Experimental demonstration of a compact epithermal neutron source based on a highpower laser

S. R. Mirfayzi,1 A. Alejo,1 H. Ahmed,1 D. Raspino,2 S. Ansell,3 L.A. Wilson,4 C. Armstrong,4, 5

N.M.H. Butler,5 R.J. Clarke,4 A. Higginson,5 J. Kelleher,2 C. Murphy,6 M. Notley,4 D.R. Rusby,4, 5

E. Schooneveld,2 M. Borghesi,1 P. McKenna,5 N.J. Rhodes,2 D. Neely,4 C.M. Brenner,4 and S. Kar1, 4, ∗

1Centre for Plasma Physics, School of Mathematics and Physics, Queen’s University Belfast, BT7 1NN, UK2ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK

3European Spallation Source, 22100 Lund, Sweden4Central Laser Facility, Rutherford Appleton Laboratory, Didcot, Oxfordshire, OX11 0QX, UK

5Department of Physics, SUPA, University of Strathclyde, Glasgow, G4 0NG, UK6Department of Physics, University of York, York, YO10 5DD, UK

Epithermal neutrons from pulsed-spallation sources have revolutionised neutron science allowingscientists to acquire new insight into the structure and properties of matter. Here we demonstratethat laser driven fast (∼MeV) neutrons can be efficiently moderated to epithermal energies withintrinsically short burst durations. In a proof-of-principle experiment using a 100 TW laser, asignificant epithermal neutron flux of the order of 105 n/sr/pulse in the energy range 0.5-300 eV wasmeasured, produced by a compact moderator deployed downstream of the laser-driven fast neutronsource. The moderator used in the campaign was specifically designed, by the help of MCNPXsimulations, for an efficient and directional moderation of the fast neutron spectrum produced by alaser driven source.

There has been over the last decades a sustained in-terest in laser-driven neutron sources, which are capableof producing sub-ns bursts of fast (MeV energies) neu-trons with high brightness[1–5]. Although fast neutronsare useful for many applications, such as imaging [6] andmaterial testing for fusion reactor vessels [7], the arenaof neutron science and applications mainly requires slowneutrons (with energies ranging from sub-meV to keV)which are provided at reactor and accelerator based facil-ities by moderating fast neutrons. In particular, sourcesof epithermal neutrons (eV-100keV) are of high inter-est for a wide range of applications, from material sci-ence [8–10] to nuclear waster transmutations [11] andhealthcare [12, 13]. However, the scale and operationalcosts involved in accelerator-based facilities not only limittheir availability to the scientific community, but alsohinder their wide promotion in the industrial, securityand healthcare sectors.

In this paper we report the first experimental demon-stration of producing epithermal neutrons from a laserdriven source. By directing laser-driven fast neutronsproduced by p-Li reactions onto a compact (10 cm×10cm×8 cm) moderator placed at ∼11 cm from the source,a significant flux of epithermal neutrons ∼ 5 × 105

n/sr/pulse in the range 0.5-300 eV was produced. Theepithermal neutrons were measured at a distance of 2.58m from the moderator using 3He detectors in ToF mode.The proof-of-principle results presented here indicate asignificant scope for further improvement in terms of at-tainable flux by optimising the setup and the fast neu-tron source, as will be discussed in the final section ofthe paper. In light of the latest development in diode-pumped, high repetition rate high power laser systems,laser-based sources in the future may offer the possibil-

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ity of developing compact beamlines, closely coupled tothe moderator, thanks to the significantly less hostile en-vironment offered by a laser-based system compared toreactor-based and pulsed-spallation facilities.

The experiment was carried out employing the 100TW arm of the VULCAN laser facility of the Ruther-ford Appleton Laboratory, STFC, UK. A schematic ofthe experimental setup is shown in Fig. 1. The ∼750fs FWHM laser pulses with energy of ∼50 J after thecompressor were focused onto 10 µm Au targets byan f/3 off-axis parabola, delivering peak intensities of∼ 5 × 1019 W cm−2 onto the target. The fast neutronswere produced by impinging the ions from the gold foil(henceforth called pitcher) onto a ∼2 cm thick block oflithium (henceforth called catcher) placed ∼5 mm awayfrom the pitcher. The spectra of the fast neutrons pro-

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duced at the catcher were diagnosed by a time of flight(ToF) technique, employing two fast scintillator detectors(EJ232Q [16] plastic scintillator coupled to HamamatsuR2083 PMT [17]). As shown in Fig. 1, the detectors(henceforth called ND-1 and ND-2 respectively) were lo-cated at 4.37 m and 4.27 m from the source, respectively,measuring fast neutrons spectra along the lines 0◦ (onaxis) and 35◦ (off axis) with respect to the ion beam axis.The scintillator detectors were absolutely calibrated forfast neutrons as reported in ref. [18].

After characterising the fast neutron spectra along 0◦

and 35◦, a pair of Reuter Stokes 3He filled (at 10 barpressure) proportional counters [19] were placed alongthe beam axis at a distance of 2.83 m from the catcher.The 3He detectors were placed inside a shielded housingwith high voltage (2 kV) applied across the anodes andcathodes. Due to the high sensitivity of the 3He detec-tors, the gamma flash produced by the laser interactionsaturated the signal at early times, and only neutronsof energy up to ∼300 eV could be detected by the 3Hedetectors in a ToF mode. The efficiency of the detec-tors with respect to the incident neutron energy is givenby the formula, η = 1−exp(−0.00482Pdλ), where P isthe gas pressure in bar, d is the tube diameter in mmand λ is the wavelength of neutrons in angstroms. Oneof the 3He detectors was wrapped with a few mm thickcadmium sheets in order to identify the background levelof stray thermal (≲ 0.5 eV [20]) neutrons hitting the de-tector. The 3He detectors were placed behind a B4Ccylindrical collimator (∼30 cm long with ∼30 cm outerdiameter and ∼5 cm inner diameter) as shown in Fig. 1.

The configuration of the compact moderator used inthe experiment is shown in Fig. 2(a). The fast neutronsfrom the catcher, were directed to impinge onto the mod-erator, which was aligned along the beam axis as shownin Fig. 1. The thickness and materials used in the mod-erator were chosen based on MCNPX [21] simulations,with the aim of optimizing the flux of epithermal neu-trons from the exit plane of the moderator. The choice ofmaterials for the experiment was restricted by radiationsafety and manufacturing constraints. Within the overallmoderator size, the main component was a 5 cm×5 cm×4cm block of high density polystyrene, which was designedto slow down ∼MeV neutrons to the epithermal range(mean free path of 1 MeV neutron is ∼3 cm [22]). Underthe condition of the experiment, the high Z front layers(2.4 cm thick lead and 1.2 cm thick tungsten) mainlyserved as reflectors to the moderated neutrons producedby the polystyrene block. However in presence of higherenergy (≥ 10 MeV) neutrons, this design would also in-crease the neutron population by (n,xn) reactions insidethe thick layers of lead and tungsten. The polystyrene,tungsten and lead blocks were housed in a cm-thick alu-minium structure, surrounded by 1.5 cm thick lead toreduce neutron leakage from the side walls of the mod-erator. In order to produce a pure epithermal beam and

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FIG. 2. (a) The design of the moderator superimposed onthe neutron (in the range 1eV-1keV) flux distribution acrossthe mid-plane of the moderator, obtained from MCNPX sim-ulation for 1 MeV neutrons entering the moderator from theleft side (shown by red arrow). (b) Comparison between neu-tron spectra at the two detectors, detector-1 and detector-2respectively (shown in (a)), placed 5 cm away from the sideand exit faces of the moderator for different incident neutronenergies as obtained from MCNPX simulations.

filter the lower energy neutrons, a 2 mm cadmium sheetwas placed at the exit plane the moderator.

As can be seen in Fig. 2(a), the moderator pro-duces an anisotropic flux distribution of epithermal neu-trons. While the front layers of lead and tungsten maycontribute by pre-moderating and multiplying high en-ergy neutrons, the sub-MeV neutrons are efficiently mod-erated to the epithermal range inside the polystyreneblock. The neutron spectra emerging from the exit planeand the side of the moderator for different input neutronenergies are shown in Fig. 2(b), which illustrates the ef-fectiveness of the design in moderating neutrons of MeVenergies, as produced in abundance in our experiment(see Fig. 3(a)).

The experiment was carried out in two stages. Sincethe moderator and the 3He detector restrained us frommeasuring the fast neutrons produced along the beamaxis, the first stage was devoted to characterising thespectra and angular distribution of the fast neutrons pro-duced by the catcher, before deploying the moderator andthe 3He detectors. The data collected by the two scintil-lator detectors over a number of dedicated shots showedfairly similar spectral profiles as shown in fig. 3(a)). Theneutron flux above 2.5 MeV was measured as ∼ 107 n/Sr,whereas the spectra indicates that the highest flux liesin the sub-MeV region (estimated to be of the order of109 n/sr between 0.1-1 MeV). The ratio of the fast neu-tron fluxes recorded by ND-1 and ND-2 was found to be∼0.6 over a number of shots taken at similar laser andpitcher/catcher conditions. When the ND-1 became re-dundant in the following stage (due to the installation ofthe on-axis 3He detector, see Fig. 1), the data obtainedby the ND-2 was used as a reference to infer the on-axisfast neutron spectra.

Placing the moderator at a distance ∼11 cm behindthe catcher produced a significant amount of epithermal

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off axis) detectors. (b), (c) Raw data (from three shots for each case) obtained in the 3He detectors without and with themoderator, respectively. (d) Raw data in presence of the moderator obtained in the 3He detectors wrapped with Cd foil, whichis a good absorber of neutrons of energies below ∼0.5 eV, as can be seen from the cross-section of 113Cd(n,total) [20] plotted inthe insert. The zoomed plots of the raw data in linear X-scale are shown in the inserts. Epithermal neutron hits are identifiedby the voltage spikes in the output signal (as shown in the insert in (b)) and the neutron energy is calculated by ToF (as shownin a separate axis under (d)). (e) Epithermal neutron spectra produced by the moderator, averaged over 16 measurements,where the red line is a eye-guide to the data points.

neutrons along the beam axis, in contrast to what de-tected in shots without the moderator. This is appar-ent from the increased number of voltage spikes in thevoltage versus time/energy plots of Fig. 3(c) and (d),compared to the plot of Fig. 3(b). Each voltage spikeis created when a neutron is absorbed in the 3He de-tector via 3He(n,p)3H reaction, and the released protonand 3H ionize the gas inside the detector generating anelectron avalanche. While the incident neutron energywas determined from the ToF, the neutron flux was cal-culated by counting the number of voltage spikes in theoutput signal. As expected, the Cd wrapped 3He detec-tor showed rapidly decreasing number of neutron hits inthe signal trace after ∼300 µs, as shown in Fig. 3(d),which is consistent with the absorption cross-section ofCd for low energy neutrons. Taking into account the de-tector efficiency (as mentioned earlier) and normalisingthe signal with respect to the incident neutron flux mea-sured by the ND-2 detector in the same shot, the spec-trum of the epithermal neutrons was obtained. Fig. 3(e)shows the epithermal neutron spectrum produced by themoderator in the detectable energy range of 0.5-300 eV,by averaging over 8 shots (i.e. 16 measurements). Thehigh energy cut-off is due to the saturation of the 3Hedetectors at early times by the gamma flash. As can beseen in Fig. 3(b), the number of neutron hits in this en-

ergy range was insignificant in shots without the moder-ator. The background-subtracted neutron flux producedby the moderator in the energy range 0.5-300 eV was(5.1±0.15)×105 n/sr/pulse.

As mentioned earlier, the experiment was designed toassess and demonstrate the feasibility of driving intensebursts of epithermal neutrons with short pulse lasers.The moderator in the current experiment was placed at adistance of ∼11 cm from the catcher to monitor shot-to-shot variations in the fast neutron spectra by the ND-2.If the moderator were placed in contact with the catcher,using a minimal (∼cm) thickness of high Z material inits front layer, the fast neutron flux per cm2 incident onthe polystyrene would have increased by two orders ofmagnitude, producing a commensurate increase in ep-ithermal flux at the moderator output. A prudent esti-mation based on MCNPX simulations suggests that anepithermal flux of ∼ 104 n/cm2/pulse in the energy range0.5-300 eV (∼ 3 × 104 n/cm2/pulse for 1 eV - 100 keV)was produced in the current experiment at a nominal dis-tance of 5 cm from the moderator (i.e. at the detector-2position in Fig. 2(a)).

While these proof-of-principle results are encouraging,there is significant scope for further optimisation of theepithermal neutrons flux by optimising the moderatordesign as well as the fast neutron source. The input

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flux of fast neutrons can be significantly enhanced byincreasing the energy and number of incident ions on thecatcher. Order of magnitude higher fast neutron fluxesthan produced in the current experiment have been al-ready reported [2–4], and, in principle, can be furtherimproved upon by taking advantage of ongoing develop-ments in laser-driven ion acceleration[23–25]. A credi-ble source of fast neutrons can also be obtained by de-ploying MeV electron jets from laser-driven explodingfoils [5]. High energy electrons efficiently produce neu-trons inside a metal converter via photo-nuclear reactionsby generating high flux of bremsstrahlung photons. Al-beit isotropic, such sources produce neutrons of energy inthe MeV range highly desirable for moderation. In termsof optimsing the moderation process, laser driven sourcesoffer greater flexibility compared to the other facilities,where the formidable heat and extraneous radiations con-strain significantly the choice of material, temperatureand geometry of a moderator. Finally, it is to be notedthat the epithermal bursts produced by the laser-drivenneutrons of sub-ns duration would also have an intrinis-cally short duration (solely determined by the charac-teristics of the moderator), beyond the capability of theaccelerator-driven spallation sources, typically set by theduration (sub-µs - ms) of their proton pulses [26–28].

Laser-based sources are fast approaching a crucialstage in their development for neutron science and ap-plications to complement large-scale facilities. Althoughsources such as the National Ignition Facility reportrecord yield in fusion neutrons per pulse [29, 30], its scaleand repetition rate are inadequate for any credible appli-cation related to neutron science. On the other hand, therepetition rates achievable on smaller scale installationscould still be too low to guarantee successful exploita-tion for the aforementioned applications. The future of alaser-based approach would be relient on the progress indiode-pumped technologies, such as the DiPOLE [31, 32]and HAPLS [33] projects, aiming towards developing 10Hz, Petawatt-class laser systems.

We gratefully acknowledge funding from EPSRC[EP/J002550/1-Career Acceleration Fellowship held byS. K., EP/L002221/1, EP/J500094/1, EP/K022415/1and EP/J003832/1] and STFC [ST/P000142/1]. Thiswork was also supported by STFC’s Business and Inno-vation Directorate. Authors would like to acknowledgevaluable discussions with J. Collier (Director of CLF,STFC, UK), S. Langridge (Head of diffraction & ma-terials division, ISIS, STFC, UK) and R. Dalgliesh (In-strument Scientist, ISIS, STFC, UK). Authors also ac-knowledge support from the members of the experimentalscience group, A. Cox (mechanical engineering) and D.Haddock (target fabrication group) of the CLF, STFC,UK.

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[33] https://lasers.llnl.gov/science/photon-science/highpowered-lasers