Research articles

By Dr. Faycal Kharfi , Ms. Naziha Hachouf , Prof. Ahmed Boucenna
Corresponding Author Dr. Faycal Kharfi
Faculty of Science, University of Setif and Nuclear Research Centre of Birine, BP.180 - Algeria 17200
Submitting Author Dr. Faycal Kharfi
Other Authors Ms. Naziha Hachouf
Nuclear Research Centre of Birine, - Algeria

Prof. Ahmed Boucenna
Faculty of Science, University of Setif, - Algeria


Dense Plasma Focus, BNCT, MCNP, Neutron Imaging

Kharfi F, Hachouf N, Boucenna A. Feasibility of BNCT and Neutron Imaging with 1Hz, 5kJ Plasma Focus Neutron Source at the ICTP-MLAB Laboratory. WebmedCentral RADIATION ONCOLOGY 2012;3(3):WMC003130
doi: 10.9754/journal.wmc.2012.003130

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Submitted on: 03 Mar 2012 03:31:11 PM GMT
Published on: 04 Mar 2012 08:01:56 AM GMT


Plasma Focus (PF) devices operated in Deuterium mode (D-D) can be advantageously used as a high intensity short-duration neutron source for many original medical and new imaging applications. At the Multidisciplinary Laboratory (MLAB) of the International Centre for Theoretical physics (Trieste, Italy), a low cost compact and efficient 1 Hz, 5kJ Plasma Focus source was designed and manufactured. This device will be used in several domains of science and basis research. The purpose of the implementation of Plasma Source Device (PFD) at the MLAB is not only to further investigate the physics of the plasma focus; its uniqueness derives from the creation of a source of radiation and particle beams for diagnostic and technology development as well as applications to interdisciplinary projects such as cultural heritage, biology and medicine. This source allows a neutron yield of ~108 per shot (pulse). The duration of the pulse is about 10 ns. In this paper, a general description of this source and its mode of operation are presented. The main characterises of neutrons produced by the Plasma Focus chamber will be outlined. A proposition and a draft design concerning a Boron Neutron Capture Therapy (BNCT) and neutron imaging exposure systems that can be implemented around this source are presented in details. The advantages and limitations such as resolution and pulse mode of operation affecting the utilisation of this kind of source for BNCT and neutron imaging are also discussed.
Keywords: Dense Plasma Focus, BNCT, MCNP, Neutron Imaging.


The Plasma Focus device (PFD) was independently developed by Filippov et al [1] and Mather [2]. The principle of functioning of this device is based on D-D or D-T fusion. From 1962 until recent years, many devices were constructed and studied by varying different parameters such as voltage, energy and anode current for different applications. PF is a pulsed device that can produce, among other emissions, short bursts of hard X-rays, fast neutron and ions. The fact that the burst duration is in the 10 - 100 ns range and the possibility of turning the device off make Plasma Focus an interesting alternative to commercially available radioisotopic sources of both neutrons and hard X-rays.
In the MLAB Laboratory of the International Centre for Theoretical Physics (ICTP, Trieste), a 5kJ dense plasma focus (DPF) device was constructed and became available for use in 2008. This Plasma Focus Source, powered by four interconnected capacitors, was designed and developed at the laboratory as part of a training program on plasma technology. Particular interest has been shown in the plasma focus (PF) device and its ability to produce rather high fluxes of fast neutrons and hard X-rays [3-6]. PFD was successfully tested and used as a source for X-rays imaging [7]. Here, the image was taken from a single shot or from superposition of a number of shot. It’s depends on the sample composition and thickness. Using PFD for BNCT and Neutron Imaging (NI) is a possible but a complicated task. This because the short duration of the PF pulse (3-20 ns) and the produced low neutron yield do not allow the production of a well exposed neutron image for NI and optimum exposed tumor site for BNCT in one pulse except for some cases. In these domains of PF applications, only few works were published so far and just some demonstration systems were manufactured until now. For example, a position sensitive imaging detector with PF neutron source (ING-103) of 20 ns duration and 3x1010 neutrons yield was proposed by DEL MAL VENTURES as pulsed neutron imaging system. Some interesting applications performed around a neutron generator (ING-07) and a PF neutron sources are presented in reference [8]. Our work is to study how a PF neutron source, especially the ICTP one, can be used for neutron imaging and BNCT. The major problem to overcome with this kind of source is the short duration of the pulse (few ns) because it’s well known that for the production of perceptible image to be detected by a high sensitivity CCD camera, a 1 to 2 lux light intensity is necessary [9]. For an ideal detection system the minimum neutrons fluence able to produce net image that can be differentiated from the camera noise is ~103 n/cm2 for an exposure time of 1 ms and a maximum camera pixel size (resolution) of 1mm x 1mm. For BNCT, a maximum fluence of 1012 n/cm2 is generally required for the treatment of some type cancers. The aim of this work is the presentation and discussion of the possibilities and limitations related to the use ICTP PF-5 neutron source for neutron imaging and BNCT. The ICTP DPF-5 neutron source has been constructed by an ICTP research team under the supervision of Pr. V.A. Gribkov [10].

ICTP PF-5 source description and capability

Plasma Focus devices flourished in the 70’s and 80’s as nuclear fusion devices based in the pinch phenomenon occurring during the path of high electric currents through the working gas. After well understanding of the operation mechanism of Plasma Focus a large variety of working gas and configurations has been studied and developed in order to increase the neutron emission yield. Actually, PF are among the cheapest available neutron generators with extremely short pulses duration of tens of ns that permit a number of specific neutrons applications. The principle of PF is based on the fusion of special kind (deuterium and or Tritium) gas between two electrodes when an intense electrical discharge is applied. The Plasma Focus phenomenon occurs at the open end of coaxial electrodes when an intense electrical discharge between them is applied. The coaxial electrodes are located inside a vacuum chamber filled with deuterium gas at low pressure. The PF-5 source being constructed at the ICTP consists of banks that discharge over coaxial electrodes through spark-gaps. The capacitor consists of Four capacitors connected in parallel (Fig.1). After starting discharge in the gap between the electrodes, the created azimuthal magnetic field produces a Lorentz force that pushes the sheath current towards the open end of the electrodes (Fig.1). On its arrival at the open end, the magnetic field starts to contract, accelerating the plasma towards the axis. Finally D-D fusion reactions process starts for a pulse of time and the sheath clashes on the axis in the form of a small dense plasma cylinder. The life time of the focus is about 10ns. Under suitable conditions the focus generates beams of ions, electrons, neutrons and X-ray. Using Deuterium gas PFD generates fast neutron beam pulses of 2.5 MeV in Energy. The emitted neutrons can be applied to perform radiographs and substance analysis, taking advantage of the penetration and activation properties of generated fast neutrons. The main characteristics of the PF-5 neutrons source are presented in table 1.
The first remark that can drawn for the PF-5 source characteristics is the fact that dynamic neutron radiography is not for interest because of the maximum pulse rate repetition (1Hz) not allowing a suitable frame rate. It's well established that for the available imaging technology an exposure time of 1ms is required to be able to investigate flow properties with a moderate speed of 1m/s with a resolution of 1mm. Another parameter affecting the dynamic imaging process is the speed (decay time of the scintillator) that should less than the exposure time in order to avoid frames (images) overlapping. As example the decay time a LiF+ZnS:Ag screen is ~100?s, an exposure time less than this value is not suitable for dynamic neutron imaging.  In order visualize this flow with best viewing conditions a frame rate of about 1000 Hz is needed. Indeed only static imaging is possible and with interest with this PF source.  
Radiography with thermal neutron is a well established non-destructive method. Neutrons of higher energy than 0.3 eV are used for neutron radiographic examinations in much smaller extent. Fast neutrons are used in neutron imaging because of their unique material penetrating properties and their relatively high source strength at which high neutron yields can be produced.    
Fast Neutron imaging will become easier as neutron yields increase. In the case of PFD the neutron yield is so important but the neutron emission duration and the pulses repetition frequency are the main barrier for their uses for neutron dynamic imaging. Neutron detector efficiency and generated neutron yield with PFD must be well optimized to produce image signal that can separated from the CCD-camera noise (background).


Boron Neutron Capture Therapy is an experimental radiotherapy technique that uses neutron beam to cancer therapy for sites such as glioblastoma multiforme, a malignant brain tumor, where conventional radiation therapies fail. The Principle of BNCT is based on the injection of substance that contains Boron into blood vessel of the patient. After approximately 30mn the substance reaches the tumor site. The patient then will be exposed to neutron beam at the level of the tumor site. Neutrons are captured by Boron fixed in the tumor cells. 10B has a very large capture cross section (3830 barns) for thermal neutrons and decays into an alpha particle and a lithium nucleus, the combined ranges of which are ~10 µm, approximately one cell diameter. These last charged particles are responsible of tumor cells elimination. For BNCT success, a thermal neutron fluence of about 1012 n/cm2 should be delivered to a tumor with 10B concentration of 30 µg/g [11-13]. Epithermal neutron are more suitable for the treatment (1 eV < E < 100 keV). This because epithermal neutron thermalize in the biological tissue at de depth of about 2.5 cm through scattering process with a low absorption probability that can cause damage to normal tissue. Therefore, they can provide a maximum thermal neutron flux density at the tumor site with a minimum damage.

The design of irradiation system to be implemented around PFD source must take into consideration that the neutrons must pass through a neutron moderator, which shapes the neutron energy spectrum suitable for BNCT treatment (epithermal neutron, E~1eV). Before entering the patient the neutron beam is shaped by a beam collimator and fast neutron are filtered. While passing through the tissue of the patient, the neutrons are slowed by collisions and become low energy thermal neutrons. In this work, the proposed design of a BNCT irradiation system is shown in fig.2.

In this work an MCNP simulation is performed in order to get the shape of the neutron spectrum just before the entrance of the collimator with respect to the proposed design and geometry (Fig.2). For a practical purpose, just the important zones of the proposed design (Fig.2) are taken into consideration in the MCNP input geometry shown in Fig.3. Results of MCNP simulation are shown in Fig.4. The available DPF performance, neutron flux intensity (MCNP) and required number of shots required to perform BNCT are presented in table 2. 

Regarding the results of table 2, a very long exposure time is required to reach the recommended neutron fluence for BNCT. To overcome this limitation, the improvement of DPF performance is necessary. With the actual performance, the studied DPF device can only be used to irradiate guinea pigs or as a test facilities. We believe that improvement in BNCT protocol and injected substance could contribute to allow the effective utilization of actual low neutron fluence DPF device for the treatment of some kind of superficial or low depth head tumor and especially for skin melanoma.

PFD for Neutron Imaging

The proposed design is based on the available technology for radiographic imaging system. Scintillator-CCD-camera based system was selected because of the variety of scintillator materials and CCD-chip that can be combined. Image intensifier associated to high dynamics CCD is more than necessary to perform ultra short exposure down to 10 ns. The most important parameters that characterize a neutron imaging system are:

- The minimum exposure time allowing the production a detectable image: this parameter depends closely on the neutron yield, detection efficiency of the scintillator and quantum detection efficiency of the camera.

- The image resolution;

- The contrast.

In the design of detection imaging system used with PF-5 source some considerations should be taken into account:

1. The detector should be well shielded in order to avoid contribution electromagnetic radiation to CCD heating and noise signal generation

2. Neutron beam should be filtered from X-ray especially in order to consider produced image due to fast neutrons essentially. The source should be shielded with Lead or Bismuth.

3. Resolving the problem of strong electromagnetic pickup;

4. Improving the produced neutron yield to maximum by choosing the most appropriate PF configuration. A yield of 1012 n/pulse is quite enough;

5. Selecting a scintillator screen of maximum detection efficiency. For example detection efficiency of 1% of LiF+ZnS:Ag (500µm) for fast neutron is insufficient with a neutron yield of 108 n/pulse.

The following relations govern the design of neutron imaging detection system:

1. Flux intensity: the neutron beam intensity at the level of the detection system is proportional to the available source intensity (Eq.1) [14].

Where Flux0 and Fluxd are respectively the neutron flux at the source and detector levels, L is the source to detector distance and D is the source or collimator inlet aperture diameter.

2. The neutron detection and light escape efficiencies: If Sintillator+ CCD-camera is used as detection system; the light escape efficiency from the scintllator will be given by Eq.2. The term (1- exp(-d/?n) represent the neutron detection efficiency which depends closely on the scintillator thickness. Where d is the scintillator thickness, ?cp , ?l and ?n are respectively the charged particle, the emitted light and the neutron  free path in the scintillator material.                   

3. The image resolution (IR): it’s generally computed according to the measurement of total image geometric unsharpness given by Eq.3 [14]. Where l is the object to image detector distance.

The minimum neutron fluence to produce detectable image is varying from 103 to 104 n/cm2 which is equivalent to 10 to 100 n/detector-pixel when an amount of 10x10 pixels per cm2 is used.

The image resolution for digital detector is defined as the full width at half maximum (FWHM) for the line spread function (LSF). Line spread function is response of completely absorbing object with an infinitesimally narrow slit [11]. LSF due to scintillator contribution can be expressed by Eq.4 [15]. Where d is the scintillator thickness and µi is the corresponding inherent unsharpness. The total line spread function is determined by the convolution of all different contributions and tends to have a Gaussian form given by Eq.5 [16]. Where x indicates the position along the measuring image axis and ?x is the standard deviation.  

The line spread function depends on the geometry of the detection system and the divergence of the neutron beam. For the case of an isotropic neutron source, the line spread function has a standard deviation of Dl/L?40 [16].

In order to get a compromize between neutron intensity and homogeneity in one hand and neutron image quality in another hand, different values of the L/D ration varying between 10 and 100 are considered. A neutron extraction window from the PF chamber of 1 cm in diameter is considered as a reasonable value according to the PF  chamber dimensions and mode of operation. This allows a circular irradiation area with a diameter varying between 1.5 cm to 6.2 cm for a 1.5° beam divergent angle. After many simulations to get a good compromize between design constraints and limitations, PFD performances and image quality, optimal design parameters are identified. The optimal design parameters and expected performances are presented in table 3.

The proposed neutron imaging system to be associated to the PF-5 source is presented in Fig.5. By placing the scintillator at 50 cm from the source and the use of 2 cm lead filter, this system can produce an optimal neutron image. The neutron extraction window should be manufactured from a neutron transparent material and accurately placed in the front side of the PF chamber.  


In this work feasibility of BNCT and Neutron Imaging with PF-5 neutron source is studied and discussed. For the BNCT case, although the produced neutron flux by PFD is not very intense, the utilization of such source for radiotherapy (BNCT) is possible. The results of this feasibility study indicates that the utilization of DPF with some improvement in terms of power could be a promising alternative neutron source to obtain relatively acceptable neutron fluence rate for BNCT application to the treatment of some kind of cancer. Better results could be reached by a dedicated facility with some amelioration in terms of neutron interaction properties of the injected substance.

For the case of Neutron Imaging, This study indicates that the static neutron imaging is possible with high sensitivity low noise CCD-Camera for biological application. The obtained image will be less or more noised depending on the used intensifier and CCD-Camera. Some others remarks can be drawn:

1. Dynamic neutron radiography is quite impossible with PF source with the actual pulse duration and repetition rate (108 n/shot, 1Hz);

2. Fast neutron imaging is possible but only for static object;

3. The obtained image with suitable X-ray filter will be 80% due to fast neutron;

4. Due to geometric and neutron yield limitation, the irradiation area will be 2.3 cm in diameter. This diameter can be increased to 10 cm but others performance parameters will be affected;

5. PFD neutron yield of 1013 allows more imaging system performance in matter of irradiation area and image resolution and purity. 

Finally, it is important to mention that the final utilization of DPF source for BNCT and fast neutron dynamic imaging depends on the progress in further increase of the produced neutron yield and neutron detection and conversion efficiencies.


D-D: Deuterium-Deuterium
D-T : Deuterium-Tritium
MLAB: Multidisciplinary laboratory
ICTP: International Centre for Theoretical Physics
BNCT: Boron Neutron Capture Therapy
NI : Neutron Imaging
PF : Plasma Focus
DPF: Dense Plasma Focus
PFD: Plasma Focus Device
eV : electron Volt
keV : kilo electron Volt
CCD : Charged Coupled Device


This work was undertaken in the framework of ICTP/IAEA STEP program under the supervision of Prof. C. Tuniz. The principle author would like to thank Prof. C. Tuniz and all the Multidisciplinary Laboratory (ICTP) staff for their help and availability.


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