors and spallation sources to work with free neutrons for use in irradiation and in neutron scattering experiments. Now, much of this work can be performed in the researcher’s home lab.
Neutron Generation
Because free neutrons are unstable, they can be obtained only from nuclear disintegrations, nuclear reactions, and high-energy reactions (such as in cosmic radiation showers or accelerator collisions).
Neutron generators are neutron source devices which contain compact linear accelerators and that produce neutrons by fusing isotopes of hydrogen together. The fusion reactions take place in these devices by accelerating either deuterium, tritium, or a mixture of these two isotopes into a metal hydride target which also contains either deuterium, tritium or a mixture. Fusion of deuterium atoms (D + D) results in the formation of a He-3 ion and a neutron with a kinetic energy of approximately 2.5 MeV. Fusion of a deuterium and a tritium atom (D + T) results in the formation of a He-4 ion and a neutron with a kinetic energy of approximately 14.1 MeV. The DT reaction is used more commonly than the DD reaction because the yield of the DT reaction is 50–100 times higher than that of the DD reaction.
D + T → n + 4He En = 14.1 MeV
D + D → n + 3He En = 2.5 MeV
Neutrons produced from the fusion reaction are emitted isotropically (uniformly in all directions). In all cases, the associated He nuclei (alpha particles) are emitted in the opposite direction of the neutron.
In comparison with radionuclide neutron sources, neutron tubes can produce much higher neutron fluxes and monochromatic neutron energy spectrums can be obtained. The neutron production rate can also be controlled.
Products
Academic and Industrial Applications
The neutron plays an important role in many nuclear reactions. Neutron applications have focused on measuring different elements in a variety of materials. Typically, neutron-generator-based approaches can provide this information much more quickly than other laboratory techniques.
Because fast neutrons have a large effective range of penetration in most materials—greater than 1-meter in many cases—neutron analysis of bulk materials has significant advantages over x-ray laboratory techniques. This is particularly true where sample collection and preparation are a problem, as when sampling is difficult or when the material is not homogeneous. The noncontact, nondestructive, and remote-measurement capability that neutron analysis techniques allow additional advantages.
Neutron capture can result in neutron activation, inducing radioactivity in a sample. This forms the basis of neutron activation analysis (NAA). NAA has been most often used to irradiate small samples of materials in a nuclear reactor, and then analyze the delayed gamma-ray emission after the sample is removed. With the introduction of Adelphi’s high-output neutron generators, which have an “off switch,” NAA becomes simple in a home lab experimental setup.
Prompt gamma neutron activation analysis (PGNAA) is a similar technique, but one where the sample is not left in an activated condition. In PGNAA neutrons excite the sample while prompt gamma-rays from elements in the sample are measured and analyzed, simultaneously. PGNAA has been most often used to analyze industrial bulk materials on conveyor belts using neutron-emitting radioisotopes. Increasingly, this standard industrial work is being performed with neutron generators.
Cold, thermal and fast neutron radiation is commonly employed in neutron scattering experiments, where the radiation is used in a similar way one uses X-rays for the analysis of condensed matter. Neutrons are complementary to the latter in terms of atomic contrasts by different scattering cross sections; sensitivity to magnetism; energy range for inelastic neutron spectroscopy; and deep penetration into matter.
Healthcare
The medical practice of radiation science is Radiology. Radiation Oncology is a segment of Radiology, and is distinguished as therapeutic rather than diagnostic.
The radiation treatment of cancer follows the general understanding that healthy tissues recovers better from exposure to any radiation as compared to diseased tissues. This phenomenon is enhanced by active cropping of the treatment beam, fractioning, and exposure angle control. In some circumstances, absorption of radiation is helped by disease-specific radiosensitizers or contrast agents.
Boron Neutron Capture Therapy (BNCT) is an alternative radiation oncology method that uses exposure to neutrons rather than gamma rays or x-rays. The technique relies on a type of radiosensitizer that couples boron atoms to a molecule with affinity to the tumor. Healthy tissue has little reaction with the neutron beam. Cancerous tissue that has boron uptake, however, is greatly affected.
Neutrons are absorbed into boron and boost the energy level of the nucleus of the atom. This higher energy state quickly drops to its normal level with the emission of an alpha particle. It is the energetic alpha particle that kills the surrounding diseased cells.
BNCT is a very active research technique, with many researchers actively seeking a portable neutron source. US., European, South American and Japanese researchers continue to pursue this approach to cancer therapy, and have actively sought a portable neutron source.
Analysis of past research efforts reveals a two-fold problem to the approach of using a research reactor as the source of neutrons. First, the spectrum of neutron energies used was not highly absorbed into boron. Secondly, the background gamma radiation coming from the core complicates the treatment plan.
Current research has focused on recurrent head and neck tumors, among other diseases. Fixed installation cyclotron neutron sources (not offered by Adelphi Technology) have been treating recurrent head and neck tumors with great success using BNCT. Other BNCT researchers from around the world have signed a letter of support for the development of less expensive, portable neutron sources of the kind produced by Adelphi Technology, Inc.
Adelphi Technology, Inc. has several proprietary designs for neutron generators and close coupled moderators. A major benefit of Adelphi Technology’s re-configurable design is that future therapy setups can be accommodated easily. Contact us to see if a simple exposure experiment in our demonstration lab will prove neutron therapy valid for your disease treatment research.
Security
Programs at several national laboratories, universities, and private companies in the U.S. and around the world are developing neutron generator-based systems for detecting high explosives, chemical weapons, contraband and special nuclear materials in a variety of concealed situations. The objectives of these programs include developing systems for border security, airline-cargo inspection, and first response in the investigation of unknown packages.
Adelphi Technology offers several neutron generator configurations for security applications. Smaller systems may be used in the inspection and screening of passenger baggage and larger systems are useful for cargo containers.
Some of our generators can perform as the neutron source in a security imaging system that can isolate concealed substances in the inspected volume. Please remember that neutrons are absorbed by different mechanisms than x-rays making neutron imaging systems complimentary to the more common x-ray imaging. In nuclear reactors and nuclear weapons, the fissioning of elements like uranium-235 and plutonium-239 is caused by their absorption of neutrons. With high-output neutron generators, this phenomenon can be exploited to find concealed special nuclear material (SNM) and may prove an enabling technology in global anti-proliferation efforts.
Neutron analysis (prompt gamma analysis and/or pulsed differential die-away analysis) allows materials identification which means that explosives and SNM may be detected and identified. The ability of neutron systems to discriminate elements gives it a significant advantage over other technologies. The high-intensity output, pulsing capability, long lifetime and serviceability of Adelphi’s generators make them ideal for these applications. In the future, we feel that Adelphi Technology’s neutron generators will be instrumental in protecting national security at national borders.