feasibility of bnct and neutron imaging with 1hz, 5kj ... · original medical and new imaging...

Article ID: WMC003076 ISSN 2046-1690

Feasibility of BNCT and Neutron Imaging with 1Hz,5kJ Plasma Focus Neutron Source at theICTP-MLAB LaboratoryCorresponding Author:Dr. Faycal Kharfi,Senior Research Scientist, Faculty of Science, University of Setif and Nuclear Research Centre of Birine, BP.180,17200 - Algeria

Submitting Author:Dr. Faycal Kharfi,Senior Research Scientist, Faculty of Science, University of Setif and Nuclear Research Centre of Birine, BP.180,17200 - Algeria

Article ID: WMC003076

Article Type: Research articles

Submitted on:24-Feb-2012, 01:43:58 PM GMT Published on: 25-Feb-2012, 10:22:40 AM GMT

Article URL: http://www.webmedcentral.com/article_view/3076

Subject Categories:RADIATION ONCOLOGY

Keywords:Dense Plasma Focus, BNCT, MCNP, Neutron Imaging

How to cite the article:Kharfi F . Feasibility of BNCT and Neutron Imaging with 1Hz, 5kJ Plasma Focus NeutronSource at the ICTP-MLAB Laboratory . WebmedCentral RADIATION ONCOLOGY 2012;3(2):WMC003076

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Feasibility of BNCT and Neutron Imaging with 1Hz,5kJ Plasma Focus Neutron Source at theICTP-MLAB LaboratoryAuthor(s): Kharfi F

Abstract

Plasma Focus (PF) devices operated in Deuteriummode (D-D) can be advantageously used as a highintensity short-duration neutron source for manyoriginal medical and new imaging applications. At theMultidisciplinary Laboratory (MLAB) of theInternational Centre for Theoretical physics (Trieste,Italy), a low cost compact and efficient 1 Hz, 5kJPlasma Focus source was designed andmanufactured. This device will be used in severaldomains of science and basis research. The purposeof the implementation of Plasma Source Device (PFD)at the MLAB is not only to further investigate thephysics of the plasma focus; its uniqueness derivesfrom the creation of a source of radiation and particlebeams for diagnostic and technology development aswell as applications to interdisciplinary projects suchas cultural heritage, biology and medicine. This sourceallows a neutron yield of ~108 per shot (pulse). Theduration of the pulse is about 10 ns. In this paper, ageneral description of this source and its mode ofoperation are presented. The main characterises ofneutrons produced by the Plasma Focus chamber willbe outlined. A proposition and a draft designconcerning a Boron Neutron Capture Therapy (BNCT)and neutron imaging exposure systems that can beimplemented around this source are presented indetails. The advantages and limitations such asresolution and pulse mode of operation affecting theutilisation of this kind of source for BNCT and neutronimaging are also discussed.Keywords: Dense Plasma Focus, BNCT, MCNP,Neutron Imaging.

Introduction

The Plasma Focus device (PFD) was independentlydeveloped by Filippov et al [1] and Mather [2]. Theprinciple of functioning of this device is based on D-Dor D-T fusion. From 1962 until recent years, manydevices were constructed and studied by varyingdifferent parameters such as voltage, energy andanode 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. Thefact that the burst duration is in the 10 - 100 ns rangeand the possibility of turning the device off makePlasma Focus an interesting alternative tocommercially available radioisotopic sources of bothneutrons and hard X-rays. In the MLAB Laboratory of the International Centre forTheoretical Physics (ICTP, Trieste), a 5kJ denseplasma focus (DPF) device was constructed andbecame available for use in 2008. This Plasma FocusSource, powered by four interconnected capacitors,was designed and developed at the laboratory as partof a training program on plasma technology. Particularinterest has been shown in the plasma focus (PF)device and its ability to produce rather high fluxes offast neutrons and hard X-rays [3-6]. PFD wassuccessfully tested and used as a source for X-raysimaging [7]. Here, the image was taken from a singleshot or from superposition of a number of shot. It’sdepends on the sample composition and thickness.Using PFD for BNCT and Neutron Imaging (NI) is apossible but a complicated task. This because theshort duration of the PF pulse (3-20 ns) and theproduced low neutron yield do not allow the productionof a well exposed neutron image for NI and optimumexposed tumor site for BNCT in one pulse except forsome cases. In these domains of PF applications, onlyfew works were published so far and just somedemonstration systems were manufactured until now.For example, a position sensitive imaging detectorwith PF neutron source (ING-103) of 20 ns durationand 3x1010 neutrons yield was proposed by DEL MALVENTURES as pulsed neutron imaging system. Someinteresting applications performed around a neutrongenerator (ING-07) and a PF neutron sources arepresented in reference [8]. Our work is to study how aPF neutron source, especially the ICTP one, can beused for neutron imaging and BNCT. The majorproblem to overcome with this kind of source is theshort duration of the pulse (few ns) because it’s wellknown that for the production of perceptible image tobe detected by a high sensitivity CCD camera, a 1 to 2lux light intensity is necessary [9]. For an idealdetection system the minimum neutrons fluence ableto produce net image that can be differentiated from



the camera noise is ~103 n/cm2 for an exposure timeof 1 ms and a maximum camera pixel size (resolution)of 1mm x 1mm. For BNCT, a maximum fluence of1012 n/cm2 is generally required for the treatment ofsome type cancers. The aim of this work is thepresentation and discussion of the possibilities andlimitations related to the use ICTP PF-5 neutronsource for neutron imaging and BNCT. The ICTPDPF-5 neutron source has been constructed by anICTP research team under the supervision of Pr. V.A.Gribkov [10].

ICTP PF-5 source descriptionand capability

Plasma Focus devices flourished in the 70’s and 80’sas nuclear fusion devices based in the pinchphenomenon occurring during the path of high electriccurrents through the working gas. After wellunderstanding of the operation mechanism of PlasmaFocus a large var iety of working gas andconfigurations has been studied and developed inorder to increase the neutron emission yield. Actually,PF are among the cheapest available neutrongenerators with extremely short pulses duration oftens of ns that permit a number of specific neutronsapplications. The principle of PF is based on the fusionof special kind (deuterium and or Tritium) gas betweentwo electrodes when an intense electrical discharge isapplied. The Plasma Focus phenomenon occurs at theopen end of coaxial electrodes when an intenseelectrical discharge between them is applied. Thecoaxial electrodes are located inside a vacuumchamber filled with deuterium gas at low pressure. ThePF-5 source being constructed at the ICTP consists ofbanks that discharge over coaxial electrodes throughspark-gaps. The capacitor consists of Four capacitorsconnected in parallel (Fig.1). After starting discharge inthe gap between the electrodes, the created azimuthalmagnetic field produces a Lorentz force that pushesthe sheath current towards the open end of theelectrodes (Fig.1). On its arrival at the open end, themagnetic field starts to contract, accelerating theplasma towards the axis. Finally D-D fusion reactionsprocess starts for a pulse of time and the sheathclashes on the axis in the form of a small denseplasma cylinder. The life time of the focus is about10ns. Under suitable conditions the focus generatesbeams of ions, electrons, neutrons and X-ray. UsingDeuterium gas PFD generates fast neutron beampulses of 2.5 MeV in Energy. The emitted neutronscan be applied to perform radiographs and substanceanalysis, taking advantage of the penetration and

activation properties of generated fast neutrons. Themain characteristics of the PF-5 neutrons source arepresented in table 1.The first remark that can drawn for the PF-5 sourcecharacteristics is the fact that dynamic neutronradiography is not for interest because of themaximum pulse rate repetition (1Hz) not allowing asuitable frame rate. It's well established that for theavailable imaging technology an exposure time of 1msis required to be able to investigate flow propertieswith a moderate speed of 1m/s with a resolution of1mm. Another parameter affecting the dynamicimaging process is the speed (decay time of thescintillator) that should less than the exposure time inorder to avoid frames (images) overlapping. Asexample the decay time a LiF+ZnS:Ag screen is ~100µs, an exposure time less than this value is notsuitable for dynamic neutron imaging. In ordervisualize this flow with best viewing conditions a framerate of about 1000 Hz is needed. Indeed only staticimaging is possible and with interest with this PFsource. Radiography with thermal neutron is a well establishednon-destructive method. Neutrons of higher energythan 0.3 eV are used for neutron radiographicexaminations in much smaller extent. Fast neutronsare used in neutron imaging because of their uniquematerial penetrating properties and their relatively highsource strength at which high neutron yields can beproduced. Fast Neutron imaging will become easier as neutronyields increase. In the case of PFD the neutron yield isso important but the neutron emission duration and thepulses repetition frequency are the main barrier fortheir uses for neutron dynamic imaging. Neutrondetector efficiency and generated neutron yield withPFD must be well optimized to produce image signalthat can separated from the CCD-camera noise(background).

PFD for BNCT

Boron Neutron Capture Therapy is an experimentalradiotherapy technique that uses neutron beam tocancer therapy for sites such as glioblastomamultiforme, a malignant brain tumor, whereconventional radiation therapies fail. The Principle ofBNCT is based on the injection of substance thatcontains Boron into blood vessel of the patient. Afterapproximately 30mn the substance reaches the tumorsite. The patient then will be exposed to neutron beamat the level of the tumor site. Neutrons are captured byBoron fixed in the tumor cells. 10B has a very large



capture cross section (3830 barns) for thermalneutrons and decays into an alpha particle and alithium nucleus, the combined ranges of which are ~10µm, approximately one cell diameter. These lastcharged particles are responsible of tumor cellselimination. For BNCT success, a thermal neutronfluence of about 1012 n/cm2 should be delivered to atumor with 10B concentration of 30 µg/g [11-13].Epithermal neutron are more suitable for the treatment(1 eV < E < 100 keV). This because epithermalneutron thermalize in the biological tissue at de depthof about 2.5 cm through scattering process with a lowabsorption probability that can cause damage tonormal tissue. Therefore, they can provide a maximumthermal neutron flux density at the tumor site with aminimum damage.

The design of irradiation system to be implementedaround PFD source must take into consideration thatthe neutrons must pass through a neutron moderator,which shapes the neutron energy spectrum suitablefor BNCT treatment (epithermal neutron, E~1eV).Before entering the patient the neutron beam isshaped by a beam collimator and fast neutron arefiltered. While passing through the tissue of the patient,the neutrons are slowed by collisions and become lowenergy thermal neutrons. In this work, the proposeddesign of a BNCT irradiation system is shown in fig.2.

In this work an MCNP simulation is performed in orderto get the shape of the neutron spectrum just beforethe entrance of the collimator with respect to theproposed design and geometry (Fig.2). For a practicalpurpose, just the important zones of the proposeddesign (Fig.2) are taken into consideration in theMCNP input geometry shown in Fig.3. Results ofMCNP simulation are shown in Fig.4. The availableDPF performance, neutron flux intensity (MCNP) andrequired number of shots required to perform BNCTare presented in table 2.

Regarding the results of table 2, a very long exposuretime is required to reach the recommended neutronfluence for BNCT. To overcome this limitation, theimprovement of DPF performance is necessary. Withthe actual performance, the studied DPF device canonly be used to irradiate guinea pigs or as a testfacilities. We believe that improvement in BNCTprotocol and injected substance could contribute toallow the effective utilization of actual low neutronfluence DPF device for the treatment of some kind ofsuperficial or low depth head tumor and especially forskin melanoma.

PFD for Neutron Imaging

The proposed design is based on the availabletechnology for radiographic imaging system.Scintillator-CCD-camera based system was selectedbecause of the variety of scintillator materials andCCD-chip that can be combined. Image intensifierassociated to high dynamics CCD is more thannecessary to perform ultra short exposure down to 10ns. The most important parameters that characterize aneutron imaging system are:

- The minimum exposure time allowing the productiona detectable image: this parameter depends closely onthe neutron yield, detection efficiency of the scintillatorand quantum detection efficiency of the camera.

- The image resolution;

- The contrast.

In the design of detection imaging system used withPF-5 source some considerations should be taken intoaccount:

1. The detector should be well shielded in order toavoid contribution electromagnetic radiation to CCDheating and noise signal generation

2. Neutron beam should be filtered from X-rayespecially in order to consider produced image due tofast neutrons essentially. The source should beshielded with Lead or Bismuth.

3. Resolving the problem of strong electromagneticpickup;

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

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

The following relations govern the design of neutronimaging detection system:

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

Where Flux0 and Fluxd are respectively the neutronflux at the source and detector levels, L is the sourceto detector distance and D is the source or collimatorinlet 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 the expression of Eq.2. The term (1- exp(-d/λn)represent the neutron detection efficiency whichdepends closely on the scintillator thickness. Where dis the scintillator thickness, λcp , λ l and λn arerespectively the charged particle, the emitted light andthe neutron f ree path in the sc int i l la tormaterial.

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

The minimum neutron fluence to produce detectableimage is varying from 103 to 104 n/cm2 which isequivalent to 10 to 100 n/detector-pixel when anamount of 10x10 pixels per cm2 is used.

The image resolution for digital detector is defined asthe full width at half maximum (FWHM) for the linespread function (LSF). Line spread function isresponse of completely absorbing object with aninfinitesimally narrow slit [11]. LSF due to scintillatorcontribution can be expressed by Eq.4 [15]. Where dis the scintillator thickness and µi is the correspondinginherent unsharpness. The total line spread function isdetermined by the convolution of all differentcontributions and tends to have a Gaussian form givenby Eq.5 [16]. Where x indicates the position along themeasuring image axis and σx is the standarddeviation.

The line spread function depends on the geometry ofthe detection system and the divergence of theneutron beam. For a case of isotopic neutron source;the line spread function has a standard deviation ofDl/L√40 [16].

In order to balance between neutron intensity andhomogeneity in one hand and neutron image quality inanother hand an L/D varying between 10 and 100were considered. A neutron pickup diameter from thePF chamber of 1 cm is reasonable according to PFDmode of operation. This allows an irradiation areadiameter varying between 1.5 cm to 6.2 cm for 1.5° ofneutron beam divergent angle. After many simulationsthrough the variation of design, performance andimage quality parameters inside the boundary limits,an optimal choice was selected. The optimal designparameters and expected performance are presentedin table 3.

The proposed neutron imaging system to beassociated to the PF-5 source is presented in Fig.5.By placing the scintillator at 50 cm from the sourceand the use of 2 cm lead filter, this system canproduce an optimal neutron image. The neutron pickup window should be manufactured from a neutron

transparent material and well placed in the front sideof the PF chamber.

Conclusion(s)

In this work feasibility of BNCT and Neutron Imagingwith PF-5 neutron source is studied and discussed.For the BNCT case, although the produced neutronflux by PFD is not very intense, the utilization of suchsource for radiotherapy (BNCT) is possible. Theresults of this feasibility study indicates that theutilization of DPF with some improvement in terms ofpower could be a promising alternative neutron sourceto obtain relatively acceptable neutron fluence rate forBNCT application to the treatment of some kind ofcancer. Better results could be reached by a dedicatedfacility with some amelioration in terms of neutroninteraction properties of the injected substance.

For the case of Neutron Imaging, This study indicatesthat the static neutron imaging is possible with highsensitivity low noise CCD-Camera for biologicalapplication. The obtained image will be less or morenoised depending on the used intensifier andCCD-Camera. Some others remarks can be drawn:

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

2. Fast neutron imaging is possible but only for staticobject;

3. The obtained image with suitable X-ray filter will be80% due to fast neutron;

4. Due to geometric and neutron yield limitation, theirradiation area will be 2.3 cm in diameter. Thisdiameter can be increased to 10 cm but othersperformance parameters will be affected;

5. PFD neutron yield of 1013 allows more imagingsystem performance in matter of irradiation area andimage resolution and purity.

Finally, it is important to mention that the finalutilization of DPF source for BNCT and fast neutrondynamic imaging depends on the progress in furtherincrease of the produced neutron yield and neutrondetection and conversion efficiencies.

Abbreviation(s)

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

Acknowledgement(s)

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

References

1. N.V. Fi l ippov, T.J. Fi l ippova and V.P.Vinogradov,‘Dense high temperature plasma in anon-cylindrical z-pinch compression’, Nuclear FusionSuppl., vol. 2, p. 577(1962).2. J.W. Mather, ‘Investigation of the high energyacceleration mode in the coaxial gun’, Phys. FluidsSuppl., vol. 7, p. 28 (1964).3. S. Lee, T.Y. Tou, S.P. Moo, M.A. Eissa, A.V.Gholap, K.H. Kwek, S. Mulyodrono, A.J. Smith,Suryadi, W. Usada and M. Zakaullah, ‘A simple facilityfor the teaching of plasma dynamics and plasmanuclear fusion’, Am J. Phys., vol. 56, p. 62 (1988).4. S. Lee, L.K. Chen, T.C. Chia, S. Kumar, A. Serban,S.V. Springham, K.A. Toh, A.C. Chew, S.P. Moo andC.S. Wong, ‘Collaborative research using the smallplasma focus’, in Proc. IAEA Tech. CommitteeMeeting on Research Using Small Tokamaks (SerraNegra, SP, Brazil, 1993), pp. 153-164.5. S.P. Moo, C.K. Chakrabarty and S. Lee, ‘Anevaluation of a 3.3kJ plasma focus for pulsed neutronactivation’, in Proc. Symp.Small Plasma PhysicsExperiments II (ICTP, Trieste, 1989), pp. 248-258.6. C.S. Wong, S.P. Moo, J. Singh, P. Choi, C.Dumeitrescu- Zoita and C. Silawatshananai,‘Dynamics of x-ray emission from a small plasmafocus’, Malaysian J. Sci., vol. 17B, p. 109 (1996).7. C. Moreno et al, ‘Using a 4.7 kJ plasma focus forintrospective imaging of metallic objects and neutronicdetection of water’. CP563, Plasma Physics; IX LatinAmerican Workshop, edited by H. Chuaqui and M.

F a v r e , A m e r i c a I n s t i t u t e o f P h y s i c s1-565396-999/01(2001). 8. V.I. Mirekov, ‘Investigation of Perspectives of fastneutron radiography on the basis of non-traditionalneutron sources: neutron generators with acceleratingand Plasma Focus Tubes, Proceeding of InternationalConference on Nuclear Energy in Central Europe,Portoroz, Slovenia, 6 to 9 September, 1999, pp.725-730 (1999).9. J. Teillet et al, ‘Feasibility of high frame neutronradiography by using steady thermal neutron beamwith 106 n/cm2s flux’, Nucl. Isntr. & Meth. in Phys.Resear section A, V.369, pp 186-194 (1996).10. V.A. Gribkov et al., MLAB ICTP Technical Report,w w w . m l a b . i c t p . i t / u p l o a d s /Dense-Plasma-Focus-BoraICTP.pdf11. V. Benzi et al., “Feasibility analysis of a PlasmaFocus neutron source for BNCT treatment oftransplanted human liver”, Nuclear Instruments andMethods in Physics Research B 213, pp.611–615(2004). 12. N.Golnik, K. Pytel, Irradiation facilities for BNCT atresearch reactor MARIA in Poland, Pol J Med PhysEng, 12(3), pp.143-153 (2006).13. S. Agosteoa et al., “Characterization of anaccelerator-based neutron source for BNCT versusbeam energy, Nuclear Instruments and Methods inPhysics Research A 476, pp.106-116 (2002).14. J.C. Domanus, 'Practical neutron radiography,'Kluwer academic publishers. Dordrecht, (1992).15. A.A. Harms, 'Mathematics and physics of neutronradiography', Klumer academic publishers, Dorrrechet(1986).16. V.O. de Haan et al, 'Conceptional desing of anovel high frame-rate fast neutron radiography facility',Nucl. Isntr. & Meth. in Phys. Resear section A, 539, pp321-334 (2005).



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