Neutron

A neutron is a subatomic particle that normally is part of the nucleus of a chemical element. When free (not bound to a nucleus), a neutron can have important physical, chemical, and biological effects.

The mass m_n of a neutron is close to, but not equal to, the mass of a proton:

m_n = 1.674 927 211 × 10⁻²⁷ kg.

The neutron consists of three quarks. A free neutron shows beta decay, breaking down into a proton, an electron, and an antineutrino. Because it disintegrates, the free neutron does not exist in nature. Neutrons do not carry electric charge, they pass unhindered through the electrical fields within liquids and solids.

History

The existence of the neutron was discovered, in 1932, by Sir James Chadwick, who received the 1935 Nobel Prize in Physics for his work. A repeatable experimental demonstration of the existence of the neutron solved a number of then-outstanding problems in physics, although the applications and significance of neutrons were in their infancy.

Health effects

From the biological standpoint, neutrons are indirectly ionizing. See acute radiation syndrome; a given dosage by particles may have greater biological effect than the same dosage from X-rays or gamma rays.

Applications

Analytical

Neutron activation analysis, which has several variants, is a widely used technique for measuring the proportions of elements in a sample, including trace elements. A basic analyzer has a neutron source (preferably a appropriate nuclear reactor, detectors for gamma rays emitted by the target, and an extensive database and technical skills to interpret the interaction and its results. Portable analyzers are restricted to radioisotope and accelerator sources.

Neutron capture method

Also called the (n,gamma) reaction, it depends on non-elastic collisions of neutrons with nuclei in the target. The nucleus struck becomes excited, with the excitation energy dependent on the particular element. The period of excitation is usually quite short, and, as the nucleus loses exciting energy, it will emit one or more gamma rays, at characteristic times and energy levels. Half-lives of excited nuclei can range from less than a second to months, and it is these times and energies that are analyzed.

Imaging

There are various industrial applications of neutrons for such purposes as inspecting the quality of welds, which are relatively straightforward with a generator on one side and an imaging detector on the other. Newer applications, such as for baggage and cargo screening in transportation safety, are both more complex and more powerful.

First, by using multiple beams and detectors, three-dimensional views of the contents of a container can be visualized. Second, neutron activation of conventional materials in the container help identify their content, such as nitrogen-rich compounds that might be explosives. Third, if fissionable materials are present, there will be a net increase of neutrons emitted when the container is irradiated.

Nuclear reactors

In nuclear reactors for power and research, neutron generation steadily increases as more and more fissionable material comes into close proximity. The challenge is less to generate them than to control their rate of flow, and the basic mechanism is to have control rods, of neutron-absorbing materials, interspersed with the rods containing the fissionables. Mechanically inserting or withdrawing numbers of control rods is the usual method of fine-tuning the neutron generation rate.

Neutron generators

There are three basic ways to generate neutrons: radioisotopes, particle accelerators, and nuclear reactors. Each has advantages and disadvantages for specific applications.

Neutron generation from radioisotopes

In the first nuclear weapons, an initiator, at the center of the fissionable material, emitted neutrons. Codenamed the "Urchin", it was a sphere of mixed polonium (₂₁₀Po) and beryllium. Without going through its complex mechanical design, the basic material was a hollow beryllium sphere, grooved on the inside, and with a solid beryllium pellet at the center. As the urchin was explosively compressed and vaporized, alpha particles emitted by the Po-210 then struck beryllium atoms, which released neutrons. The bomb also depended on a number of other mechanical aids to generate enough neutrons for the critical mass, such as neutron-reflecting outer shells that redirected neutrons into the core

Neutron generation from particle accelerators

Later neutron sources, based on linear particle accelerators, which is a cylinder with an ion source at one end and an ion target at the other end. The space between them contains deuterium, tritium, or some mixture depending on the specific generator design. Electrical current supplied to the source causes an electrical arc and generates hydrogen ions, which are then accelerate using electromagnetic force from another, accelerating electrode, which sends the accelerated cloud into the target. Individual neutrons (i.e., not a beam) are generated by the ions hitting the target, which has one or more hydrogen isotopes on its surface.

The most obvious difference between the unclassified generators used in industry, and the classified detectors used in weapons, is size and ruggedness. Both types do have a superficial resemblance to a household hair dryer, with the ion source at the motor/heater end. The first tube used titanium hydride targets, but the standard in the indusctry uses scandium hydride.

One unclassified design described by Sublette is the Milli-Second Pulse (MSP) tube developed at Sandia. "It has a scandium tritide target, containing 7 curies of tritium as 5.85 mg of ScT2 deposited on a 9.9 cm² molybdenum backing. A 0.19-0.25 amp deuteron beam current produces about 4-5 x 10⁷ neutrons/amp-microsecond in a 1.2 millisecond pulse with accelerator voltages of 130-150 KeV for a total of 1.2 x 10¹⁰ neutrons per pulse. For comparison the classified Sandia model TC-655, which was developed for nuclear weapons, produced a nominal 3 x 10⁹ neutron pulse." The neutrons are not produced as a burst, as a stream that triggers successive neutron multiplication cycles in the ultimate target. In the design of an ENI, the critical parameters are the beam intensity, and the speed and shape of the initial ionization pulse.

In a weapon, the ENI can be placed wherever mechanically convenient, as long as it is within 1-2 meters of the core and not separated by a neuttron absorber. Since the neutron generator usually contains tritium, radioactive decay of the tritium means that the generators are components that need periodic replacement.

New, more compact and long-lived ENIs are available. Obviously, an ENI for a bomb does not need a long service life once active, but industrial generators have tended to exhaust their ions. New generator designs, however, provide the target with a source of fresh ions. The lifetime of these new devices may be in the thousands of hours.

Neutron generation from nuclear reactors

While nuclear reactors are the least portable source of neutrons, they also can be the most powerful, and offer a range of neutron characteristics that are especially useful in analysis. By using different reactors, different placement of targets with respect to the reactior, and the moderators used in the reactor, the widest possible range of neutron fluxes can be obtained. The flux distributions contain thermal, epithermal, and fast neutrons.