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Answer to Question #9368 Submitted to "Ask the Experts"

Category: Accelerators

The following question was answered by an expert in the appropriate field:

Q

What kind of controls would be needed for a DT (deuterium-tritium) neutron generator with a source strength of 1.5 × 108 neutrons per second?

A

DT neutron generators are small accelerators that utilize the large fusion cross section (5 barns) of the deuterium (D) and tritium (T) ions in the following reaction to produce 14.2 MeV neutrons:

D + T --> n + 4He.

Either one of the isotopes can be the projectile and the other the target. Both isotopes are usually in the form of metal hydrides (more on this later). For example if a metal hydride form of deuterium is used for the source, it is heated until the deuterons form a plasma and then they are accelerated, using high voltage, to 130 keV. The 130 keV deuterons then interact with the target tritium atoms to produce alpha particles and 14.2 MeV neutrons. The total energy released from this fusion reaction is 17 MeV. The neutrons and alpha particles produced in the DT reaction are emitted isotropically. Of course, the alpha particle range is very short and may not exit the target material.

There are several types of DT generators with either pulsed or continuous output. Since you have not identified the type, I assume the neutron emission is continuous or one pulse per second where each pulse is of the strength you have given. Using the neutron weighting factors in the latest version of 10 CFR 835, which is based on International Commission on Radiological Protection 60, and fluence to dose conversions (see Health Phys 96(2009)617-628), a source emitting 1.5 x 108 14-MeV neutrons per second would produce a dose rate of about 1.8 x 10-2 Sv h-1. Of course this constitutes a High Radiation Area. This source will require shielding. Compact shielding for fast neutrons should have at least two component, the first layer uses high Z material, such as iron or tungsten, to slow down the neutrons and then hydrogenous material to thermalize and absorb the thermal neutrons. Usually concrete, polyethylene, or even water are used. Thermal neutrons are captured on hydrogen through the

H + n --> 2.2 MeV gamma-ray + D.

The emitted gamma ray is another source of radiation from the hydrogenous shields. The fluence of these photons can be reduced by adding boron compounds to any of these shield materials. In the borated shielding material, thermal neutrons are captured on boron-10 which has a large capture cross section (3,837 barns). The reaction products are an alpha particle and lithium-7 ion, both of which have very short ranges. A 0.48 MeV gamma ray is emitted 94 percent of the time in this reaction, which has a shorter attenuation length than the 2.2 MeV gamma ray.

As for controls, for the radiological issues, shielding, interlocks, training, and access controls are needed. For a DOE facility, 10 CFR 835 specifies the requirement.

You should assume that whenever the DT generator is operating, neutrons of all energies from 14.2 MeV down to thermals will be produced. In principal these neutrons can activate materials including air. Of course, the amount produced depends on the number of neutrons produced. Depending on the form of tritium used, a damaged generator could leak tritium. Since this is also a high-voltage device, electrical safety considerations such as interlocks may be needed. If the device is malfunctioning and sparks, there is the possibility of x-ray emissions.

Finally, metal hydrides are used in order to have a large storage of deuterium and tritium in very compact form. Although deuterium is not radioactive, both of these materials are sensitive nuclear materials (e.g., for tritium threshold quantities, check DOE-STD-1027-92). Nuclear material accountability becomes important. This becomes more serious if the metal used for the deuterium hydride is lithium. Lithium deuteride is fuel for thermonuclear weapons.

Kamran Vaziri, PhD
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