High flux neutron production from 12C beams on heavy targets

Radiation Measurements.Vol. 28. Nos I-6, pp. 313-316, 1997 O 1997ElsevierScienceLtd Printedin GreatBritain.All rights reserved 1350-4487/97 $17.00 + 0.00 PlI: S 1350-4487(97)00090-$

Pergamon

HIGH

FLUX NEUTRON PRODUCTION FROM ON HEAVY TARGETS

12C B E A M S

J.C. ADLOFF (1), R. BRANDT (2), V.S. BUTSEV (4), M. DEBEAUVAIS (1), F. FERNANDEZ (3), B.A. KULAKOV (4), M.I. KR/VOPUSTOV (4), M. OCHS (2), A.N. SOSNIN (4) AND M. ZAMANI (5) (1) Centre de Recherches Nucl~aires, Strasbourg, France, (2) Kernchemie, PhilippsUniversit~t, Marburg, Germany, (3) Universitat Autonoma, Barcelona, Spain, (4) Lab. of High Energy JINR, Dubna, Russia, (5) Aristotle University, Thessaloniki, Greece ABSTRACT Spallation neutrons produced from ~2C ions at 18 and 44 GeV on Cu and Pb targets were studied as well as thermalization in appropriate moderators. The irradation were performed at the Dubna LHE Synchrophasotron. Results are given for thermal and fast neutrons estimated and compared with different experimental methods. KEYWORDS

High energy 12C ions, Cu and Pb targets, gamma spectroscopy, Track etch method, Thermohtminescence. INTRODUCTION

Above a few hundred MeV, most proton energy is lost through nuclear interactions. In targets heavy enough to exhaust the hadronic cascade, the multiple production called spaUation produces heavy fragments, alphas, protons, neutrons, muons, etc. A large part of spallation is accounted for by evaporation of light particles and neutrons. Their energies are lower than about 10 MeV and most have energies around 2 MeV. A fraction of the light particles and neutrons have energies around half that of the beam energy. A smaller fraction of the order of 10% are high energy cascade particlesneutrons preduced by direct intranuclear collisions. Concerning the neutron production, spallation neutron sources have already contributed much to science and have been widely discussed recently for their applications in reaction-hased investigations now and for the future (Bowman et al., 1992). In the present work, neutron production by t2C ions at 18 and 44 GeV on Cu and Pb targets was measured. The experiments were performed at the Synchrophasotron accelerator at the LHE, Dubna. The study concerns neutron production in extended targets and in the moderator. EXPERLMENTAL ARRANGEMENT Irradiations were performed with a total of 10t°-10~2 ~2C ions. The experiment layout is presented in Fig. 1. The thick target of Cu or Fo was surrounded by a paraffin moderator. For the neutron detection, various methods were applied to estimate the numbers of thermal and fast neutrons. NEUTRON DETECTION

1) Gamma-ray spectroscopy In Fig. 1, the positions indicated with 1, 2, 3... correspond to holes in the moderator filled with plastic vials containing approximately 1 g La (in the form LaCI3.7H20) in one experiment or 1 g U (as

313

314

PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE

UO3.1H20) in another experiment, used for the neutron measurements. Secondary neutrons were detected via the (n,y) reactions :

a) '39La(n,y) '4°La

b) 23SU(n,y) 239U -~ 239Np For reaction a) the ~4°La activity was mesured using the y-ray lines at 487.0 and 1596.5 keV. For the case of reaction b) 239Np was measured by the y-ray of 277.6 keV. Details concerning corrections for the measurements and the efficiencies as well as the beam control are given in Abdullaev et al., 1995; Bisplinghoff e/al., 1995; Brandt et al., 1995. lJ.l~ neuLroB d e t P e l o r s

•

• Paraffin

•

•

•

- Moderator I

Fig. 1. Experimental arrangement indicating the target, the moderator and neutron detector positions.

Fig. 2. Detail of containers.

2) Thermoluminescence We have used TLD600 which is sensitive to both thermal neutrons and y-rays and TLD700 which is sensitive only to y-rays. The thermal neutron calibration was made in the CEA Laboratory at Cadarache in the Sigma facilities. The response of TLD600 was found to be 10.03 mV.°C.mg~.llSv "~ and so this value was applied for thermal neutron dose estimation. For the conversion of dose to number of neutrons, I.C.R.P.21 (1973) conversion factors were used. From the difference in readings of the two detectors the thermal neutron flux was estimated. 3) Track etch method Two different track etch techniques were applied. One is based on systems calibrated in equivalent dose. Then appropriate conversion factors transform the dose to neutron flux. The other method is based on the U (n,f) reaction. a) One technique used consisted of CR39 detectors on which 6LiF (Zamani et al., 1996) was evaporated on one surface of the detector. The particles detected were a particles and 3H from the 6Li(n,a)t reaction and proton recoils from (n,p) scattering. It has been shown that the system responds well to thermal neutrons as well as to fast ones. For fast neutrons measurements, 1 mm thick Cd foils were used to absorb thermal neutrons. Another sample without the Cd absorber gave the total neutron flux. From the difference of the readings of the two systems, the thermal and fast neutron components were estimated. For the conversion of the track count to equivalent dose, the response of 5000 tr.cm2mSv * was taken for thermal neutrons and 210 tr.cm2mSv "~ for fast neutrons. The conversion factors of I.C.R.P.21 were used to transform equivalent dose to number of neutrons. For fast neutrons the conversion factor for 2 MeV was taken. The etching was done with NaOH (5 N) at 70°C for 150 min. These conditions also develop trajectories for protons from 300 keV to 3 MeV. In ad_dition, a CR39 sample was used with a polyethylene moderator of 0.5 cm to estimate the response coming from the proton recoils. The response of 510 tr.cm'2mSv* was used since it is known from the calibration. As conversion factor, the same as for 2 MeV was taken. b) For the determination of slow and fast neutrons, fission fragments of the U(n,f) reaction were registered in a Lexan detector. A standard etching with NaOH (5N) at 60°C for 60 min was utilized. For thermal neutrons, a target of 97.7% 23SU and for fast neutrons a target of 99.65% 238U was employed. About 100 lag/cm2 of uranium as determined by weighing and a-particle spectrometry was

315

PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE

evaporated directly on a Lexan sheet. A sandwich was made by covering it by another Lexan sheet. Such sandwiches were placed in containers (Fig.2) at three distances separated by paraffin (4, 7 and 10 cm) from the middle of the Cu or Po. In Fig. 1 we can see that one container was placed at 90 ° to the beam midway along the length of the Cu or lab target and the other in front of the target at 0 °. To determine the number of neutrons from the number of fission tracks, we took a fission cross section of 500 b for thermal neutrons on 235U and 0.6 b for >2 MeV neutrons on 23SU. A correction for the 0.35% of 235U in the 23Su was made.

RESULTS AND DISCUSSION For the determination and the comparison of neutron fluxes obtained with the different methods we give only the results obtained at 90 °. The total number of neutrons is the product of the number of neutrons determined per cm 2 times the total surface area of the cylinder at the different distafices. For the calculation to be valid, the distribution of the neutrons has be isotropic, which is only approximately the case. In table 1, the results of thermal neutron production per incident ~2C are presented for Cu and Pb targets for the two beam energies used. The uncertainties in the neutron numbers are about 30 %. R is the distance from the middle of the Cu or Pb target to the neutron detector at which a measurement was performed. Table 1. Thermal neutrons per mcident t2C (estimated from measurements at the outer surface of the moderator). Method

R, cm

12C - Cu 18 GeV

12C - lab 18 GeV

12C - Cu 44 GeV

12C - lab 44 GeV

235U(n, f)

10.0

31

40

91

161

6Li(n, a)t

10.0

31

40

60

167

139La(n, y)14°La

9.8

66

172

184

423

y)239Np

9.5

65

173

185

439

-"~U(n, f)

7.0

100

120

349

553

TLD600

7.0

78

172

223

713

23SU(n'

The results among the three detection methods are in good agreement in view of the different energy response of each method and the uncertainties in the reference cross sections. From Table l it can be concluded that 1.5-2.5 times more neutrons are produced by the lab target than by the Cu target. In going from 18 to 44 GeV incident ~2C, the neutron production increases 2-3 times for Cu and 3.5-4.5 times for the Pb target. The same influence of beam energy is also observed in direct reactor measurements with proton beams (Andriamonge et el., 1995). In Fig. 3 we present the results for thermal neutrons at the three distances inside the moderator. The flux behind 3 cm of paraffin (R = 7 cm) for both targets and both energies are more than 3.5 times higher than at R = l0 cm and R = 4 cm. This shows that 3 cm of paraffin is the best moderator thickness for obtaining the highest thermal neutron yield. It is known that 3 cm of paraffin is a good thickness for thermalization of 2-3 MeV neutrons. Table 2. Fast neutrons per incident t2C (__ 30 % error) Method

IL cm

12C - Cu 18 GeV

12C - Pb 18 GeV

12C - Cu 44 GeV

12C - Pb 44 GeV

23SU(n,2n ) 7 MeV _
9.5

16

19

37

60

p-recoils 300 keV < E < 3 M e V

I0.0

13

44

55

13 5

316

PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE 500 450 400 350 "o • g

18 Gev C+CU

300 44 Gev C * Cu 250

........................................

. .................................................................... ,t.

z

20o

18 GevC-Pb 4 4 G e v C + Pb

150 100 50 0

i iiiii I

I

I

4

7

10

Distance in c m

Fig. 3 Thermal neutron per mcidmt Z2Cas a function of the distance from the beam axis inside the paraffin moderator for Cu and Pb at 18 and 44 GeV. In Table 2, the number of fast neutrons generated in the target per incident ion is given. In the case of fast neutrons the results are taken from track etch measurements and from radiochemical measurements in the 23SU(n, 2n) reaction using a cross section of 1 barn for 7 MeV < E= ~ 20 MeV. For proton recoils in CR39, the neutron energies lie between 300 keV and 3 MeV. The energy dependence gives the same result for both targets, i.e. 2-3 times more neutrons for the higher bombarding energy. The results for fast neutrons as a function of the three distances inside the paraffin are all about the same. This is a confirmation that the energy of most neutrons emitted was greater than 5 MeV. Further results for this experiments are described in (Ochs et al., 1996). The results of this work should be useful for the design of a neutron source with high neutron multiplication from heavy ion production. REFERENCES Abdullaev I.G. et al., (1995) Neutron Production in Extended Cu-Targets Irradiated with Relativistic 12C-Ions at Dubna, as Studied with SSNTD and Nuclear Chemistry. Rad. Meas 25, 219-230 Andriamonge S. et al., (1995) Experimental Determination of the Energy Generated in Nuclear Cascades by a High Energy Beam. Phys. Lett. B-348, 697-709 BisplinghoffB. et al., (1995) Neutron Generation in Massive Cu-Targets during the Irradiation with 22 and 44 GeV Carbon Ions. Radioanalyt. andNucl. Chem. 189, 191-206 Bowman C.D. et al., (1992) Nuclear Energy Generation of the Waste Transmutation Using an Ao,~,eselerator - Driven Intense Thermal Neutron Source. Nucl. Inst. Meth. A 320, 336 Brandt R. et al., (1995) Further Evidence for Enhaced Nuclear Cross-section observed in 44 GeV Carbon Interactions with Copper. JINR.-Preprint JINP,, Dubna, Russia I.C.R.P.21, Data for protection against Ionizing Radiation for external sources. Pep. 21, Oxford, Pergamon Press (1973) Ochs M. et al., (1997) SSNTD and Radiochemical Studies on the Transmutaion of Nuclei Using Relativistic Ions. Rad. Meas (this volume) Zamani M. et al., (1996) An Individual Dosimeter with (n,(x) (n, p) converters. Rad. Meas 26, 87-92