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EXPERIMENTAL STUDY OF NEUTRON FIELDSPRODUCED IN PROTON REACTIONS WITH

HEAVY TARGETS

A. Kugler, V. Wagner

Nuclear Physics Institute AS CR, 25068 RezCzech Republic

I. Introduction

One of important aspects of intensive neutron source design for ADS is thoroughlyknowledge of neutron field around the production target. To get experimental data forbenchmarking of corresponding simulation codes we lunched series of experiments in 1997.Bellow we report first results obtained in experiment with thick targets of tungsten and leadcarried out by us at 1998 in Laboratory of High Energy (LHE), Joint Institute for NuclearResearch (JINR), Dubna1, as well as results of analysis of experimental data obtained duringTAPS2 campaign at Kernphysisch Versneller Institute (KVI), Groningen, in 1997 withrespect to azimuthal and energetic spectra of neutrons emitted from thin tungsten target

II. Model of ADS neutron-production target

Model of ADS neutron-production target was constructed at Nuclear Physics Institute(NPI). Cylinder with diameter of 2 cm and length of 60 cm is made from high purity 74W.Target was fixed at back using special aluminum holder. Let us mention, that the activation ofaluminum by relativistic protons is rather low, hence the manipulation with the whole setupwas much easier. Intensity of produced neutron field was measured by threshold activationmethod. The target was surrounded by 14 thin foils from 197Au and 28Al which were fixedat surface of the special holder, thin-wall (4 mm) aluminum tube with outer diameter of 36mm. The holder can be removed using distance manipulator without touching the target andhence it was possible to install new set of foils shortly after irradiation. Dimension of Aufoils were 1.6 ×× 2.0 cm, their thickness was 100 µµm and they were fixed with step of 4 cmin direction along the target. Al foils have thickness of 50 µµm, their width was 1.6 cm andthey surround whole circumference of the holder.

We also irradiated cylindrical lead target with diameter of 9.6 cm and length of 50cm placed into corresponding hole in box with dimension about 1 ×× 1 x 1 m made fromboron plastic. The Au and Al foils were placed on the surface of lead target, for their positionalong the length of target see the Table II.

1 collaboration with M. I. Krivopustov, C. A. Novikov, P. A. Rukoyatkin, Ts.Tumendelger, D. Chultem (LHE JINR) and J. Adam (LNP JINR).

2 TAPS is a collaboration of GSI Darmstadt, GANIL Caen, University of Giessen,University of Mainz, KVI Groningen, NPI Rez, and University of Valencia.

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III. Threshold activation method

Interaction of neutrons within Au and Al foils leads to the reaction with main characteristicsgiven bellow:

Reaction Halftime (hours) Gamma lines (KeV)

27Al (n,α) 24Na 14.959 1368.633

197Au (n,γ) 198Au 64.68408 411.8197Au (n,2n) 196Au 148.392 333.0 , 355.72 , 426.0197Au(n,4n) 194Au 39.5 328.45

Corresponding cross sections as function of neutron energy were deduced from literature and

are demonstrated in Fig.1. The isotope 198Au is produced mainly by low energy neutrons,cross section is very high (1.3-2.1 barn) between 5-10 keV, it fells down approximatelylinearly in log-log scale from value of 1 barn at 15 keV to value of 0.01 barn at 4 MeV .Production of 24Na starts from the neutron energy of 6.5 MeV, while production of 196Austarts from the neutron energy of 8 MeV and production of 194Au starts from the neutronenergy as high as 28 MeV. Hence, foils of 197Au a 27Al can be used as energy thresholddetectors of neutrons.

Figure 1: Cross section for production of indicated isotopes inAu an Al induced by neutrons with indicated energy.

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IV. Thick target experiment

Both targets were irradiated by collimated beam of protons (diameter of 18 mm) with anenergy of 1.5 GeV accelerated by synchrophasotron at the Laboratory of High Energies, JointInstitute for Nuclear Research, Dubna, Russia. The range of protons in tungsten was 54.5 cm,hence they were completly stopped in our 60 cm long tungsten target in contrast with the caseof lead target. Gamma decay of isotopes produced in foils were measured using two high-purity Ge-detectors to remove systematical errors due to possible error in detection efficiency.Dead time corrections were checked using measured yield of photons corresponding to. K-line (1460 KeV) as its intensity is constant in time as it belongs to natural backgroundradiation. Corresponding gamma spectra were analyzed using PC-code DEIMOS, whichcarries out gaussian fit of gamma peaks. Obtained areas bellow peak were corrected to:

• Time interval between moment of the irradiation and of the measurment: C (time) = e t/ττ. , here 'τ ' is corresponding half-live

• Weight of the foil: C (weight) = 1 / weight

The absolute efficiency of detector as well as correction due to accidentally summing ofphoton cascade leading to misinterpretation of two photon hit as one photon with summedenergy was taken into account, too. Results obtained from both detector did not differ morethan 1%. Number of protons hitting the tungsten target was 1.86 x 1012 . It was deduced fromthe activation of aluminum foil placed in front of target. The back-scattering of protons fromthe target was estimated using LAHET based simulation. The error of beam dose isestimated to be around 20%. Number of protons hitting the lead target was 42.2 x 1012 . Thisnumber was reported to us by operator staff as deduced from the beam current integrator. Itserror is estimated to be around 30%. Resulting numbers of radioactive nuclei produced perone proton are given in tables I and II and are presented at Fig. 2-4. The errors in number ofprotons are not included!Let us mention, that values corresponding to detection of high energy neutrons coincide forboth targets. In other words, amounts of neutrons produced at "equivalent" depth of bothtargets are almost the same despite of different diameters of targets. Here the "equivalentdepth" means that primary protons passed by same number of target atoms. For low energyneutrons the situation is completely different The corresponding intensity of neutron fieldsurrounding lead target is more than two orders of magnitude higher in comparison withtungsten target, see Fig.4. Detailed LAHET simulations, see bellow, revealed that the sourceof these low energy neutrons is rescattering of high energy neutrons produced in target inhuge "moderator", which surrounded lead target. This finding points out possiblemisinterpretation of experimental data about transmutation cross section as well as other lowenergy (or even thermal) neutron induced reaction in vicinity of target surrounded bymoderator.

V. Thin target experiment

Barium fluoride was shown to be an efficient detector material for neutrons in a wide range ofenergies [1,2]. Therefore the high acceptance BaF2 photon detector system TAPS is aneffective tool for exclusive neutron measurements. We analyze in this direction data obtainedduring irradiation of thin tungsten target by the beam of 190 MeV protons fromsuperconducting cyclotron AGOR of KVI at 1997. Neutrons were detected by TAPS

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Figure 2: Number of produced radioactive nuclei per one proton hittingtungsten target as function of thickness of tungsten to be passed by primaryprotons to achieve corresponding foil position. Symbols correspond to

Figure 3: Number of produced radioactive nuclei per one proton hitting leadtarget as function of thickness of lead to be passed by primary protons toachieve corresponding foil position. Symbols correspond to experimentaldata, curves to results of simulation, see text.

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modules, which were configured into 6 blocks of 64 BaF2 scintillators shielded in front by thin chargeveto plastic detectors. Modules were positioned around the target to cover the polar angles from 60 up

Figure 4: Number of produced radioactive nuclei per one proton hitting lead(upper part) or tungsten (lower part) target as function of thickness of lead ortungsten, respectively, to be passed by primary protons to achievecorresponding foil position. Symbols correspond to experimental data, curvesto results of simulation, see text.

Figure 5: Experimental setup of thin target experiment carried out by TAPSat AGOR KVI proton beam.

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to 170 degrees, see Fig.5. The energy of neutrons were derived from their Time-Of-Flight over thedistance of about 60 cm, see Fig.6.

VI. Simulations

Simulations were carried out using LAHET (Bertini) + MCNP4B code combination. As anexample, see energy spectra of neutrons passing through foils positioned at 3.6, 27.6 and 47.6cm distance from the beginning of tungsten target presented in Fig.7. Let us mentionenhancement of high energy part of these spectra with growing distance from the beginning oftarget due to strongly forward peaked preequilibrium high energy neutrons emitted inspallation reaction in first part of target, compare specta at Fig.6. These energetic spectra wereat each foil's position convoluted with corresponding cross sections, see upper part of Fig.7.

Figure 6: The neutron production on the W target. The energy spectra fordifferent angles (left side) and angular distributions for the low and highenergy component (right side).

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Figure 7: Results of simulations: number of produced neutrons per oneproton hitting tungsten target passing through three different foil's positionas function of neutron energy (lower part). Cross sections corresponding toused actiavtion reactions in foils deduced from literature (upper part).

Figure 8: Results of simulations: number of produced radioactive nuclei perone proton hitting tungsten target as function of thickness of tungsten to bepassed by primary protons to achieve corresponding foil position.Contribution due to activation induced by scattered and secondary protonsare indicated, see text.

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Table I: Experimental yield of radioactive nuclei produced in Au and Al foils placed alongthe W target per one primary proton. Numbers have to multiplied by 10-3 . Errors given arestatistical only, see text.

PositionX[cm]

X.ρρ[g/cm2]

198Au 196Au 194Au 24Na

1.6 31 5.92(13) 16.36(15) 4.64(11) 1.35(5) 5.6 108 6.09(13) 17.53(15) 5.34(11) 1.44(5) 9.6 185 4.06(9) 12.26(14) 3.83(12) 1.11(4)13.6 262 2.40(15) 7.97(16) 2.75(12) 0.710(29)17.6 340 1.45(8) 5.13(9) 2.02(10) 0.456(28)21.6 417 0.98(9) 3.29(8) 1.27(10) 0.304(17)25.6 494 0.58(8) 2.38(9) 0.85(9) 0.236(20)29.6 571 0.48(8) 1.74(8) 0.50(8) 0.173(19)33.6 648 0.25(8) 1.38(8) 0.58(7) 0.153(25)37.6 726 0.10(6) 0.86(6) 0.35(7) 0.096(20)41.6 803 0.10(7) 0.61(6) 0.31(7) 0.062(20)45.6 880 - 0.40(8) 0.15(9) 0.051(19)49.6 957 - 0.24(6) 0.11(7) 0.017(15)53.6 1034 - 0.13(6) 0.06(7) 0.052(16)

Table II: Experimental yield of radioactive nuclei produced in Au and Al foils placed alongthe Pb target per one primary proton. Numbers have to multiplied by 10-3 . Errors given arestatistical only, see text.

PositionX[cm]

X.ρρ[g/cm2]

198Au 196Au 194Au PositionX[cm]

X.ρρ[g/cm2]

24Na

1.0 11 1706(5) 10.79(8) 2.86(8) 1.0 11 1.14(9) 5.0 57 1673(5) 16.12(8) 4.73(8) 5.0 57 1.74(12) 7.0 79 1678(5) 16.32(8) 5.02(9) 7.0 79 1.56(10) 9.0 102 1682(5) 15.82(8) 4.78(9) 9.0 102 1.69(11)13.0 148 1618(5) 14.01(9) 4.42(8) 13.0 148 1.56(10)17.0 193 1580(5) 11.85(10) 3.67(7) 17.0 193 1.26(8)21.0 238 1554(4) 9.93(9) 3.29(7) 21.0 238 0.99(9)25.0 284 1504(4) 7.88(5) 2.67(7) 24.0 272 0.83(6)29.0 329 1441(4) 6.22(7) 1.98(7) 28.0 317 0.72(5)33.0 375 1359(4) 4.83(5) 1.59(7) 32.0 363 0.48(5)37.0 420 1268(4) 3.68(5) 1.17(7) 35.0 397 0.46(4)41.0 465 1140(4) 2.90(5) 1.04(6) 40.9 464 0.30(3)45.0 511 1044(4) 2.14(5) .65(7) 46.9 533 0.158(24)49.0 556 992(4) 1.47(4) .52(8) - - -

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Emission of secondary protons due to spallation reaction as well as scattering out of primaryproton beam were taken into account, too. To judge these effects, see Fig. 8. Results areindicated in Fig.2.-4. by plotted curves.

VI. Conclusions

• Presence of massive shielding close to the target has dominant role in spatial distributionof low energy neutrons

• Phenomenological model can describe shape of spatial distribution of high energyneutrons along the target

• However detailed simulations revealed significance of proper accounting of scatteredand secondary protons and azimuthal distribution of neutrons produced in spallationreaction as well.

• Detailed simulation by LAHET(Bertini)+MCNP4B lead to absolute values compatiblewithin 30% with experimental data

References

[1] T. Matulewitz et al., Nucl. Instr. and Meth. A 274(1989)501[2] V. Wagner et al., Nucl. Instr. and Meth. A 394(1997)332