Neutron Generators: How they work

Principles of neutron generator operation

Adelphi’s neutron generator products operate by producing a plasma of either pure deuterium gas or a mixture of deuterium and tritium. These gasses are isotopes of hydrogen (hydrogen consists of 1 proton and no neutrons, deuterium contains 1 proton and 1 neutron and tritium contains 1 proton and 2 neutrons). The ions exit a small hole in the ion source (called the iris or ion source exit aperture) and are accelerated by a high voltage, typically of 80kV to 225kV depending on the product). These ions implant into the neutron generator’s target. Eventually during this ion bombardment/implantation, an ion of deuterium will collide with an ion or deuterium or tritium. One of the ions will already be implanted in the target and the other one will have been accelerated to such an energy that nuclear fusion occurs on the neutron generator’s target. A range of nuclear reactions occur, the ones that lead to neutrons are:

• D+D=He-3 + n (2.45 MeV)
• D+T=He-4 + n (14.1 MeV)

Where D denotes deuterium (1 proton + 1 neutron), T denotes Tritium (1 proton and 2 neutrons), “He” indicates a helium nucleus either the isotopes He-3 (2 protons and 1 neutron) or He-4 (2 protons and 2 neutrons), n denotes a neutron and the energy of the neutron is stated in MeV.

Neutron generator components/subsystems

The neutron generator consists of a number of subsystems which enable this reaction:

Subsystem: Ion source (Electron Cyclotron Resonance, ECR)

Adelphi uses Electron Cyclotron Resonance (ECR) ion source in all of its current products. These ion sources efficiently produce atomic species of ions: Deuterium or tritium molecules contain 2 atoms (the isotopes of Hydrogen are H2, D2 and T2, the number denoting the number of atoms in the molecule). In these ion sources a Radio Frequency (RF) source excites a plasma, a magnetic field is applied to the ion chamber volume. The frequency of the microwaves is matched to the magnetic field such that the electrons move in circles. These electrons that are undergoing circular motion then collide with molecules and can nock of an electron to make a molecular ion (a charged molecule), with sufficient energy (as is true in our ECR ion sources), the electron can split the molecule into atoms, the motion of the electrons nocks off any electrons bound to the D or T nuclei leaving a plasma that contains deuterium and tritium ions and electrons. The key thing is that the ion sources are engineered to ensure that the ECR resonance conditions occurs within the volume of the ion source. If 2.45 GHz radiation is used, then the magnetic field needs to be 875 Gauss to satisfy the ECR condition.

Subsystem: Radio Frequency (RF) source

Adelphi presently uses semiconductor RF sources to excite the plasma. Depending on the product, and on supply chain availability, these may be RF sources that are designed and built at Adelphi, or they may be purchased. Another option is magnetrons, very similar to the magnetrons inside microwave ovens. RF sources offer greater control and stability (magnetrons suffer from ‘mode hopping’ which causes the ion source to momentarily be on and off resonance causing fluctuations in beam current on the sub second timescales. This in itself is not an issue, but the ion source runs more efficiently when run precisely on resonance, so for magnetron systems greater cooling of the ion source (and also the RF source/magnetron) is required.

The ion sources can be pulsed (typically by asserting a digital signal), this allows the neutron production of the generator to be pulsed.

Subsystem: Hight Voltage (HV)

The ions are acceleration by a high voltage. Adelpi typically uses commercial off the shelf (COTS) power supplies for most of it’s products. However, Adelphi has been building its own high votage power supplies for around 8 years which we typically use on custom generators for miniaturization applications. We are also phasing-in the use of our own high voltage power supplies on some of our lower yield commercial products.

Subsystem: Gas system (gas source)

In “open” systems the generator is actively pumped and deuterium gas is supplied via a gas bottle. A regulator is connected to the bottle to regulate the pressure and a Mass Flow Controller (MFC) meters the deuterium gas into the system. It also measures the gas flow, alerting the system when the bottle is empty.

In “sealed” systems, the gas is pre-loaded into the neutron generators getter (non evaporable getter, NEG). The getter is heated and it then evolves gas. The generator can then be operated when the generator reaches its operating pressure, which is measured using various vacuum gauges.

Subsystem: Gas system (vacuum system)

The “open” systems employ an active pumping system, they are usually left continuously running and are never switched off. These consist of a turbo pump and roughing pump. The roughing pump is typically exhausted to the room. During typical operation 1 to 5 standard cubic centimeters of deuterium are released into the environment every minute, these quantities are so low that they do not cause any problems, theoretically the vacuum systems could be exhausted to the outside world if this was considered an issue.

Subsystem: Target cooling

The metal target is bombarded with ions. For our low yield generators, this corresponds to a few watts of heat energy. For our high yield generators, this corresponds to a few kilowatts of heat energy. Target cooling is therefore a key issue for the high yield generators. The neutron generators include a closed-loop cooling system that keeps the target below the temperature at which the deuterium or tritium boils off the target. When operated we typically measure the neutron yield as a function of RF source duty cycle and plot the graph. If the neutron yield is proportional to the duty cycle then cooling is sufficient, if the yield rolls-off at high duty cycles then this indicates that the target is getting too hot, causing ions to “boil off” the target. This is an indication that cooling should be improved to increase yields.

Subsystem: Ion source cooling

The high yield generators also need to have ion sources that produce a large number of ions. Cooling is necessary on these. Sometimes the same cooling system is used on the RF/ion source cooling as on the target. Other times they are separate systems. The details depend on the specific generator and upon the customer’s application/preference.

Start-up and operation.

To operate the generator it must be brought to it’s operating pressure, microwaves must be applied to produce ions, and the high voltage system must be energized. Ions are then accelerated towards the target and then the target loads with ions. This loading process can take a few minutes, the first time the generator is operated, and the generator’s yield ramps up over this time. When the target is loaded. The cooling systems must be operational to prevent damage to the generator. This is especially important for the high-yield generators. Adephi’s control system provides an extensive set of interlocks that allow full control over the neutron generator’s parameters while preventing operation of the generator under any “unsafe” (to the generator) conditions, such as when the cooling systems are not operational.

Prior to applying voltage for the first time, generators need to undergo high voltage conditioning. During this process, the high voltage is slowly and carefully ramped up to full voltage over the course of around 15 minutes. It is good practice to condition the generator’s high voltage system prior to operating the generator each day (this isn’t necessary if the generator is operated 24-7, as some customers do). Sealed generators generally require less conditioning because during manufacturing they are baked out, so there is minimal gas adsorption onto the generator’s internal surfaces. Adelphi’s generators can be conditioned by hand, or have automated conditioning routines.

The generators are provided with an interlock input switch closure. Customers typically connect a series of switches to doors and “EMO – Emergency Machine Off” buttons in series. This is the user’s Personal Protection System (PPS) interlock input. When the electrical circuit is broken the generator cannot operate. Adelphi’s control system has outputs for tower lights that are configurable. Typically

• Red= System Energised: High voltage is present so X-rays could be produced and and neutrons will be made when ever the ion source is energized.
• Amber = All of the interlocks are satisfied so the generator will make neutrons if commanded to do so.
• Green = At least one of the interlocks is not satisfied so the neutron generator will not allow the high voltage to be applied, so the system is safe and cannot produce radiation.

Some customers want the external interlock to also inhibit the microwaves/ion source, and the system can be configured in that way, or in many other ways.

Not all customers use the PPS interlock, and just “loop-it-out” (by providing a wire link across the interlock input) but it is most common to have at a minimum to provide the EMO switch to the PPS input (usually a red button).