New Mini Neutron Tubes with Multiple Applications

Abstract

Recent experimental investigations have demonstrated that a substantial amount of H⁻/D⁻ ions can be formed by thermal desorption processes. Based on these new findings, new mini axial and coaxial-type neutron tubes have been developed for the production of high or low-energy neutrons via the d-d, d-¹⁰B, d-⁷Li or p-⁷Li nuclear reactions. By operating these mini neutron tubes with a high frequency AC high-voltage supply, short pulses of high intensity neutron beams can be generated. Multiple applications, such as carbon and well logging, neutron imaging, cancer therapy, medical isotope production, fission reactor start-up, fusion reactor material evaluation, homeland security and space exploration can be performed with the subcompact neutron generator system. It is shown that the performance of these new mini neutron tubes can exceed those of the conventional plasma-based neutron sources.

1. Introduction

Compact and high flux neutron generators are useful for many applications, such as cancer therapy, medical isotope production, material analysis, carbon and well logging, and cargo screening. Conventional neutron generators employ a plasma source, from which positive deuterium and tritium ions are extracted and accelerated to energies higher than 100 keV, resulting in 2.45 MeV d-d or 14 MeV d-t neutrons produced on the surface of the target electrode. However, there are several drawbacks in using positive ion-based neutron generators. First, the plasma source produces both atomic (D⁺) and molecular (D₂⁺ and D₃⁺) ions. Although the D⁺ ions are useful for neutron production because they react with the deuterium atoms on the target surface with full acceleration energy, the molecular ions will dissociate on the target and each deuterium atom will carry only one-half or one-third of the acceleration energy, resulting in a reduced neutron yield. Second, there are secondary electrons, produced along the acceleration column and at the target electrode, that accelerate back to the plasma source, forming part of the high voltage power supply drain current and generating unwanted X-rays. Third, the plasma source occupies a large volume of the neutron generator and it normally requires special power sources (such as a Radio-Frequency (RF) generator with a matching network) for operation. Eliminating the plasma source will make the neutron generator system smaller and can greatly simplify the operation process. Fourth, positive ions can charge up the target if it is not a good electrical conductor, and be deflected away from the target without producing neutrons. All these issues can be eliminated if negative deuterium (D⁻) ions are employed.

Recent experimental investigations demonstrated that a substantial amount of H⁻/D⁻ ions can be produced by thermal desorption processes [1]. Based on these new findings, mini axial- and coaxial-type neutron tubes have been developed, which can eliminate most issues from positive deuterium ion-based neutron generators. These “plasma-less” neutron tubes are very small in size, similar to that of the X-ray tubes. Since the operation employs thermally emitted H⁻/D⁻ ions, no molecular negative ions or electrons are present in the accelerated beam. By using a high frequency alternating current (AC) high-voltage supply, short pulses of high intensity neutron beams can be formed by using the d-d, d-¹⁰B, d-⁷Li or p-⁷Li nuclear reactions. Important applications, such as carbon and well logging, neutron imaging, cancer therapy, medical isotope production, fission reactor start-up, space exploration and homeland security can now be performed with a very compact neutron generator system. This article describes the design principles and potential applications of the new mini neutron generator.

2. The Mini Neutron Tube Design

A conventional neutron generator contains three main components: an ion source, an acceleration column and a beam target electrode [2]. In the new mini neutron tube, the ion source is a thin titanium foil where the H⁻/D⁻ ions are emitted. Once emitted, these ions are accelerated to the target electrode where neutrons are produced. Without an ion source chamber, the volume of the neutron generator is significantly reduced. In addition, complicated power sources, such as an RF or microwave generator and its matching network, are eliminated, and a simple heater power supply is all that is required to produce the H⁻/D⁻ ions. The complete neutron generator now takes the form of a small tube. The mini neutron tube can have two different configurations: the axial or the coaxial arrangement.

2.1. The Axial-Type Mini Neutron Tube

Figure 1 is a schematic diagram showing the axial-type neutron tube concept. This neutron tube is made of Pyrex glass ~2.5 cm in diameter and ~8 cm in length. A slotted thin titanium foil (~0.1 mm thick), used as a H⁻/D⁻ ion emitter, is located at one end of the tube, while a beam target electrode is located at the opposite end. The beam target electrode is positively biased with respect to the emitter foil. To produce H⁻/D⁻ ions by thermal desorption processes, hydrogen/deuterium gas and cesium vapor are initially introduced into the glass tube. The temperature of the titanium foil can be varied by adjusting the heating current. When the temperature of the titanium foil reaches 250 °C, copious amounts of D⁻ ions are emitted [1]. These D⁻ ions are accelerated to the target electrode. Depending on the reaction at the target, the neutrons produced can have different energies. Thus, 2.45 MeV neutrons are generated by the d-d fusion reaction on a titanium target. Likewise, 6 MeV neutrons are produced by the d-¹⁰B reaction, and 10 and 13 MeV neutrons can be formed via the d-⁷Li reaction, as previously reported [2].

2.2. The Coaxial-Type Mini Neutron Tube

The coaxial-type neutron tube works on the same principle as the axial-type neutron tube, except the D⁻ ion emitter and the beam target electrode are both cylindrical in form. Figure 2 is a schematic diagram showing that the target electrode is surrounded by the D⁻ emitting electrode. For some applications, such as reactor start-up, a small diameter neutron source is desired, and the coaxial-type mini neutron tube shown will be the better choice. This arrangement can produce a higher D⁻ ion current density and therefore a higher neutron flux in the vicinity of the beam target. If a higher neutron output is needed, then the multi-layer coaxial tube (Figure 3) can be used. In this neutron tube configuration, the cylindrical D⁻ ion emitter is located between the outer and the central target cylinders. This arrangement can produce a higher D⁻ ion current and therefore a higher neutron yield. In the coaxial-type neutron tubes, gas filling and cesium dispensing are performed before the tube is sealed.

For low ion beam current, both axial and coaxial-type neutron tubes can be operated with a direct current (DC) high-voltage (~100 kV) power supply. For some applications, a H⁻/D⁻ ion beam energy as high as 1 MeV is needed. In this case, a high frequency AC high-voltage supply should be employed. These compact high-voltage power sources are commonly found in portable dental X-ray machines. With this approach, the mini neutron tube can provide an instantaneous high flux of neutrons suitable for cancer therapy, neutron imaging, material analysis, carbon and well logging measurements. The production of a high neutron flux with various energies by the mini neutron tube and some of its applications are presented in the following sections.

3. Mini d-d Neutron Tubes for 2.45 MeV and Thermal Neutron Production

The d-d fusion reaction produces 2.45 MeV neutrons, which are useful for cancer therapy [3] and well-logging purposes [4]. By moderating the 2.45 MeV neutrons to thermal neutrons, d-d neutron generators have also been used for neutron activation analysis (NAA) or prompt gamma activation analysis (PGAA) [5]. The new mini d-d neutron tube, coupled with a high frequency AC high-voltage power supply, can enhance the performance and greatly reduce the size of the neutron generator system. The following sections describe some important applications of the new mini d-d neutron tube.

3.1. Axial (d-d) Neutron Tube for Intraoperative Radiotherapy (IORT) and Skin Cancer Treatment

An RF-driven D⁺ ion-based d-d neutron generator has recently been applied for intraoperative radiotherapy (IORT), for the irradiation of the tumor bed with 2.45 MeV d-d neutrons after removal of the solid cancer tumor [3]. Monte Carlo simulations have demonstrated that operating the d-d neutron generator with a beam power of 100 kV, 10 mA can generate a neutron yield of 3.3 × 10⁹ n/s with a flux of 8 × 10⁷ cm⁻² s⁻¹ at the center of the irradiation window. The delivered dose rate is about 2 Gray (RBE)/min, resulting in 4 to 9 min of treatment time [3].

The new mini d-d neutron tube design can provide a high peak neutron yield in pulsed mode operation. By operating the mini neutron tube at 500 kV and 10 mA of D⁻ ion current, the peak neutron yield is 5 × 10¹⁰ n/s. The average neutron yield becomes 5 × 10⁹ n/s for 10% duty factor (1 ms, 100 Hz) operation. Thus, the treatment time should be about the same as the larger D⁺ ion-based d-d neutron generator. By positioning the beam target at the center of the tumor bed, one can further increase the neutron flux on the cavity wall. The near isotropic neutron emission will permit the irradiation or “sterilization” of the surrounding side walls of the cavity, therefore reducing the chance of cancer recurrences. With the neutron beam produced in short pulses, the concept of FLASH therapy [6] can be tested with this neutron radiotherapy tool. The mini d-d neutron tube and its associated power supplies can all be mounted on a robotic arm similar to a low-energy dental X-ray machine.

Neutron radiation therapy is a high linear energy transfer (LET) type of radiation, with a larger relative biological effectiveness (RBE) compared to conventional X-ray therapy. However, a fission reactor or an accelerator-based neutron source is not easily available for cancer treatment. For this reason, most of the superficial cancers are treated with low and high-energy X-ray machines. With the successful treatment of refractory Merkel cell carcinoma (MCC) by neutron radiation [7], the newly developed mini d-d neutron tube should be suitable for the neutron radiation therapy treatment of MCC. Other cancers that are refractory to standard radiation therapy should also be considered for neutron radiation therapy. By placing the axial d-d neutron tube adjacent to the skin or breast cancer, one can deliver the highest available neutron flux to the cancer cells. With the diameter of the beam target electrode properly optimized, one can control the neutron irradiation area without performing beam scanning. The mini neutron tube radiation therapy system is very compact and convenient for use in most clinical centers.

3.2. Axial (d-d) Neutron Tube for Porosity Measurement in Well Logging

Americium-Beryllium (Am-Be) and ²⁵²Cf neutron sources are commonly used by the well logging industry to perform compensated porosity measurements. The Am-Be and ²⁵²Cf sources are small and do not require electrical power for operation. However, because of homeland security issues, accelerator-based neutron sources (such as d-d, d-t or d-⁷Li neutron sources) are being explored to replace the Am-Be and ²⁵²Cf sources. Monte Carlo simulations have shown that a d-d neutron source has a greater sensitivity for measuring the formation porosity than either Am-Be or d-t sources when normalized to the same source intensity [4]. To be useful for compensated porosity nuclear well logging, the intensity of a d-d neutron source must be improved from 10⁶ to 10⁸ n/s with the same power for operation as the d-t neutron sources. By operating the mini neutron tube at 1 MV and 2 mA of D⁻ ion current, the peak neutron yield is 4 × 10¹⁰ n/s. For 1% duty factor (1 ms, 10 Hz) pulsed operation, the average neutron yield becomes 4 × 10⁸ n/s. The size of the mini d-d neutron tube is smaller than the Penning-type d-t neutron source used by the well-logging industry. Its operation is quite simple and the required total electrical power is about 20 W. The performance of this mini d-d neutron tube should meet the compensated porosity logging requirement. However, there are some drawbacks for the d-d neutron tube. Since the energy of the d-d neutron is below 2.5 MeV, it is not suitable for other well-logging measurements, such as fast neutron inelastic scattering. In that respect, the d-¹⁰B neutron tube (see Section 4) is a better choice for well-logging measurements.

3.3. Multi Axial (d-d) Neutron Tubes for High Flux Thermal Neutron Generation

There are many applications that require a high flux of thermal neutrons with energies of about 25 milli-electron-volts (meV). Neutron Activation Analysis (NAA) and Prompt Gamma Activation Analysis (PGAA) are proven methods of elemental analysis [5]. With NAA, samples are irradiated with thermal neutrons and then removed to a counting area where the induced radioactive decay products are analyzed. PGAA utilizes an external thermal neutron beam to irradiate a sample, which then emits prompt gamma rays with energies characteristic of all elements. NAA/PGAA can be performed by using thermal neutrons from research reactors, isotopic neutron sources or neutron generators. In general, the thermal neutron flux produced by isotopic sources or neutron generators is low (~10⁶ n/cm²/s). However, with the new mini d-d neutron tubes, the intensity of the thermal neutron beam can be significantly improved. Figure 4 shows a thermal neutron irradiator design based on three mini d-d neutron tubes for NAA/PGAA purposes. In this irradiator, high density polyethylene (HDPE) is used to moderate the d-d neutrons from 2.45 MeV to a thermal energy of 25 meV. Monte Carlo (MCNP) calculations show that the maximum thermal neutron flux occurs at 4 cm from the neutron emitting target electrode for a 20 cm thick HDPE slab (Figure 5). The sample to be irradiated is placed at the center of a 32 cm diameter by 20 cm high cylindrical HDPE block. Three axial d-d neutron tubes are embedded inside the HDPE block as illustrated in Figure 4. The distance of the beam target for each neutron tube is maintained at 4 cm from the sample. By operating each neutron tube at 1 MV, 10 mA and 1% DF, the peak d-d neutron yield is 2 × 10¹¹ n/s. The peak thermal neutron flux at the sample is calculated to be about 2 × 10⁹ n/cm²/s or an average flux of 2 × 10⁷ n/cm²/s. This new irradiator, with enhanced thermal neutron flux, should be useful for NAA/PGAA measurements.

4. Mini d-¹⁰B Neutron Tube for 6 MeV Neutron Production

Neutrons with 6 MeV energy have many important applications, including well logging, underground carbon storage monitoring, special nuclear material (SNM) detection, positron emission tomography (PET) isotope production and fission reactor startup. The 6 MeV neutrons can be produced via the d-¹⁰B reaction [2]. Pure ¹⁰B or its compounds such as LaB₆ or B₄C can be used as a thick target for the D⁻ ion beam. Using the known cross-sections for the d-⁷Li and d-¹⁰B reactions [8] and the measured neutron yield for the d-⁷Li reaction [9], the thick target yield for the d-¹⁰B neutron at 500 keV and 1 MeV ion beam energies is estimated at about 5 × 10⁹ and 7 × 10¹⁰ n/mA, respectively [2]. Using the mini (d-¹⁰B) neutron tube, one can perform the following important applications with a very compact system.

4.1. Axial (d-¹⁰B) Neutron Tube for SNM Detection

Neutrons can be used to detect special nuclear materials (SNM) hidden inside cargo containers by stimulating detectable SNM signatures. When the container is probed with a pulsed beam of fast neutrons, they become thermalized and induce nuclear fission in the SNM. The fissions generate β-delayed high-energy gamma radiation (< 7 MeV), which is a signature for SNM [10,11]. Unfortunately, both the d-d and d-t neutrons conveniently produced by a compact neutron generator are unsuitable for this interrogation scheme. The 2.45 MeV d-d neutrons have a low penetrating power, while the 14 MeV d-t neutrons produced a strong background of interfering gamma rays by being above the ¹⁶O(n,p)¹⁶N reaction threshold energy (10 MeV) [10]. The best neutron energies should be in the 5 to 8 MeV range to achieve good penetration while limiting the activation of oxygen in the air and materials in the cargo container [11]. The d-¹⁰B reaction produces neutrons with energies at 6 MeV, which falls within the desired energy range. The thick target yield for the d-¹⁰B neutron at 1 MV, 250 mA is estimated at 1.8 × 10¹³ n/s for continuous-wave (cw) operation. If operated at pulsed mode with 10% DF, the average neutron yield becomes 1.8 × 10¹² n/s. The axial (d-¹⁰B) neutron tube can be simply located at the bottom of the cargo container. Assuming isotropic emission, the peak neutron flux at the center of a 235 cm (wide) × 239 cm (high) container is approximately 1 × 10⁸ n/cm²/s, which should satisfy the interrogation requirement [12]. Thus, besides producing neutrons with the desired energy, the d-¹⁰B neutron tube also provides the required neutron flux for the active interrogation of cargo containers at a port of entry.

4.2. Mini Axial (d-¹⁰B) Neutron Tube for Carbon or Well Logging

To combat climate change, it is essential to remove the carbon in the atmosphere. Part of the decarbonization effort is to extract carbon from the atmosphere in the form of CO₂, which can then be buried deep underground and for storage. To ensure the CO₂ will not escape back to the atmosphere, some remote monitoring systems need to be installed in the storage reservoir. Neutrons with the proper energies can stimulate the carbon atoms, resulting in the emission of gamma radiation. Figure 6 shows the inelastic scattering cross-sections of carbon (C), oxygen (O), and nitrogen (N) [13]. To detect carbon, the neutron energy must be greater than 4 MeV. When irradiated by the 6 MeV d-¹⁰B neutrons, specific gamma photons with an energy of 4.43 MeV will be produced from the carbon atoms. Thus, by monitoring the 4.43 MeV neutron response gamma line, the presence of carbon can be evaluated [14]. Besides using it for monitoring carbon in underground storage, the (d-¹⁰B) neutron tube should be suitable for compensated porosity well logging. Unlike the d-d neutrons, the 6 MeV d-¹⁰B neutrons can perform the inelastic neutron scattering for the detection of carbon and other elements. Therefore, the (d-¹⁰B) neutron tube can provide spectroscopic measurement which is similar to the d-t neutron generator. The (d-¹⁰B) neutron tube is a better choice, because the neutron yield is higher and the boron target is more suitable for high temperature operation than a tritium target. In addition, there is always an environmental safety concern for the usage of radioactive tritium. By operating the mini (d-¹⁰B) neutron tube at 1 MV, 1 mA and 1% DF, the peak neutron yield is 7 × 10¹⁰ n/s (the average neutron yield is 7 × 10⁸ n/s). The average beam power is about 10 W for 1% DF operation. A performance study will be made for using the d-¹⁰B neutrons for both carbon and well-logging purposes, and the result will be reported.

4.3. Mini (d-¹⁰B) Neutron Tube for ¹¹C PET Radioisotope Production

Positron emission tomography (PET) imaging is an important clinical tool for staging and monitoring treatment response in cancer patients. Volkovitsky et al. have proposed the production of a ¹¹C PET radioisotope from the ¹⁰B(d,n)¹¹C reaction by using a 2 MeV deuteron linear accelerator [15]. Alternatively, one can employ a (d-¹⁰B) neutron tube to produce this ¹¹C isotope [16]. By operating the mini neutron tube at 1 MV, 10 mA, the ¹¹C yield is ~7 × 10¹¹ atoms/s. For 1% DF pulsed operation, the average production rate for ¹¹C is ~7 × 10⁹ atoms/s. The amount of ¹¹C produced by the mini neutron tube should be adequate for the manufacturing of positron-labeled radiopharmaceuticals for PET [15]. The system is quite compact and its operation is simple. It has a much smaller footprint than a deuteron linear accelerator or a cyclotron, and it can be conveniently located in a medical clinic or a hospital. For a short-lived positron emitter, such as ¹¹C (half-life is 20.4 min), it is essential to prepare the isotope near the PET machine. Besides the ¹¹C isotope, the d-¹⁰B reaction also produces 6 MeV neutrons, which can be used for making other medical isotopes [2]. Thus, the (d-¹⁰B) neutron tube can have multiple isotope manufacturing purposes.

4.4. Long Axial or Coaxial (d-¹⁰B) Neutron Tube for Reactor Startup Operation

The startup operation in fast reactors during initial fuel loading and the first approach to criticality, or after normal shutdown, may encounter a very low count rate with inherent core neutrons. In this case, some auxiliary neutron sources such as antimony-beryllium photo-neutrons are needed to obtain sufficient count rates [17]. Instead of using the antimony-beryllium source, one can employ the mini (d-¹⁰B) neutron tube to generate additional neutrons. Either the axial- or the coaxial-type neutron tube can be installed inside a titanium tubing with a diameter as small as 1 cm. Fuel loading operations and criticality campaigns are normally performed at an ambient temperature between 180 to 250 °C. The beam target of B¹⁰ can easily survive in such high temperatures. Operating the (d-¹⁰B) neutron tube at 1 MV, 2 mA with a 1% duty factor, the average output neutron yield is 1.4 × 10⁹ n/s. If desired, the neutron output can be adjusted by varying the duty factor or the D⁻ ion beam power.

5. Mini (d-⁷Li) Neutron Tube for 10 and 13 MeV Neutron Production

The d-⁷Li reaction produces neutrons with energies 10 and 13 MeV, similar to 14 MeV d-t neutrons. However, one can avoid the use of tritium and can easily achieve a thick-target condition for high neutron production. With a thermally emitted D⁻ ion-based neutron tube, the generator system becomes very compact both in the axial and coaxial configurations. The following sections describe three important applications of the mini (d-⁷Li) neutron tube.

5.1. A Point High-Energy Neutron Source for Imaging Application

For fast neutron imaging applications, neutrons with high energy (such as 10 and 13 MeV d-⁷Li neutrons) are preferred due to their penetration capability. In some applications, it is advantageous to employ very high intensity short neutron pulses to capture the fast reaction events. To achieve high image resolution, the source size for neutron emission must be small. The coaxial neutron tube arrangement shown in Figure 7 can meet these requirements.

Table 1. Coaxial (d-⁷Li) neutron tube for imaging application.

Operation Parameter	Coaxial (d-7Li) Neutron Tube
Dimensions of D− emitter	120 mm (diam) by 2.5 mm (height)
Peak D− ion current (mA)	30
Peak D− ion beam energy (MeV)	1
Peak ion beam power (kW)	30
Average beam power for 1% DF (kW)	0.3
Dimensions of Li target	2 mm (diam) by 2 mm (height)
Surface area of Li target (cm2)	0.13
Average beam power density for 1% DF (kW/cm2)	2.3
Peak neutron yield (n/s)	1.2 × 1013
Average neutron yield for 1% DF (n/s)	1.2 × 1011

provides a summary of the operation parameters of a (d-⁷Li) neutron tube that has a neutron emission source size no larger than 2 mm. Operating at 1 MV, 30 mA of D⁻ ion beam and 1% DF (1 ms, 10 Hz), the peak neutron yield is 1.2 × 10¹³ n/s. The average neutron yield is 1.2 × 10¹¹ n/s. The average beam power density on the Li target is 2.3 kW/cm². To maintain the lithium target below its melting point temperature (180 °C), this power density must be reduced by spreading the ion beam onto a larger area. This can be accomplished by oscillating the Li target tubing as illustrated in Figure 7. The required traveling speed is estimated to be 10 cm/s.

5.2. Coaxial (d-⁷Li) Neutron Tube for Radioisotope Production

The (n, 2n) reaction can be used to produce many medical radioisotopes [2]. A good example is the production of ²²⁵Ra via the ²²⁶Ra(n, 2n)²²⁵Ra reaction, which has the highest cross-section of a neutron energy range between 8 and 13 MeV [18]. The ²²⁵Ra will subsequently decay to ²²⁵Ac, which is an alpha particle emitter and can be readily conjugated with molecular carriers. Alpha particles are of considerable interest for radiotherapy applications, as their short range in soft tissue is limited to only a few cell diameters. It has been shown that useful quantities of ²²⁵Ra can be produced by using the d-⁷Li neutrons from a compact neutron generator and that the unwanted ²²⁷Ra impurities is about 1% [19]. By using the new coaxial (d-⁷Li) neutron tube (Figure 7), one can use a much smaller amount of ²²⁶Ra. After 100 h of neutron irradiation, 2.1 mCi of ²²⁵Ra can be achieved. Most importantly, the ²²⁷Ra impurity content is expected to be less than 1%.

Table 2. Radioisotope production by using the mini coaxial neutron tube.

	(d-7Li) Neutron Tube for 225Ra Production	(p- 7Li) Neutron Tube for 99Mo Production
Production reaction	226Ra(n, 2n)225Ra	98Mo(n, γ)99Mo
Volume of sample	2.5 mm diam × 10 mm long	2.5 mm diam × 10 mm long
Number of atoms (N)	6.7 × 1020	3.1 × 1021
Reaction cross-section (σ)	2.25 × 10−24 cm2	7 × 10−24 cm2
Ion beam power (10% DF)	1 MV, 100 mA of D− ions	1.9 MV, 100 mA of H− ions
Average neutron yield	4 × 1012 s−1	1.5 × 1011 s−1
Average neutron flux on sample (Φ)	2.6 × 1012 cm−2 s−1	1.9 × 1011 cm−2 s−1
Radioisotope yield (Y = N*σ*Φ)	3.9 × 109 s−1	4.2 × 109 s−1
Decay constant (λ)	5.38 × 10−7 s−1	2.9 × 10−6 s−1
Activity of radioisotope (λ*Y*3.6 × 105) (after 100 h irradiation)	2.1 mCi 227Ra impurities < 1%	120 mCi

provides a summary of the operation.

5.3. High Intensity (d-⁷Li) Neutron Irradiator for Fusion Reactor Wall Material Study

The fusion community recognizes that a dedicated facility is needed to develop suitable wall material for advanced fusion reactors. The International Fusion Materials Irradiation Facility (IFMIF) has been established to address this issue [20]. The IFMIF is a large 40 MeV accelerator-based neutron irradiator. The neutron energy spectrum for the IFMIF extends to 40 MeV, which is higher than the 14 MeV d-t neutron energy in a fusion reactor. On the other hand, the tabletop-size coaxial (d-⁷Li) neutron generator can provide a modest neutron power density to irradiate small quantities of sample material. The 10 and 13 MeV d-⁷Li neutrons are about the same energy as the 14 MeV d-t neutrons. With a 30 cm diameter coaxial (d-⁷Li) neutron generator, a 1 cm diameter by 1 cm high sample is placed at the center of a hollow Li target tube, as shown in Figure 7. By operating the coaxial neutron irradiator at 1 MV and a 500 mA D⁻ ion beam current, the average neutron power density on the sample is about 10 W/cm² for a 10% duty factor pulsed operation. To reduce the ion beam power density on the Li target to a reasonable level (<1 kW/cm²), the Li target tube moves up and down to increase the effective beam target area. The sample located at the center of the hollow Li target tube remains stationary. As no tritium is used, the operation of this compact neutron irradiation system is safe and can be performed in any fusion research laboratory.

6. Mini (p-⁷Li) Neutron Tubes for Direct Production of Epithermal Neutron

Epithermal neutrons in the range of 0.4 eV to 20 keV are desirable for use in cancer treatment and for the production of medical radioisotopes. They can be formed by moderating the 2.5 MeV d-d neutrons or the 10 and 13 MeV d-⁷Li neutrons [2]. Alternatively, they can be directly produced by employing the ⁷Li(p,n)⁷Be reaction at near the threshold of the interaction energy (1.881 MeV) [21,22]. This approach of forming epithermal neutrons offers several advantages. First, as no moderation is needed, the epithermal neutron flux is higher. Second, the epithermal neutrons formed at near threshold energy are directional, with a mean neutron angle of approximately 20°. Therefore, shielding is not needed on the side and at the back of the neutron tube. Third, the d-d and the d-⁷Li neutrons are emitted isotropically; therefore, more than 50% of these neutrons produced are wasted, but not the near-threshold produced p-⁷Li neutrons. The following sections describe the design of the mini neutron tubes for direct epithermal neutron production and their applications.

6.1. Axial Epithermal Neutron Tube for Boron Neutron Capture Therapy (BNCT)

Boron neutron capture therapy (BNCT) is based on the high probability for a stable isotope ¹⁰B capturing a thermal neutron, thereby releasing two high-energy ions (⁴He²⁺ and ⁷Li⁺) [23]. Because of the high linear energy transfer (LET) and relative biological effectiveness (RBE) of these ions, only cells in close proximity to the reaction ¹⁰B(n, α)⁷Li are damaged, leaving adjacent cells unaffected. The enhanced uptake of the boron-labeled agent in tumor cells versus normal cells results in the selective killing of tumor cells. Using the mini axial (p-⁷Li) neutron tube (Figure 1), a high flux of epithermal neutrons can be directly produced by operating the generator near the threshold interaction energy. On entering the patient’s body, these epithermal neutrons are moderated to thermal neutrons when they arrive at the tumor site. The distribution of epithermal and thermal neutron flux inside the body has been studied in detail by Nakagawa et al. [23]. At 1.9 MV, 10 mA, and with H⁻ ion beam diameter of 2 cm, the total neutron yield is 1.5 × 10¹¹ n/s with a mean neutron energy of 38 keV and a mean neutron angle of 23° [21]. For a H⁻ ion beam diameter of 5 cm and with an ion beam current of 65 mA, the average neutron yield for 1% DF operation is approximately 1 × 10¹⁰ n/s, or an average epithermal neutron flux of 5 × 10⁸ n/cm²/s, which is the recommended value for BNCT treatment [24]. With recent advancements in boron drug development [25], the mini axial (p-⁷Li) neutron tubes should be useful for the treatment of cancer via boron neutron capture therapy.

6.2. Coaxial Epithermal Neutron Tube for the Production of Medical Radioisotopes

⁹⁹Mo is an important medical isotope because of the widespread use of its daughter nuclide ^99mTc as a medical tracer in diagnostic procedures. As reported in Ref. [2], a useful amount of ⁹⁹Mo can be produced by using the ⁹⁸Mo(n,γ)⁹⁹Mo reaction. The ⁹⁹Mo yield can be significantly enhanced by irradiating ⁹⁸Mo targets with resonance neutrons. The neutron energy for resonance capture occurs between 500 and 20 keV, which falls within the range of the epithermal neutron energy. By using the coaxial-type (p-⁷Li) neutron tube shown in Figure 7, a small volume of ⁹⁸Mo sample can be enclosed inside a Li target tube. To reduce the beam power density on the Li layer, the Li target tube is oscillated up and down so as to increase the effective beam target area. The ⁹⁸Mo sample on the other hand remains stationary. For 1.9 MV acceleration voltage and 100 mA of H⁻ beam current, the total p-⁷Li epithermal neutron yield is 1.5 × 10¹² n/s [20]. For a 10% DF pulsed operation, the average epithermal neutron flux on the ⁹⁸Mo sample is 1.9 × 10¹¹ n/cm²/s. With this arrangement, the activity for ⁹⁹Mo after 100 h of irradiation is estimated to be about 120 mCi (see Table 2). Since a single medical procedure uses 2 to 10 mCi of ⁹⁹Mo, the amount of ⁹⁹Mo produced by this coaxial (p-⁷Li) neutron tube should be adequate for most hospital use.

7. Conclusions

Mini neutron tubes based on the thermal emission of H⁻/D⁻ ions can provide a high flux of neutrons in cw or in pulsed operation. The axial or coaxial mini neutron tube can be operated from hundreds of kilo-volts to 1.9 mega-volts to generate a high flux of high and low energy neutrons via the d-d, d-¹⁰B, d-⁷Li or p-⁷Li nuclear reactions. These new mini neutron tubes have found important applications in intraoperative radiation therapy (IORT), skin and breast cancer treatment, carbon and oil well logging, neutron imaging, medical isotope production, fusion reactor material evaluation, startup of fission reactor, homeland security and lunar and Mars exploration. With further development, the plasma-less neutron tube can be miniaturized to form a handheld neutron generator unit similar to the portable handheld X-ray machines. This will open up further applications, including medical applications such as proton and neutron brachytherapy.