Portable Neutron Generators

Read this first: US NRC regulations require that buyers have a Type A Broadscope specific license authorizing possession of tritium in the applicable quantities before purchase of tritium neutron generators or you must apply for an engineering safety sealed source and device (SS&D) review and have a specific license authorizing the possession of these devices. Visit U.S. Nuclear Regulatory Commission (NRC) website for details.

Neutrons may be produced using a number of techniques including radioactive isotopic sources, electrophysical neutron generators and large research accelerators.

Isotopic neutron sources produce continuous fluxes of neutrons. Typical isotopic sources are Californium-252 (252Cf), with a half-life of about 2.6 years (one mg 252Cf produces about 2.3106 n/s), or Americium-Beryllium (AmBe), which produces neutrons via the 9Be(a,n)12C reaction (241Am has a half-life of 458 years). Isotopic neutron sources have the advantage of having a long useful life and producing a relatively constant flux of neutrons. They are relatively inexpensive for low flux (<108 neutrons per second) sources. However, isotopic sources have several disadvantages. The neutron output can not be turned off; requiring that they be contained within bulky shielding at all times. Isotopic neutron sources cannot be pulsed and the energy spectrum of the emitted neutrons is broad and peaks at energies below the threshold for some important reactions.

Over the last few years accelerator based neutron sources have evolved in the two main directions. On one side substantial funding resulted in creation of advanced neutron sources in many parts of the world with a SNS to be the most powerful. The Spallation Neutron Source (SNS) is an accelerator-based neutron source being built in Oak Ridge, Tennessee, by the U.S. Department of Energy. The SNS will provide the most intense pulsed neutron beams in the world for scientific research and industrial development. At a total cost of $1.4 billion, SNS construction began in 1999 and will be completed in 2006.

On the other side tabletop neutron generators have evolved from a large, expensive instrument to a compact, affordable product. Small neutron generators using the deuterium (2H) - tritium (3H) reaction are the most common accelerator based (as opposed to isotopic) neutron sources. Creating deuterium ions and accelerating these ions into a tritium or deuterium target produces neutrons. Deuterium atoms in the beam fuse with deuterium and tritium atoms in the target to produce neutrons.

d + t→ n + 4He              En = 14.2 MeV

d + d→ n + 3He              En = 2.5 MeV

d + t reaction has the largest maximum cross-section of 5.0 Barn (10-24cm2) of all fusion reactions. Maximum cross-section of this reaction for energies of incoming particle below 1 MeV is reached at the energy of 130keV. Table below shows fusion energy release, maximum cross section (in Barns) for energies below 1 MeV and the energy of incoming particle corresponding to maximum cross-section (in MeV), for most important fusion reactions used in portable neutron generators.

 

Reaction

Fusion energy release, MeV

Maximum cross-section, Barn*

Energy of incoming particle at smax, MeV

1

d + d → n +  3He

  3.3

  0.09

1.0

2

d + t → n + 4He

17.6

5.0

   0.13

3

t + d → n + 4He

17.6

5.0

    0.195

*A unit of area sometimes used to measure cross section in nuclear interactions involving incident particles. It is equal to 10-28 square meter. The name comes from the phrase `side of a barn' (something easy to hit).

Neutrons produced from the d-t reaction are emitted isotropically from the target. Neutron emission from the d-d reaction is slightly peaked in the forward (along the axis of the ion beam) direction. In both cases, the He nucleus (a particle) is emitted in the exact opposite direction of the neutron. In order to estimate neutron flux density n (neutrons per square cm per second) we can use a simple relation:

n = N/(4pR2)

where N is a total neutron output from neutron generator (in neutron per second), R is a distance from the deuterium or tritium target inside neutron tube to the location where we measure neutron flux. For example for N=3*1010 neutrons/s and distance of R = 1m neutron flux n = 0.24 * 106 neutrons/(cm2s). At R=50m neutron flux is only about 102neutrons/(cm2s).

Most small d-t accelerators are sealed tube neutron generators. The ion source, ion optics and the accelerator target are enclosed in within a vacuum tight enclosure. Either glass or ceramic insulators provide high voltage insulation between the ion optical elements of the tube. The neutron tube is, in turn, enclosed in a metal housing, the accelerator head, which is filled with a dielectric media to insulate the high voltage elements of the tube from the surroundings. The accelerator voltage is normally between 80 and 180 kilovolts.

The accelerated ions strike the target. The target is made of titanium, scandium, or zirconium that form stable chemical compounds called metal hydrides when combined with hydrogen or its isotopes. These metal hydrides are made up of two hydrogen (deuterium or tritium) atoms per metal atom and allow the target to have extremely high densities of hydrogen. This is important for maximizing the neutron yield of the neutron tube. The gas reservoir element also uses metal hydrides as the active material.

The neutron generator does not create any radiation when it is switched-off. They may be operated either as continuous or pulsed neutron sources. The neutrons produced are mono-energetic (2.5 MeV or 14 MeV).

Del Mar Ventures supplies a variety of neutron generators, both standard and custom made. Two main components of the neutron generator are ion source and target. Three different types of ion sources are used: Penning Ion Source, Spark Ion Source and Plasma Focus.

In the table below we provide brief comparison of those ion sources:

 

Penning Ion Source

Spark

Plasma Focus

Frequencies

up to 20kHz and continuous

1 - 100

0.01

Neutron Output, n/s

109

3*1010

1010 per pulse

Lifetime at max yield

500

100

500 pulses

Pulse duration, ms

10-200

0.8

0.010

All three types of ion sources can operate in a pulsed mode. Plasma Focus ion source provides shortest neutron pulses about 10ns long. Spark ion source provides longer pulses with a typical duration of 800ns, but it can operate at much higher repetition rate than Plasma Focus. Highest repetition rate can be achieved using Penning Ion Source with typical pulse duration of 10 200 ms.

Penning ion source is the only ion source of those shown in this table that can provide continuous neutron flux during operation (continuous mode, or CW). The Penning ion source is a low gas pressure, cold cathode ion source which utilizes crossed electric and magnetic fields. The ion source anode is at a positive potential, either dc or pulsed, with respect to the source cathode. The ion source voltage is normally between 2 and 7 kilovolts. A magnetic field, oriented parallel to the source axis, is produced by a permanent magnet. Heating or cooling the gas reservoir element regulates the gas pressure in the source.

Plasma is formed along the axis of the anode that traps electrons, which, in turn, ionize gas in the source. The ions are extracted through the exit cathode. Under normal operation, the ion species produced by the Penning source are over 90% molecular ions. Penning Ion Source is used in continuous mode neutron generators including new line on neutron generators with built-in a particle detector (aNG).

Lifetime shown in the table is a minimum lifetime guaranteed for operation at maximum yield. For example 100 hours shown in the table for spark ion source neutron generators corresponds to total of 100Hz* 3600*100hours = 3.6*106 pulses. In fact actual life time at maximum yield is even higher - about 107 pulses (10 Million pulses) and limited by the ion source lifetime. Lifetime will be longer is operational frequency is less. For example for 10 Hz it would be at least 1000 hours (guaranteed), >2000 hours (actual).

Neutron Generator with built in Multipixel Alpha Particle Detector

Neutron generator with built in alpha particle detector uses an interesting physical property of the deuterium-tritium reaction, namely that the target emits an alpha particle simultaneously with, and at exactly at 180 degrees opposite to, each fast 14 MeV neutron produced. The α-particles with energy of 3.5 MeV flight back-to-back with the neutrons in the c.m. (center of mass) system. In a modified form of the d-t generator these alpha particles can be detected internally with a position sensitive segmented (multipixel) array detector. By localizing the α particle trajectory the direction of the corresponding neutron is determined. A multipixel detector will provide alpha particle time of generation and therefore that of the accompanying neutron as well as the direction relative to the target and therefore also the direction of the accompanying neutron, since its line of travel is opposite to that of the alpha particle. The fast neutrons thus produced are therefore also defined as "tagged" (by the alpha particle), in time as well as in direction.

Neutron generators with built-in a particle detector are becoming very popular for identification of the content of different objects with high sensitivity and position resolution. The neutron collides with a nucleus of the material under study and produces a g-ray, whose time of arrival at the detector can be precisely measured. This is therefore a Time-of-Flight (TOF) method, allowing determining the distance traveled by the neutron (as both the speed of the neutron and of the gamma are known). As its direction is also known, three-dimensional spatial resolution of targets can be provided.

The speed v/c of a neutron of kinetic energy K, or actually of any particle, is equal to:

v2/c2 = 1 - m02/m2 = 1 - m02/(m0 + K/c2)2, where m0 is the particle rest mass (939.57 MeV/c2 for a neutron) and m = m0 + K/c2 is its relativistic total mass. A neutron with a kinetic energy of 14 MeV, for example, would therefore have a total mass of 953.57 MeV/c2 and therefore a speed of 0.17c = 0.511010 cm/s = 5.1 cm/ns. To obtain 5cm spatial resolution acquisition electronics with nanosecond resolution is used.

In the most advanced option a multipixel a particle detector is used. The combination of fast neutron incident on a target volume and high-energy gamma rays exiting the target volume makes the technique very penetrating. The combination of the multipixel detector of a particles and multichannel analyzer of the gamma rays makes the techniques very sensitive and ensures high resolution. Multipixel detector of accompanying particles will increase the directionality of the neutron detector system providing more effective active shielding from the surrounding materials.

Due to the use of alpha particle signal as a natural trigger, there is no need to use pulsed neutron generator. Instead, compact D -T neutron generator with continuous Penning ion source can be used without expensive and heavy high-voltage pulse electronics. The neutron production rate might even have to be kept low so neutron interactions do not interfere in order to limit random coincidences.

Detection of the associated alpha particle requires that an alpha detector be built into the neutron tube.  This places stringent requirements on the alpha detector material and design in that it must withstand the temperatures used in processing the neutron tube without introducing contaminants into the tube. Alpha detector should also have very high radiation hardness for reliable operation. Del Mar Ventures team has substantial experience in developing neutron generators with built in alpha particle detectors.

This brochure provides basic information about neutron generators and helps to the reader to identify main features and specification of different models available from Del Mar Ventures. The aim of this brochure is to help to our potential customers to choose the best model for their specific needs. When sending us your inquiry or request for quotation, please indicate the following: average total neutron output (n/s), mode of operation (pulsed or CW), for pulsed mode required pulse duration, lifetime at required yield, any requirements or limitations on dimensions of the neutron tube. Detailed description of proposed application would be very also helpful.

Del Mar Ventures> Product List> Neutron Products Index>

Del Mar Ventures
4119 Twilight Ridge
San Diego, CA 92130
tel (858) 755-6727
fax (858) 755-6771
v/efax (858) 630-237
v/efax: (509) 752-0123
delmar@sciner.com
http://www.sciner.com/
http://www.femtosecondsystems.com/

eyemail.gif (2917 bytes)