Butler, H M

Fulmer, C B

Wallace, K M

CERN Document Server

Particle Accelerators, 1972, Vol. 4, pp. 63-68 © GORDON AND BREACH, SCIENCE PUBLISHERS LTD. Printed in Glasgow, ScotlandRADIATION LEAKAGE THROUGH THIN CYCLOTRON SIDELDWALLStc. B. FULMER, H. M. BUTLER AND K. M. WALLACEOak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USAThick targets of carbon, aluminum, copper, and tantalum were each bombarded with beams of protons, deuterons,ex-particles, and carbon ions. The target station was 5.5 m from an 80 cm thick ordinary concrete shield wall.Measurements were made of radiation levels of fast neutrons, thermal neutrons, and y-radiation on the shielded sideof the wall for each target-beam combination. Total radiation levels due to leakage through the wall were thusdetermined as a function of beam intensity for targets of a wide range of atomic mass and for a variety of incidentbeam particles with energies in the range of 8-66 MeVjamu. Deuteron beams result in the highest radiation levels(for a given beam energy). Measurements on both sides of the wall indicate that neutrons are the principal radiationleakage through the wall. Neutron induced reactions in the wall, however, result in significant levels of y-radiationon the shielded side of the wall which are "150 per cent of the total rem dose.1. INTRODUCTIONThe Oak Ridge Isochronous Cyclotron! (ORIC)is a variable energy accelerator for a variety ofcharged particles. The beams are used in a widerange of experiments. The radiation shieldingrequirements for such an installation are much morevariable than for a nuclear reactor or less versatileaccelerator. If the maximum requirements areprovided in the fixed shielding it may be needlesslyexpensive both in installation cost and in valuablespace occupied by the shield. It may be morepractical in some situations to use more modestpermanent shields that meet the requirements mostof the time and use administrative control and localshielding around the target when target and beamconditions require it.In the work reported here measurements weremade of radiation leakage through a thin (80 em)concrete block wall that was erected in one of thetarget rooms at ORIC. The wall was erected todivide the room into two target rooms (north andsouth) for different types of experiments. Whenbeam is on target in one of the rooms it is oftendesirable for experimenters to have access to theother room. The north roonl will be used exclu-sively for experiments with low intensity beams ofheavy ions and radiation leakage through the con-crete block wall will be low. In the south roomtargets are bombarded with high intensity beamst Research sponsored by the U.S. Atomic Energy Com-mission under contract with Union Carbide Corporation.P.A. A(up to 10 pA) of light particles. Thus the importantradiation leakage is from a target in the south roomthrough the 80 em thick concrete block wall intothe north room.The shield wall thickness requirements ataccelerator installations are usually determined byneutron production rates, especially the higherenergy neutrons. Estimates based on the data ofJohnson and Ohnesorge2 and of Wadman3 andneutron attenuation data4 indicated that a concretewall 100-120 em thick would provide adequateshielding for most situations. Because of spacelimitations it was decided to erect a wall 80 emthick, make radiation leakage measurements, andto use these data to guide administrative control ofaccess to the north room and in the design of a localshield at the target in the south room.Measurements were made of the radiation doserates of fast neutrons, thermal neutrons, and beta-gamma in the north room for beams of protons,deuterons, alpha particles and carbon ions on thicktargets of carbon, aluminum, copper, and tantalumat target station A.2. DESCRIPTION OF THE MEASUREMENTSFigure I shows the plan of target room CIII .atORIC, with the stacked block wall that was recentlyerected. It is apparent that a significantly thickerwall would render inaccessible one of the beamlines that are directed to the room. In principle,material of more effective shielding properties64 C. B. FULMER~ H. M. BUTLER AND K. M. WALLACEFIG. 1. Target room CIII at ORIC. The block wall divides the rOOln into two target rOOlns. For the measurementsreported in this paper targets were bombarded at A and a fast neutron monitor was located at B.could have been used in the wall. Ordinary con-crete blocks, however, were available from otherbuilding modifications and could be used to erectthe wall at minimum cost.For the measurements of radiation leakagethrough the wall the cyclotron beam was focusedto an 1'-1 1 em diameter spot at the target stationlabeled A in Fig. 1. For all of the measurementsthe targets were thick enough to stop the beam.A portable fast neutron survey meter was posi-tioned at location B on the target side of the wall5.5 m from the target. This meter was observedvia television and used by the cyclotron operatorsto determine' the beam intensity needed on thetarget, which was electrically insulated from thebeam line and served as a Faraday cup to monitorthe incident beam intensity.The radiation levels in the north room are due toradiation leakage from the target at A in the southroom through the 80-cm wall and from the cyclotronroom through the 213-cm concrete wall at the topof Fig.. l The latter do not significantly affect themeasurements, however. About 25 per cent of theaccelerated beam is extracted and delivered to thetarget at A. Fast neutrons resulting from beamlosses in the cyclotron room and leaking into thenorth room of Clll are attenuated by a muchlarger factor than those from a target at A.For most of the measurements the beam intensitywas adjusted to yield a dose rate of 2 rem/hr on thefast neutron monitor at position B. Then with thebeam intensity maintained, suitable health physicssurvey meters were used to obtain measurements ofdose rates of fast neutrons, thermal neutrons andy~radiation5 in the north room on the shielded sideof the wall. These measurements were madedirectly across the wall from B and at several otherlocations in the room. The variation of themeasured dose rates in the room was usually lessthan 10 per cent.The fast neutron measurements were used todetermine half-value thicknesses6 of ordinaryconcrete for the overall neutron spectra resultingfrom the various target-beam combinations. ForRADIATION LEAKAGE THROUGH SHIELD WALLS 65the first two series of data runs measurements withthe portable fast neutron meters were supplementedby data obtained with integrating monitors whichemployed tissue equivalent gas in the ionizationchambers. The integration feature removed errorsdue to beam intensity fluctuations during themeasurements. In all cases agreement was within5 per cent and usually within 2 per cent. The fastneutron survey meters used in these measurementshave a negligible response (less than 1 per cent) tobackground gamma radiation. 7The dose rate measured by the fast neutronmonitor at location B is due to a combination ofradiation directly from the target and from reflec-tions from the walls of the room. An estimate ofthe contribution due to reflection was obtained bymoving the detector to a position equidistantbetween A and B (see Fig. 1). If reflections wereunimportant the measured dose rate (per unit beamintensity on the target at A) would be a factor of 4larger than at B. The measured deviation fromthis indicated that about one third of the radiationlevel of fast neutrons at B is due to reflections. Thisdemonstrates that a modest shield around the targetwill significantly reduce the shield wall thicknessrequirements.3. RESULTS AND DISCUSSIONTable I lists the beam currents needed on thicktargets at station A (Fig. 1) to yield a fast neutrondose rate of 2.0 rem/hr at B. The uncertainty ofthese measurements is ±10 per cent except forcarbon ions where the low beam intensity availableat the time of the measurements resulted in un-certainties of ±20 per cent. For convenience ofcomparing results of the measurements for variousincident particles beam intensities are expressed inparticle microamperes, where 1 particle J1A is6.25 x 101'2 particles/second.The proton data cover the largest energy rangeand show a strong trend of increased neutronemission with bombarding energy. There is also atrend for neutron yield to increase with targetatomic mass with the largest increase in the lowmass region. The increase of neutron yield withbombarding energy is similar for all targets.The measurements reported here were obtainedby using health physics survey instruments. Theincrease of neutron yield with target mass thusmeasured for 66 MeV protons is very similar to theresults (for 63 MeV protons) of Johnson andOhnesorge2who used threshold detector techniques.The neutron measurements from targets bombard-ed with deuterons and ex-particles also show an in-crease ofyieldwith incident particle energy. Therateof increase for both particle types, however, is largerthan that of the proton data. The a-particle datashow the principal increase of neutron yield withtarget mass it?- the low mass region. The 25-MeVdeuteron data show a somewhat less systematicbehavior of neutron yield with target mass whilethe 40-MeV data show an almost constant neutronyield over the wide range of mass.For a number of target-beam combinations y-radiation levels were also measured at B (Fig. 1) atthe location of the fast neutron measurements onthe target side of the shield wall. In all cases they-ray rad dose rate was 10 per cent as large as thefast neutron rem dose rate which is a rad dose ratetimes a quality factor. Thus the rad dose rates atB for fast neutrons and for y-radiation are aboutequal.Data were obtained with carbon ions at onebombarding energy. The neutron yields aresignificantly lower for all targets than when incidentTABLE IBeam current needed on thick targets at station A to result in 2.00 remjhr due to fast neutrons at location B 5.5 m fromtarget. Beam currents are in particle /lA.Protons Deuterons Alpha Particles Carbon IonsTarget 20 MeV 31 MeV 40 MeV 66 MeV 25 MeV 40MeV 50MeV 80MeV 100 MeVCarbon 2.0 1.0 0.20 0.42 0.20 11 3.0 160Aluminum 1.5 0.80 0.50 0.12 0.30 0.20 4.2 1.3 58Copper 0.60 0.40 0.25 0.090 0.50 0.18 4.0 1.0 48Tantalum 0.60 0.30 0.23 0.040 0.74 0.21 4.2 0.95 80P.A. A2c. B. FULMER, H. M. BUTLER AND K. M. WALLACET;.;..7'(71-'-------=1./.1-/I-- /~//r/F CARBON TARG~11T----- 0 PROTONS ------1 /' ______ 6 DEUTERONS ______/ .. a - PARTICLES/ • CARBON IONS ------///7~ ~.//'7--r-I .7V/lV~! ~/'1 ALUMINUM TARGET-- o PROTONS/ 6 DEUTERONS-/7 A a- PARTICLES• CARBON IONS6621025~ 2.c""-EQ)101E-lW> 5w-lZ0i= 2"<i0<ia:: 1005210 20 30 40 50BEAM ENERGY ( MeV/ amu)60 702102.c 5""-EQ)E-l 2w>w1b-lz0~« 50«a::210°10 20 30 40 50BEAM ENERGY (MeV/ amu )60 70FIG. 2. Radiation levels in the north room with 1particle /lA of beam on a carbon target at A in thesouth room. The radiation levels are the sum ofmeasured dose rates of fast neutrons, thermalneutrons, andp-y.FIG. 3. Radiation levels in the north room with 1particle /lA of beam on an aluminum target at A inthe south room. The radia tion levels are the sumof measured dose rates of fast neutrons, thermalneutrons, and p- y.5,...----,----~---.----..,.----...----~-~2i___-+----+----+---+----+-------j------170.6020 30 40 50BEAM ENERGY ( MeV/emu)10T/r/1/ ...I..//.//'1/~/...1.. TANTALUM TARGETV7 V -0 PROTONS6 DEUTERONSA a - PARTICLES• CARBON IONS////VI I52~102EQ)E 5--lW>W--l 2z0~ 1d(5«0::5210°7060TARGET ---+-__~20 30 40 50BEAM ENERGY (MeV/amu)102 f---+--_--+-__+--_--+-__-I---_--I----".,TA-----t~ 102 f--+-_~-__+__-I----_____:OO.-4_/_ _+/'_l---j~E J~ 5 1---+----+-----+=I,..:r-...,.,./!:....--I---~-____ji___-____1-l T I /~/~ , 01-1 T/ 11. COPPER~ 2~ /Z ffi / 0 PROTONSQ 1 6 DEUTERONS~ 10 ~= .. a- PARTICLES~ --7- ---+----1---. CARBON IONSa:: 5 I--.It-l----+-----+----+-----+----I------lFIG. 4. Radiation levels in the north room with 1particle /lA of beam on a copper target at A in thesouth room. The radiation levels are the sum ofmeasured dose rates of fast neutrons, thermalneutrons, and p-y.FIG. 5. Radiation levels in the north room with 1particle !LA of beam on a tantalum target at A inthe south room. The radiation levels are the sumof measured dose rates of fast neutrons, thermalneutrons, and P- y.RADIATION LEAKAGE THROUGH SHIELD WALLS 67beams of protons, deuterons, and a-particles wereused. Similar to the a-particle data the carbon iondata show the largest variation of neutron yieldwith target mass between carbon and aluminum.The fast neutron dose rate measurements ob-tained at B (Fig. 1) and measurements in the northroom directly across the 80-cm concrete wall wereused to obtain neutron half-value thicknesses forthe various target-beam combinations. The results,which are reported in Ref. 6, vary less than 10 percent from the average value of 9.8 cm although theneutron yields vary appreciably over the range ofbeam-target combinations used.Figures 2-5 show radiation levels in the northroom for the various beam-target combinationsused at station A in the south room. These dataare the sums of measured dose rates of fastneutrons, thermal neutrons, and y-radiation andare normalized to 1 particle flA of beam on thetarget. For each target the data for protons andcarbon ions fit a smooth curve on the graph withthe radiation levels for a-particle and deuteronbeams above the smooth curve. Preliminarymeasurements with a beam of 160 MeV neon ionson an aluminum target also fit the smooth curve inFig. 3. For all targets the radiation levels fromdeuteron bombardments are larger than frombombardments with the other beam particles used.The smooth curves for aluminum, copper, andtantalum overlap at low beam energies and divergeat higher beam energy with the highest radiationlevels observed for the targets of highest atomicmass. Over the entire energy region radiationlevels from carbon targets bombarded with protonsor carbon ions are lower than those of aluminum,copper and tantalum.The radiation levels from deuteron bombard-ments show the least systematic behavior. Themost pronounced trend is the increase of radiationlevel with beam energy. The measured radiationlevels for 40 MeV deuterons are almost the samefor all target materials studied.Our data did not include measurements ofneutronangular distributions. For incident beams ofdeuterons direct reactions such as proton strippingwould result in forward peaked neutron angulardistributions. The neutron dose rate measurementsat B (Fig. 1) were made at a direction of '" 90 degwith respect to the incident particle beam. Onemay be inclined to attribute the less systematicbehavior of radiation levels from deuteron bombard-ments to the forward peaked neutron angulardistributions. We believe, however, that this is nota satisfactory explanation. One reason is that ourmeasurements show that a large fraction of themeasured neutron intensity at B is due to reflec-tions. Another reason is that the neutron yielddata of Johnson and Ohnesorge2 for proton bom-bardments show a large variation of neutron energyand angular distributions with target mass. Theradiation levels we observe show a more systematicbehavior for proton bombardments than fordeuteron bombardments.The data shown in Figs. 2-5 were obtained bysumming the measured dose rates of fast neutronsand other radiation in the north room. For all ofthe measurements the fast neutron component was35-60 per cent of the total radiation level. For alltargets bombarded with carbon ions the fastneutron component was 45 per cent, or less whilefor 66 MeV protons it was more than 50 per cent.For the other target-beam combinations the fastneutron component of the total radiation levelvaried between 38 and 55 per cent. In all of thedata the thermal neutron component was less than20 per cent of the total radiation level.5. CONCLUSIONSThe reaso.nably constant composition of theradiation in the north room is consistent with amuch larger attenuation of y-radiation than of fastneutrons by the 80 cm concrete wall. On the targetside of the wall the rad dose rates were about equalfor the two types of radiation. On the shielded sidethe rad dose rate of y-radiation is a factor of '" 10larger than the rad dose rate of fast neutrons (therem dose rates are about equal). The attenuationby the wall of the dose rates of fast neutrons rangesfrom 200-500 for the various beam-target combina-tions, while the attenuation of 80 cm of concretefor 3 MeV l'-rays is '" 6000 and larger for lowerenergy l'-rays. Thus much of the l'-ray radiationlevel in the north room results from neutroninduced reactions in the wall rather than l'-rayleakage through the wall. The principal radiationleakage through the wall is fast neutrons. One can68 C. B. FULMER, H. M. BUTLER AND K. M. WALLACEthus use the data in Figs. 2-5 and the fast neutronhalf-thickness data of Ref. 6 to extrapolate radiationleakage from cyclotron targets through. concretewalls of a range of thicknesses.6. ACKNOWLEDGEMENTSThe authors gratefully acknowledge the co-operation and assistance of the ORIC operationsstaff in obtaining the measurements reported in thispaper.REFERENCES1. R. S. Livingston and F. T. Howard, Nucl. Ins!r. and Meth.,6,1(1960).2. D. R. Johnson and W. F. Ohnesorge, Atompraxis, 14,Heft 4j5 (1968).3. W. W. Wadman, Health Physics, 11, 659 (1965).4. R. Wallace, Nucl. Instr. and Meth., 18, 19,405, (1962).5. The y-radiation levels were measured with an ion chambersurvey meter which is sensitive to both p and yradiation.6. H. M. Butler,K. M. Wallace and C. B. Fulmer, submittedfor publication in Health Physics.7. D. M. Davis and E. D. Gupton, Health Physics InstrumentManual, ORNL-332 (1970 ed.).Received 11 July 1972;and in final form 11 August 1972

Radiation leakage through thin cyclotron shield walls

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