Development of high flux thermal neutron generator for neutron activation analysis

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Abstract

The new model DD110MB neutron generator from Adelphi Technology produces thermal (<0.5 eV) neutron flux that is normally achieved in a nuclear reactor or larger accelerator based systems. Thermal neutron fluxes of 3–5 · 107 n/cm2/s are measured. This flux is achieved using four ion beams arranged concentrically around a target chamber containing a compact moderator with a central sample cylinder. Fast neutron yield of ∼2 · 1010 n/s is created at the titanium surface of the target chamber. The thickness and material of the moderator is selected to maximize the thermal neutron flux at the center. The 2.5 MeV neutrons are quickly thermalized to energies below 0.5 eV and concentrated at the sample cylinder. The maximum flux of thermal neutrons at the target is achieved when approximately half of the neutrons at the sample area are thermalized. In this paper we present simulation results used to characterize performance of the neutron generator. The neutron flux can be used for neutron activation analysis (NAA) prompt gamma neutron activation analysis (PGNAA) for determining the concentrations of elements in many materials. Another envisioned use of the generator is production of radioactive isotopes. DD110MB is small enough for modest-sized laboratories and universities. Compared to nuclear reactors the DD110MB produces comparable thermal flux but provides reduced administrative and safety requirements and it can be run in pulsed mode, which is beneficial in many neutron activation techniques.

Introduction

The deuteron–deuteron fusion reaction (D–D) has been a utilized extensively in recent years in various Adelphi Technology neutron generators [17], [19], [4] to produce neutrons without using radioactive material. Without the need for tritium, the design and maintenance of D–D generators are much more simple and flexible, ultimately driving down the cost of very high flux neutron generators. The DD110MB manufactured by Adelphi Technology creates high intensity thermal neutron flux to irradiate samples for neutron-activation analysis (NAA).12H+12H22He+01n+3.27MeV

The neutron generator consists of four radial deuteron ion sources surrounding a central target cavity. Deuteron ions are generated by electron cyclotron resonance (ECR) [2], [6], [12] which are then accelerated to 120 keV and bombard a titanium target. Fast neutrons (2.45 MeV) are produced via the D–D reaction (Eq. (1)) at the rectangular cavity walls and then thermalized immediately by an integrated polyethylene moderator on the back of the target. To further enhance the thermal neutron flux at the sample area, the target cavity is surrounded by a neutron reflector. The moderator is designed such that the thermal flux peaks at the center of the target cavity where the radiated sample is located. A sample rabbit transfer system was implemented to minimize the time between irradiation and counting.

The DD110MB (Fig. 1) pushes the boundary of achievable neutron fluxes from compact D–D neutron generators. Thermal neutron flux on the order of 0.5–1·108 n/cm2/s with less than 12 kW of beam power have not been available on compact D–D fusion based neutron generator systems. DD110MB provides compact and simple solution with minimal regulatory burden for NAA applications that previous required reactor-based or traditional accelerator-based systems.

Section snippets

Generator design

The DD110MB is considered a low energy beam transport (LEBT) system because of the low energy ion acceleration needed to achieve neutron production, the following considerations are critical in order to design a neutron generator that has stable operation and high neutron production efficiency [2], [11], [14], [8], [9], [18]:

  • Ion beam envelope steers clear of contact with the puller electrode.

  • Electric field gradients remain below design limit of 5 MV/m on the extraction gap [11].

  • Power density and

Characteristic curves

The characteristic curves are a set of curves that relates the current and voltage at various neutron generator operating parameters. These curves are invaluable for determining the operable range of the neutron generator. Normally, the ion current stays relatively constant as the voltage is decreased. When the voltage is low enough such that the ion beam begins to make contact with the puller electrode, the current-voltage relationship begins to exhibit a sharp inverse proportionality. As seen

High voltage arcing

One of the most challenging issue for compact, high flux neutron generators is the problem of high voltage arcing. Such arcing behavior not only reduces the average neutron flux by decreasing the uptime, but also causes instability and potential damage to the high voltage insulation. The most common form of arcing for compact generators is surface flashover on the insulator surface between the high voltage electrode and ground. The flashover strength across this insulator-bridged gap can be up

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