ISSN 1062�8738, Bulletin of the Russian Academy of Sciences. Physics, 2011, Vol. 75, No. 11, pp. 1544–1548. © Allerton Press, Inc., 2011.Original Russian Text © S.S. Verbitskii, V.N. Emokhonov, A.M. Lapik, V.G. Nedorezov, A.V. Rusakov, G.V. Solodukhov, M.A. Tikhanov, A.A. Turinge, A.N. Tselebrovskii, 2011,published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2011, Vol. 75, No. 11, pp. 1640–1645.

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INTRODUCTION

The problems of managing public safety in differ�ent areas of life are becoming increasingly relevant.The most important of the many aspects of such man�agement is preventing the unauthorized transportingof fissile substances that can be used to create a nuclearweapon or a simple “dirty” bomb. It is thus necessaryto consider developing nondestructive methods fordiscovering camouflaged nuclear materials in largecargo containers. A container bearing spontaneouslyfissionable materials is a source of an elevated radia�tion background that is only slightly higher than thenatural background, due to the use of specially assem�bled shielding. Using the active control methods asso�ciated with the irradiation of cargos by deeply pene�trating radiation such as neutrons or γ quanta allows usto enhance the detecting capability of such devices.

The choice of a detection method is based on acomparison of the following characteristics: sensitiv�ity, or the threshold amount of detected substance witha given probability; the time required to detect orinvestigate a single object; the cost of the device and itslifetime; the probability of producing a false alarm;and the radiation load on personnel and the residualradioactivity, if it does arise in the investigated cargo.

Upon the division of actinide nuclei, two fragments

are formed with average masses of = 95 and =140, a total kinetic energy of ~167 MeV, and two tothree momentary neutrons with a kinetic energy of~2 MeV. Their number tends to increase as the energyof the neutrons responsible for the division grows(Fig. 1) [1]. At the same time, around five momentaryγ quanta with a total energy of ~7 MeV [1] and aroundseven β particles with a total energy of 8MeV areformed. About 12 MeV per act of division is removedby antineutrinos.

A1 A2

The outputs of neutrons normalized to the act ofdivision upon the spontaneous division of nuclei ofdifferent isotopes of actinides are virtually identical toone another (from 1.43 to 2.4 for nuclei in the range ofthorium to plutonium). Absolute yields differ due tothe considerable differences between the half�life peri�ods of different nuclides; the reliability of detectingnuclides by this method is therefore different if allother conditions are equal. Note that registering thepower distributions of neutrons and γ radiation allowsus to improve the signal/background ratio for passiveand active control methods alike.

The probing irradiation of large�sized cargos suchas containers must be performed with pulsed penetrat�ing neutron or γ radiation, since a high background isgenerated at the moment of the pulse; this is associatedwith the scattering of the initial beam and nuclear

Applying the Photonuclear Technique to Fissile Materials DetectionS. S. Verbitskiia, V. N. Emokhonova, A. M. Lapikb, V. G. Nedorezovb, A. V. Rusakovb,

G. V. Solodukhovb, M. A. Tikhanova, A. A. Turingeb, and A. N. Tselebrovskiia

aInstitute of Energy Problems in Chemistry Physics, Russian Academy of Sciences, Leninskii pr. 38, Moscow, 117334 RussiabInstitute for Nuclear Research, Russian Academy of Sciences, pr. 60�let Oktyabrya 7a, Moscow, 117312 Russia

e�mail: lapik@inr.ac.ru

Abstract—Application of a photonuclear technique is considered as a nondestructive method for detecting theunauthorized transporting of camouflaged fissile nuclear material in cargo containers. The advantages of thetechnique (the use of time selection and the analysis of energy distributions of delayed neutrons) are demon�strated. The operating modes of the accelerator were optimized by modeling. A version of a neutron scintillationspectrometer based on a stylbene monocrystal is presented along with its schematic and main parameters.

DOI: 10.3103/S1062873811040411

8

0 10 50En, MeV

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Fig. 1. Average number of neutrons per one divisionduring the excitation of a nucleus by neutrons of differentenergies En: curve 1, 239Pu; 2, 233U; 3, for 232Th; 4, 235U.

avnN

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reactions in the cargo materials. Blocking registrationduring a pulse allows us to reduce this background. Itis, however, difficult to use momentary neutrons andγ quanta from divisions to monitor the presence of fis�sile substances in containers of large size because ofthe high level of background radiation from accompa�nying nonfissionable materials, since the thresholds ofreaction (γ, n) for most nuclei lie in the range of<10 MeV. The registering of γ quanta to detect fission�able substances was proposed in [2], but the efficiencyof these methods is low. It falls sharply when samplesof fissionable substances are placed inside protectivecontainers, are highly radioactive, or are being trans�ported in containers together with γ sources.

The division of actinide nuclei is accompanied by aunique phenomenon: the production of the mothernucleus of the neutron and γ radiation, delayed relativeto the moment of division. The detection of thesedelayed products is irrefutable evidence of the pres�ence of fissionable substances in a cargo. It is impor�tant that in a time of about 10–3 s from the moment ofirradiation, the intensity of the background radiationis reduced by almost four orders of magnitude [1], andthe γ background in the detector falls during the regis�tration of delayed neutrons.

The yield of delayed neutrons with energies ofabout 1 MeV is 1.8% for 235U and 0.67% for 239Pu rel�ative to the total number of neutrons irradiated uponthe division of a nucleus [3]. Depending on the delaytime (from fractions of second to dozens of minutes),they are usually split into six groups [3]. The fastestneutron groups, e.g., from the third to the sixth, con�tain about 60% of the delayed neutrons and carry 74%of the radiation energy. They must be studied usingpulse beams and analyzed between the operatingpulses of the accelerator.

We shall consider the possibility of creating a unitfor detecting fissionable substances in containers byregistering the delayed neutrons formed during thephotodivision of nuclei.

We propose using beams of γ quanta with averageenergies of up to 50 MeV that possess enough penetra�bility at the minimum level of the background. This islogical, due particularly to the greater environmentalsafety and lower cost (relative to using neutrons fromneutron generators as a source). Our method is espe�cially important for detecting fissionable materials incontainers with shielding against characteristic γ� andX�ray radiation. The energy of the γ beam is set quitehigh, but the beam remains purer than a neutronbeam.

The cross sections of complete photoabsorption fornuclei with Z ≥ 90 grow rapidly from around 8 MeVand reach their maximum at about 400 mb, atγ�quanta energies of 12–16 MeV [1]. Division crosssections are close to those of complete photoabsorp�tion for actinide nuclei since their fissionabilities areclose to 1 by order of magnitude.

The effective energy of a braking spectrum is usuallytaken as equal to one third of the maximum. It thereforefollows from the shape of a photodivision cross sectionand the shape of the braking spectrum that it is appro�priate to use a burst of braking radiation from a beam ofelectrons with an energy of 40–50 MeV in order toobtain the optimum yield of delayed neutrons.

OPTIMIZATION OF ACCELERATOR MODES

In order to select the parameters of the accelerator,a simulation was performed using the GEANT pro�gram [4]. A linear electron accelerator or a microtronwith an energy of 50–100 MeV was considered thesource of high�power γ radiation. The thickness andmaterial of the braking target, the distance between thebraking target and the sample, the frequency of accel�eration pulse repetition, and the container wall thick�ness were varied.

We thus established that the optimum thickness ofthe braking target is close to 0.6 of the radiation length.

Simulation allowed us to determine the dose ofradiation required to detect a localized object made ofprocessed uranium with of weight of 1 kg (a cube withan edge size of 3.75 cm), placed inside a steel con�tainer with a wall thickness of 2 mm and a volume of2 × 2 × 2 m3.

We considered an organic scintillator 100 × 100 ×10 cm3 in size placed above the container as the neu�tron detector. The spatial angle was ~10% with respectto 4π. The efficiency of registering delayed neutrons inthe scintillator was ~50%. The measurement time wasoptimized on the basis of the half�life period ofdelayed neutrons, 100 ms (for short�lived groups) dur�ing periods between pulses of a beam with a frequencyof 10 Hz.

At the average accelerator current of 1 μA,~105 delayed neutrons will be registered per second,which is greater than the background level by twoorders of magnitude. It can be seen that this methodhas great potential with regard to many parameters,but they must be optimized depending on the particu�lar sizes of localized objects and their location in thecontainer.

THE SPECTROMETER

The delayed neutron detector is one element of theunit whose settings influence its detecting characteris�tics. The threshold sensitivity can be raised by using adetector with greater efficiency in registering a charac�teristic feature (in our case, neutrons with energies ofhundreds of keV) and by reducing the detector’s sensi�tivity to background neutrons and γ quanta. The firstcan be accomplished by selecting the material and sizeof the detector; the second, by using antibackgroundcoincidence schemes and γ background suppressionschemes according to the pulse shape.

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The main neutron background on the Earth’s sur�face is thermalized neutrons from the cosmic back�ground and the reactions caused by it; the backgroundcan therefore be a sharply reduced by using hydrogen�containing organic scintillators that are not sensitive tothermal neutrons. This class of scintillators is, how�ever, sensitive to the γ background, which is blocked bymeans of particle discrimination according to theshape of the scintillation pulse (based on the differ�ence between those for neutrons and γ quanta) [4, 5].

The shape of these pulses differs due to the differentionization density of scintillator material with regardto secondary particles (recoil protons and secondaryelectrons). For energies of tens to hundreds of keV, thenumber of photons registered from a single scintilla�tion is also tens to hundreds; due to statistic fluctua�tion, the quality of division thus declines and it is nec�essary to use the best particle discrimination methodwith respect to the shape of the scintillation pulse forthese energies, DSI.

Choosing from among the several modifications ofDSI methodology [5–8] is not necessarily easy. Exper�imental results obtained using different DSI methodsdiffer considerably and can even contradict oneanother [9, 10] due to differences in the experimentalunits (loading, detector types, etc.).

In order to select the most suitable method for theproblem under consideration, we compared the mostoften used methods of particle discrimination accord�ing to the pulse shape via modeling by the MonteCarlo method.

The power dependence on the contribution from theslow component to the scintillation amplitude wasdetermined from measurement data obtained using theneutron spectrometer described in [11, 12]. Figure 2shows the distribution of the number of events per

channel relative to the zero intersection time of thetemporary axis at a particle energy of ~2.5 MeV accord�ing to the neutron scale (scattering by Еn, <10%).

To estimate the quality of division, we used theparameter M = Δ(δγ + δn)–1, which characterizes thequality of division for neutron and γ peaks where Δ isthe difference between the positions of the n andγ peaks of the distribution of zero intersection timesδγ, and δn is the peak widths at half�height for γ quantaand neutrons. It can be seen from Fig. 2 that in thispower range, the discrimination of particles obtainedusing the zero intersection method is quite good (М =1.46).

Figure 3 shows the dependence of the portion of lostneutrons Nn on the number of photoelectrons in a scin�tillation for several values of the parameter Krej. It can beseen that for bursts with an energy release equivalent to25–50 keV from Compton electrons, a suppressioncoefficient for reasonable losses of neutrons can be pro�vided only at the levels of 10–3 and 10–4.

Simulation results allow us to estimate the extremepossibilities of the zero intersection method withoutallowing for the noise of electronics, overlays, etc. Theobtained data represent the extreme values for the qual�ity of particle separation by this method. We can see thatthe method for separating particles according to theshape of a pulse by registering zero level cross sectionscan suppress the γ background at a level of 10–4 at ener�gies higher than several hundreds of keV of electronicequivalent. For energies in the region of the delayeddivision of neutrons, the γ�background suppressionlevel falls to 10–1–10–2 and the method becomes ineffi�cient.

Due to unsatisfactory results from DSI accordingto zero intersection time in the region of interest, weperformed a simulation for a method based on the use

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Fig. 2. Distribution of events by zero intersection time: Δ isthe difference in the positions of the n and γ peaks of thedistribution of events by zero intersection time; δγ and δnare the widths of the peaks at half�height for γ quanta andneutrons.

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Fig. 3. Dependence (in %) of the number of lost neutronsNn on the number of photoelectrons Ne: curve 1, Krej =10–1; 2, Krej = 10–2; 3, Krej = 10–3; 4, Krej = 10–4.

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of linear filters that was a modification of the versionfirst suggested in [6].

We modeled the division of particles with a filterthat was not optimal, but considerably easier to use. Thefilter consisted of two constants: a brief part with a dura�tion of Ts (a few tens of nanoseconds) and a consider�ably longer part with a duration of Tl (100 ns–10 μs).These constants have opposite signs and are adjustedso that the filter on average yields equal and oppositelysigned quantities for neutron and γ scintillations. Ts isadjusted with a step of 1ns in order to ensure the bestparticle division quality. For stylbene, the optimumvalue is 30 ns. Total time Тt, which is equal to the sumTs + Tl, influences the quality of particle separationappreciably. Simulation results are shown in Fig. 4.

The most important characteristic of the neutronspectrometer in registering delayed division neutronsis its ability to discern particles by the shape of thescintillation pulse. In this case, the flux of neutrons islow and the background contains many γ quanta thatcreate light scintillations in the same range of integra�tion amplitudes in the detector.

To optimize the parameters of the neutron spec�trometer, we selected a single�crystal variant based onstylbene monocrystal that allows us to discern neu�trons and γ quanta by the pulse shape, which is neces�sary for suppressing the background. Other scintilla�tors can be used instead of stylbene (antracene, p�ter�phenyl, or liquid scintillator), but stylbene is the mostsuitable material due to its relatively low cost,mechanical strength, and thermal stability in the tem�perature range of 0 to 60°С. There are two disadvan�tages that limit the use of stylbene: its high sensitivityto temperature gradients and its light yield anisotropy,or the dependence of the scintillation amplitude ofheavy charged particles on the direction of motion(20% variation). The latter property could, however,turn out to be beneficial in localizing a neutron sourceand reducing the background.

The scheme of the unit includes a high�voltagesource that is synchronized with the operation of theaccelerator and allows us to shut down the photomul�tiplier for a period of time while the accelerator beampasses through the investigated object. This is neces�sary to obtain high quality spectrometric data in thetime interval beginning 250 μs after the end of theaccelerator beam pulse and until the start of the nextaccelerator beam.

Unlike the classic version of integrating the wholecharge and slow component, in our case three selec�tions are integrated with respect to time: the wholecharge and two slow components. This allows us toimprove particle separation by the pulse shape andpartially suppresses overlapping signals by rejectingthem as they change their shape. Figure 5 shows an

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Fig. 4. Division quality of particles Mnγ with stepwise Gattifilters Nt, in dependence on the number of registered pho�tons Ne in one scintillation: curve 1, Nt = 400; 2, Nt = 600;3, Nt = 1000; 4, Nt = 2000; 5, Nt = 4000; 6, Nt = 6000; 7,Nt = 10000.

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Fig. 5. Amplitude distribution A (a) and two�dimensionaldistribution of scintillation according to the pulse shape instyblene (b) on identical scale along the Y axis. Along theX axis: (a) number of events per channel, N; (b) portion ofthe contribution from slow scintillation component Nrel.

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example of the two�dimensional distribution used toseparate types of scintillations.

Figure 6a shows the results from measuring thepower distribution of delayed neutrons during irradia�tion of a target made from natural uranium by a beamof braking γ quanta with a maximum energy of50 MeV. For the sake of comparison, Fig. 6b shows asimilar spectrum of delayed neutrons from 235U,according to ENDF data. The obtained structure inthe power distributions of delayed neutrons correlateswith the structure in the ENDF data. The power reso�lution in the measured data is better and almost equalto that of the individual groups, e.g., ENDF group 2(Fig. 6b). Figure 6a shows the result of special process�ing of the low�power portion of the obtained spec�trum. The number of peaks and their energy positionalso corresponds to the ENDF data, allowing for thehigher resolution of our spectrometer. This figureenables us to determine the minimum energy fromwhich we can reconstruct the power spectra of neu�trons, 0.27 MeV.

CONCLUSIONS

A stream of photons of braking radiation from anelectron accelerator with an electron energy of

50 MeV was used to excite a division reaction. Delayedneutrons were registered between the operating cycles ofthe pulse accelerator. This method provides a uniqueopportunity to considerably reduce the delay time forthe start of counting after activation to ~1–2 millisec�onds (this period is necessary to lower the intensity ofinterfering γ radiation from momentary division) and toobtain data on the short�lived component of the flux ofdelayed neutrons in the appropriate irradiation modes.Our set of tools can be used to separate the contribu�tions from separate groups of delayed neutrons and tostudy the spectra inside each group through the adjust�ment and attaining of special operation modes of theaccelerator (single acceleration cycles at high intensitywith a repetition period of 1–10 s) and increasing thesolid angle of registering.

The photodivision method used in our projectenables us to raise measuring sensitivity by consider�ably improving the background conditions and betterstudying the neutron spectra in the low�energy region(less than 250 keV).

REFERENCES

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5. Baker, J.H., Glunov, N.Z., Kostin, V.G., et al., Probl.Atom. Sci. Technol. Ser. Nucl. Phys. Invest., 2007,vol. 48, no. 5, p. 126.

6. Gatti, E. and De Martini, F, Nucl. Electron., 1962,vol. 2, p. 265.

7. Roush, M.L., Wilson, M.A., Hornyak, W.F., Nucl.Instrum. Methods Phys. Res., 1964, vol. 31, p. 112.

8. Sabbah, B. and Suhami, A., Nucl. Instrum. MethodsPhys. Res., 1969, vol. 58, p. 102.

9. Ranucci, G., Goretti, A., and Lombardi, P., Nucl.Instrum. Methods. A, 1998, vol. 412, p. 374.

10. Wolski, D., Moszynski, M., and Ludzievski, T., et al.,Nucl. Instrum. Methods. Phys. Res. A, 1995, vol. 360,p. 594.

11. Verbitsky, S.S., Nucl. Instrum. Methods, 1978, vol. 151,p. 117.

12. Verbitskii, S.S., Lapik, A.M., Minaev, A.I., et al.,Prib. Tekh. Eksp., 1992, no. 2, p. 135.

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Fig. 6. (a) Measured power distribution of delayed neu�trons for natural uranium. Inset: The low�energy portionof a natural uranium spectrum; (b) a similar power distri�bution for uranium 235 from the ENDF.