This work represents the first systematic study at a high energy accelerator of the depth dependence of neutron fluence in longitudinal shielding

Ciampa, D. P.

Davis, W.

Elwyn, A. J.

Fein, D.

James, E.

Johns, K.

Mierkiewicz, E.

Torres, M. M.C.

English

Analyses of both lateral shielding measurements and Monte Carlo calculations for beam stop geometry for incident hadrons at energies between 10 GeV and 10 TeV suggests that the dose equivalent can be represented by the expression H = H{sub 0}(E)e{sup -r/{lambda}}/r{sup 2} where H, is the source term, r is the radial distance to the point of interest in the shield, and {lambda} is the effective interaction length, or absorption mean free path. However, unlike the lateral shielding case, there is no similarly simple analytical expression that can be used to describe the on-axis longitudinal cascade development. In this study the results from the measurement in the forward direction of neutron fluence spectra (and the derived quantity dose equivalent) for 25 to 150 GeV pions incident on an iron beam stop as a function of thickness of iron are presented. The observed dependence of both fluence and dose equivalent on shield thickness and hadron energy was then quantified in terms of an expression in which a build up factor as well as an attenuation term was included. On the basis of this analysis the conversion factor from fluence to dose equivalent was also determined for these forward going neutrons. This work represents the first systematic study at an high energy accelerator of the depth dependence of neutron fluence in longitudinal shielding

Torres, M.M.C.

Elwyn, A.J.

Ciampa, D.P.

UNT Digital Library

Fermi National Accelerator Laboratory FERMILABCod96/333 Experimental Results of Neutron Fluence Outside an Iron Shield in the Forward Direction M.M.C. Torres et al. Fermi National Accelerator Laboratory P.O. Box 500, Batavia, Illinois 60510 September 1996 Subktted ta Health Physics Society 1997 Midyear Topical Meeting, San Jose, California, January 5-8, 1997. eQ= rntnd by Universities Rem Association Inc. under Contract No. DE-ACCQ-76CNO3000 vih the United States Department d Energy Disclaimer This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof nor any of their employees. makes any warranty. express or implied. or assumes any legal liability or responsibilic for the accuracy. completeness or usefulness of any information. apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process or service by trade name, trademark, manufacturer or otherwise, does not necessarily constitute or imply its endorsement, recommendation orfavoring by the United States Government or any agency thereof The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof Distribution Approved for public release: further dissemination unlimited. Experimental Results of Neutron Fluence Outside an Iron Shield in the Forward Direction M.M.C.Torresl, A. J. Elwyn2, D.Fein”, E.Jarnes3, K.Johns3,W.Davis”, D.P.Ciampa’, E.Mierkiewicz6 ‘Argonne National Laboratory 2Fermi National Accelerator Laboratory 3University of Arizona 4sall State University %Jniversity of Minnesota %orthern Illinois University Introduction Analyses of both lateral shielding measurements and Monte Carlo calculations for beam stop geometry for incident hadrons at energies between 10 GeV and 10 TeV [Fasso et al. 19901 suggests that the dose equivalent can be represented by the expression H = HJE)e? r2 where H, is the source term, r is the radial distance to the point of interest in the shield, and h is the effective interaction length, or absorption mean free path. However, unlike the lateral shielding case, there is no simiharly simple analytical expression that can be used to describe thz on-axis longitudinal cascade development. In this study the results from the measurement in the forward direction of neutron fluence spectra (and the derived quantity dose equivalent) for 25 to 150 GeV pions incident on an iron beam stop as a function of thickness of iron are presented. The observed dependence of both fluence and dose equivalent on shield thickness and hadron energy was then quantified in terms of an expression in which a build up factor as well as an attenuation term was included. On the basis of this analysis the conversion factor from fluence to dose equivalent was also determined for these forward going neutrons. This work represents the first systematic study at an high energy accelerator of the depth dependence of neutron tluence in longitudinal shielding [Torres 19961. Experimental Set Up and Measurements The measurements were performed during the 1991- 1992 fixed target run at Fermilab in the NWA enclosure at the NW beam line. The beam stop was constructed out of a series of iron slabs. A gap was left in the iron region to accomodate the detectors, and the movement of the individual iron slabs permitted measurements at different depths in iron. The iron was shielded on three sides by concrete blocks to help reduce background due to adjacent beam lines. The particles incident on this beam stop were negative pions with energies of 25 to 150 GeV. Neutron tluence spectra were obtained with a Bonner sphere spectrometer at 10 depths in iron from 40.6 cm to 264 cm in intervals of 20.3 cm in the forward direction. The Bonner sphere spectrometer consisted of polyethylene spherical moderators with diameters of 5.1 (2”), 7.6 (3”), 12.7 (5”), 20.3 (S’), 25.4 (lo”), and 30.5 (12”) cm at the centers of which were placed 12.7 mm long by 12.7 mm diameter cylindrical %iI(Eu) scintillators. These detectors respond to thermal neutrons via the %(n, a)3H reaction (Q=4.78 MeV). A bare (unmoderated) 6LiI crystal was also used. Most of the data was obtained with the all the spheres placed simultaneously into the neutron field as shown in Figure 1. The signal from each photomultiplier tube was transmitted through an amplifier to a multiplex-router, an ADC. and finally into a PC. Software developed by Canberra [ 19881 allowed the use of the computer as a multichannel analyzer. Neutron spectra were unfolded from measured sphere responses (normalized as indicated below) by use of the sphere response matrices of Sanna [ 19731 with the computer codes BUNKI [Lowry et al. 19841 and LOUHI [Routti et al. 19801. The shape or profile of the neutron beam at the position of the detectors was determined by moving the 12.7 (5”) cm diameter sphere to the locations of the other spheres at a number of iron thicknesses and pion energies. The beam profile was found to be independent of both incident energy and thickness in iron. The data from all spheres measured simultaneously at each energy and thickness were normalized to the neutron tluence at the beam position. Making measurements with all spheres simultaneously implicitly assumes that the spectral shape does not change with transverse position along the detector array in back of the iron shield. To check this, measurements were performed (at one pion energy and one iron thickness) by placing each detector at the same position (that of the 20.3 (8”) cm diameter sphere), and comparing the results to that from a simultaneous measurement. The spectral shapes unfolded from the data were found to be essentially identical; any difference in spectral shape with position along the array must be small. Furthermore, measurements were also performed to check the effect of neutrons backscattered from the iron blocks situated in the back of the spherical detectors. It was concluded from these studies that such backscattering contributes little to the overall properties of the radiation field at the detector position. NWA Results at Longitudinal Direction Figure 2 shows some examples of neutron spectra unfolded by use of BUNK1 for pions incident on 40.6 cm of iron. Neutron fluence per unit of log energy, or lethargy, is plotted as a function of energy. The spectra peak at energies in the hundreds of keV range, typical of neutron leakage through iron [Elwyn et al. 19861. As seen, the spectral shape is roughly independent of incident beam energy. (There are some differences in shape at small neutron energies but the neutron tluence is in any case very small in this region). These results are similiar at other depths in iron as well. In all cases, there is little evidence of the existence of neutrons with energy greater than 10 MeV. This may be because of the poor response of Bonner spheres to such neutrons. However, a preliminary measurement that utilized “C activation also did not provide any definitive information on the neutron spectrum at energies above 20 MeV. The contribution from such neutrons to total fluence would have to be greater than about 10% to have been detected in those preliminary studies. The data at all energies and depths in iron were unfolded by use of both the BUNK1 and LOUHI codes, giving values of fluence and dose equivalent is close agreement with each other. Figures 3 and 4 show tluence and dose equivalent from BUNKI plotted as a function of iron thickness at each pion energy, and as a function of pion energy at each iron thickness, respectively. As seen, both tluence and dose equivalent increase with increasing energy at each thickness, and decrease as a function of thickness at each energy. Discussions and Analysis of Longitudinal Results at NWA Following a suggestion of Thomas et al. [1988], the observed dependence of both fluence and dose equivalent in the forward direction on shield thickness and energy was described by an expression : R(E,z) = %(E,z)e-“’ (2) Here Rrepresents either fluence or dose equivalent and R. , the source term can be seperated into both depth and energy dependent factors: R, (E, z) = R, (E)B( z) . B(z), a build up factor, can be represented approximately as an exponential, and the energy dependence R,(E) is assumed to be represented as a power law. Thus we write R = R;Eme”ie-‘l” (3) The results of the fitting, first the depth dependence, and then the dependence on energy, gives values for the quantities m, a, and h. One obtains, finally, for both tluence and dose equivale R=R;E (0 83+0 OS,eO.OX: e -:/?I6 (4) where e”.028z represents the build up factor and e-z’21.6 is the attenuation. R, the source term, is (1.7WO.71)~10-~ n-cm-2-pion-’ for fluence and (2.02&1.42)~10-‘~ Sv-pion-’ for dose equivalent. The ratio of these two values gives a conversion factor between dose equivalent and tluence of (1.13~0.91)x10~10 Sv-cmW2. IMeasurements, not described here [Torres 19961, at one iron thickness and three pion energies in the lateral shielding direction were analyzed by use of Eqn. (1). The results were consistent with a value of h, the effective interaction length in iron, of 2 1.6 cm, which is in agreement with that obtained in a least-squares analysis of the results from Monte Carlo calculations [Thomas et al. 19881. The energy dependence was found to be consistent with a power law, Em, with m=0.86&0.02, in good agreement with the “standard” value of 0.80&O. 10 quoted by Thomas et al.[ 19881. The conversion factor obtained in the analysis was (1.29&0.62)~10-‘~ Sv-cm2. Conclusions From the longitudinal shielding studies reported here, the attenuation mean free path is consistent with results for lateral shielding as long as a build up factor is included in the analysis- Alternatively, combining the two depth dependent factors in Eqn. (4), one finds an attenuation term with a generalized interaction length of 55.5f7.7 cm, larger by a factor of 2.5 than the “standard” lateral shielding value. The energy dependence is essentially the same for both lateral and longitudinal shielding directions, and the exponent of E is consistent with the “standard” value of 0.8. Furthermore, when only neutrons are measured, the source strength, or conversion factor, for longitudinal shielding is the same, within the rather large errors, with that for the lateral case. Acknowledgments- Marcia Torres would like to thank Argonne National Laboratory, Fermi National Accelerator Laboratory and the Brazilian Government-CNPQ for the financial support during this research. This work was performed at Fermi National Accelerator Laboratory under contract DE-AC02-76CH03000 and Argonne National Laboratory under contract W-31-109-ENG-38 with U. S. Department of Energy. References Canberra Software, S-lOO-System 100 Version 2.0; 1988. Canberra Industries, Inc. Meridien, CT 06450 USA. Elwyn, A.J, Cossairt, J.D. A Study of Neutron Leakage Trough an Fe Shield at an Accelerator. Health Phys. 5 1:723-735; 1986. Fasso, A., Goebel, K., Hofert, M., Ranft, J., Stevenson, G.R. Shielding Against High Energy Radiation. Landolt-Bernstein Numarical Data and Functional Relationships in Science and Technology, Group I: Nuclear and Particle Physics, Springer-Verlag, Vol. 11; 1990. Lowry, K.A., Johnson, T.L. Modification to Iterative Recurssion Unfolding Algorithms and Computer Codes to Find More Appropriate Neutron Spectra. U.S. Naval Research Laboratory, Report NRL-5340, Washigton, D.C.; 1984. Routti, J.T., Sandeberg, J.V. General Purpose Unfolding LOUHI with Linear and Non-Linear Regularization. Comp. Phy. Commun. 21:119-144;1980. Sanna, R.S. Thirty One Group Response Matrices for the Multisphere Neutron Spectrometer over the Energy Range Thermal to 400 MeV. USAEC, HASL-267; 1973. Thomas, R.H., Stevenson, G.R. Radiological Safety Aspects of the Operation of Proton Accelerators. Technical Report Series No 283, IAEA, Vienna; 1988. Torres, M.M.C., Neutron Radiation Fields Outside Shielding at the Fermilab Tevatron, Ph.D Dissertation, University of Michigan, Unpublished: 1996. .,. .::.:.::. . . . . .s.i.:.i’i l:~:i-iij.ii:i:i,::i j ,::i’:‘::.::.:::::.‘::::::i:::.~::-:-:”: . . . . . . . . .,.,. . . . . .:.... . . . . . . . I I # + W I50 GeV 0 IOOGeV A 75 GeV 0 0 50 GeV 0 25 GeV =Q: + -0. 10 ” 111’ ’ ’ “‘II’ ’ ‘llilll’ ’ .lllll’ ’ “I IL’ ’ 111111’ ’ “I’ ’ l*fti ’ 111111’ ’ -5 10” d IO4 1o-jl 1o-2 10-l 1 10 IO2 Neutron Energy (MeV) Figure 2. Unfolded Neutron Spectra For Incident Pions on 40.6 cm of Iron. Shown are the neutron spectra obtained using BUNKI for pions at 5 different energies incident on 40.6 cm (16 in, 2.4 h) of iron. For each energy, the fluence per unit lethargy per pion is plotted as a function of neutron energy. 10-l I i / / '1 J -+-25GeV ; -50 GeV - +75 GeV - - 100 GeV - + 150 GeV 0 50 100 150 200 250 30( 1o-9 I---- A- 3 0 5 ‘Z \ c q $J lo-lo N b d G z! s ‘S 1o-” 4” % 6 1 Thickness in Fe (cm) I Thickness in Fe (cm) 11 11 ii -a-25GeV - -5OGeV - -I- 75 GeV - + 100 Gev - - 150 Gev 1 L 1 1 I 1 250 300 Figure 3. Fluence and Dose Equivalent vs. Thickness of Iron. Shown are fluence and dose equivalent plotted as a function of thickness in the longitudinal direc- tion for incident pions at five energies. The fluence and dose equivalent de- crease as a function of thickness at a specific energy. lo-” 40.6cm. 2.4 k 20 40 + 81.3cm, 4.8 J. - 121.9cm, 7.3 A + 142.2cm. 8.5 h 60 80 100 120 + 162.6cm. 9.7i. incident Pion Energy (GeV) ~--Cl- 182.9cm. 10.9 k --f 243.&m, 14.5 A .+ 264.2~111, 15.7 b ? 5; ‘54 & lo-lo 7 0 d - ; a -- gw w lo-l2 1 1 ’ I ’ ’ ’ ’ ’ 20 40 60 80 100 120 incident Pion Energy (GeV) ~+ 40.6cm, 2 4 + Cllcm, 3 6 2. + 81.3cm, 4.8 i. + 121 9cm. 7.3 i - 142.2cm, 8.5 2, + 162 ha-n, 9 7 I, -@--- 182.9cm, 10.9 k + 243.8cm. 14 3 X --+--- 264.2cm. 15.7 b c Figure 4. Fluence and Dose Equivalent vs. Incident Pion Energy. Shown are fluence and dose equivalent plotted as a function of incident pion energy in the longitudinal direc- tion The fluence and dose equivalent increase as a function of incident energy at a specific thickness. 

Experimental results of neutron fluence outside an iron shield in the forward direction

Embry-Riddle Aeronautical University

Publications 9-1996 Experimental Results of Neutron Fluence Outside an Iron Shield in the Forward Direction M. M.C. Torres Argonne National Laboratory A. J. Elwyn Fermi National Accelerator Laboratory D. Fein University of Arizona E. James University of Arizona K. Johns University of Arizona See next page for additional authors Follow this and additional works at: https://commons.erau.edu/publication  Part of the Astrophysics and Astronomy Commons Scholarly Commons Citation Torres, M. M., Elwyn, A. J., Fein, D., James, E., Johns, K., Davis, W., Ciampa, D. P., & Mierkiewicz, E. (1996). Experimental Results of Neutron Fluence Outside an Iron Shield in the Forward Direction. , (). Retrieved from https://commons.erau.edu/publication/221 This Report is brought to you for free and open access by Scholarly Commons. It has been accepted for inclusion in Publications by an authorized administrator of Scholarly Commons. For more information, please contact commons@erau.edu. Authors M. M.C. Torres, A. J. Elwyn, D. Fein, E. James, K. Johns, W. Davis, D. P. Ciampa, and E. Mierkiewicz This report is available at Scholarly Commons: https://commons.erau.edu/publication/221 Fermi National Accelerator LaboratoryFERMILAB-Conf-96/333CDPf-Experimental Results of Neutron FluenceOutside an Iron Shield in the Forward DirectionM.M.C. Torres et al.Fermi National Accelerator LaboratoryP.O. Box 500, Batavia, Illinois 60510NOV 1 8 1996OSTJSeptember 1996Submitted to Health Physics Society 1997 Midyear Topical Meeting, San Jose, California, January 5-8, 1997.OF THIS DOCUMENT IS UNLIMTOiXOperated by Universities Research Association he. under Contract No. DE-AC02-76CHO3000 with the United States Department of EnergyDisclaimerThis report was prepared as an account of work sponsored by an agency of the United States Government,Neither the United States Government nor any agency thereof, nor any of their employees, makes anywarranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completenessor usefulness of any information, apparatus, product or process disclosed, or represents that its use wouldnot infringe privately owned rights. Reference herein to any specific commercial product, process or serviceby trade name, trademark, manufacturer or otherwise, does not necessarily constitute or imply itsendorsement, recommendation or favoring by the United States Government or any agency thereof. Theviews and opinions of authors expressed herein do not necessarily state or reflect those of the United StatesGovernment or any agency thereof.DistributionApproved for public release: further dissemination unlimited.DISCLAIMERPortions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocumentExperimental Results of Neutron Fluence Outside an Iron Shield in theForward DirectionMMLCTonres1, A.J. Elwyn2, D.Fein3, E James3, K Johns3, W.Davis4, D.P.Ciampa5,E.Mierkiewicz1Argonne National Laboratory2Fermi National Accelerator LaboratoryUniversity of Arizona''Ball State UniversityUniversity of MinnesotaNorthern Illinois UniversityIntroductionAnalyses of both lateral shielding measurements and Monte Carlo calculations for beam stopgeometry for incident hadrons at energies between 10 GeV and 10 TeV [Fasso et al. 1990]suggests that the dose equivalent can be represented by the expressionH=H°Ktle (1)where Ho is the source term, r is the radial distance to the point of interest in the shield, and X isthe effective interaction length, or absorption mean free path. However, unlike the lateralshielding case, there is no simihariy simple analytical expression that can be used to describe theon-axis longitudinal cascade development. In this study the results from the measurement in theforward direction of neutron fluence spectra (and the derived quantity dose equivalent) for 25 to150 GeV pions incident on an iron beam stop as a function of thickness of iron are presented.The observed dependence of both fluence and dose equivalent on shield thickness and hadronenergy was then quantified in terms of an expression in which a build up factor as well as anattenuation term was included. On the basis of this analysis the conversion factor from fluence todose equivalent was also determined for these forward going neutrons. This work represents thefirst systematic study at an high energy accelerator of the depth dependence of neutron fluence inlongitudinal shielding [Torres 1996].Experimental Set Up and MeasurementsThe measurements were performed during the 1991-1992 fixed target run at Fermilab in theNWA enclosure at the NW beam line. The beam stop was constructed out of a series of ironslabs. A gap was left in the iron region to accomodate the detectors, and the movement of theindividual iron slabs permitted measurements at different depths in iron. The iron was shieldedon three sides by concrete blocks to help reduce background due to adjacent beam lines. Theparticles incident on this beam stop were negative pions with energies of 25 to 150 GeV.Neutron fluence spectra were obtained with a Bonner sphere spectrometer at 10 depths in ironfrom 40.6 cm to 264 cm in intervals of 20.3 cm in the forward direction.The Bonner sphere spectrometer consisted of polyethylene spherical moderators with diametersof 5.1 (2"), 7.6 (3"), 12.7 (5"), 20.3 (8'), 25.4 (10"), and 30.5 (12") cm at the centers of whichwere placed 12.7 mm long by 12.7 mm diameter cylindrical ^iltEu) scintillators. Thesedetectors respond to thermal neutrons via the ^Lifa, a)3H reaction (Q=4.78 MeV). A bare(unmoderated) 6LiI crystal was also used. Most of the data was obtained with the all the spheresplaced simultaneously into the neutron field as shown in Figure 1. The signal from eachphotomultiplier tube was transmitted through an amplifier to a multiplex-router, an ADC, andfinally into a PC. Software developed by Canberra [1988] allowed the use of the computer as amultichannel analyzer. Neutron spectra were unfolded from measured sphere responses(normalized as indicated below) by use of the sphere response matrices of Sanna [ 1973] with thecomputer codes BUNKI [Lowry et al. 1984] and LOUHI [Routti et al. 1980].The shape or profile of the neutron beam at the position of the detectors was determined bymoving the 12.7 (5") cm diameter sphere to the locations of the other spheres at a number ofiron thicknesses and pion energies. The beam profile was found to be independent of bothincident energy and thickness in iron. The data from all spheres measured simultaneously at eachenergy and thickness were normalized to the neutron fluence at the beam position. Makingmeasurements with all spheres simultaneously implicitly assumes that the spectral shape does notchange with transverse position along the detector array in back of the iron shield. To check this,measurements were performed (at one pion energy and one iron thickness) by placing eachdetector at the same position (that of the 20.3 (8") cm diameter sphere), and comparing theresults to that from a simultaneous measurement. The spectral shapes unfolded from the datawere found to be essentially identical; any difference in spectral shape with position along thearray must be small. Furthermore, measurements were also performed to check the effect ofneutrons backscattered from the iron blocks situated in the back of the spherical detectors. It wasconcluded from these studies that such backscattering contributes little to the overall propertiesof the radiation field at the detector position.NWA Results at Longitudinal DirectionFigure 2 shows some examples of neutron spectra unfolded by use of BUNKI for pions incidenton 40.6 cm of iron. Neutron fluence per unit of log energy, or lethargy, is plotted as a function ofenergy. The spectra peak at energies in the hundreds of keV range, typical of neutron leakagethrough iron [Elwyn et al. 1986]. As seen, the spectral shape is roughly independent of incidentbeam energy. (There are some differences in shape at small neutron energies but the neutronfluence is in any case very small in this region). These results are similiar at other depths in ironas well. In all cases, there is little evidence of the existence of neutrons with energy greater than10 MeV. This may be because of the poor response of Bonner spheres to such neutrons.However, a preliminary measurement that utilized 1JC activation also did not provide anydefinitive information on the neutron spectrum at energies above 20 MeV. The contribution fromVsuch neutrons to total fluence would have to be greater than about 10% to have been detected inthose preliminary studies.The data at all energies and depths in iron were unfolded by use of both the BUNKI and LOUHIcodes, giving values of fluence and dose equivalent is close agreement with each other. Figures 3and 4 show fluence and dose equivalent from BUNKI plotted as a function of iron thickness ateach pion energy, and as a function of pion energy at each iron thickness, respectively. As seen,both fluence and dose equivalent increase with increasing energy at each thickness, and decreaseas a function of thickness at each energy.Discussions and Analysis of Longitudinal Results at NWAFollowing a suggestion of Thomas et al. [1988], the observed dependence of both fluence anddose equivalent in the forward direction on shield thickness and energy was described by anexpression:R(E,z) = R0(E,z)e-*x (2)Here R represents either fluence or dose equivalent and Ro, the source term can be seperated intoboth depth and energy dependent factors: R0(E,z) = R0(E)B(z). B(z), a build up factor, can berepresented approximately as an exponential, and the energy dependence RO(E) is assumed to berepresented as a power law. Thus we writeR = R0Emeaze-za (3)The results of the fitting, first the depth dependence, and then the dependence on energy, givesvalues for the quantities m, a, and X. One obtains, finally, for both fluence and dose equivaleD — »' p(083±0.08) 0.028j -j/21.6 / «where e 0 0 2 8 z represents the build up factor and e"z/21-6 is the attenuation. Ro the source term, is(1.79±0.71)xl0"3 n-cm^-pion*1 for fluence and (2.02±1.42)xl0"13 Sv-pion"1 for dose equivalent.The ratio of these two values gives a conversion factor between dose equivalent and fluence of(1.13±O.91)xlO"10 Sv-cnf2.Measurements, not described here [Torres 1996], at one iron thickness and three pion energies inthe lateral shielding direction were analyzed by use of Eqn. (1). The results were consistent witha value of X, the effective interaction length in iron, of 21.6 cm, which is in agreement with thatobtained in a least-squares analysis of the results from Monte Carlo calculations [Thomas et al.1988]. The energy dependence was found to be consistent with a power law, Em , withm=0.86±0.02, in good agreement with the "standard" value of 0.80+0.10 quoted by Thomas etal.[ 1988]. The conversion factor obtained in the analysis was (1.29±0.62)xl0'10 Sv-cm2.ConclusionsFrom the longitudinal shielding studies reported here, the attenuation mean free path isconsistent with results for lateral shielding as long as a build up factor is included in the analysis.Alternatively, combining the two depth dependent factors in Eqn. (4), one finds an attenuationterm with a generalized interaction length of 55.5±7.7 cm, larger by a factor of 2.5 than the"standard" lateral shielding value. The energy dependence is essentially the same for both lateraland longitudinal shielding directions, and the exponent of E is consistent with the "standard"value of 0.8. Furthermore, when only neutrons are measured, the source strength, or conversionfactor, for longitudinal shielding is the same, within the rather large errors, with that for thelateral case.Acknowledgments- Marcia Tones would like to thank Argonne National Laboratory, Fermi National AcceleratorLaboratory and the Brazilian Govemment-CNPQ for the financial support during this research. This work wasperformed at Fermi National Accelerator Laboratory under contract DE-AC02-76CH03000 and Argonne NationalLaboratory under contract W-31-109-ENG-38 withU. S. Department of Energy.ReferencesCanberra Software, S-100-System 100 Version 2.0; 1988. Canberra Industries, Inc. Meridien, CT06450 USA.Elwyn, A.J, Cossairt, J.D. A Study of Neutron Leakage Trough an Fe Shield at an Accelerator.Health Phys. 51:723-735; 1986.Fasso, A., Goebel, K., Hofert, M., Ranft, J., Stevenson, G.R. Shielding Against High EnergyRadiation. Landolt-Bernstein Numarical Data and Functional Relationships in Scienceand Technology, Group I: Nuclear and Particle Physics, Springer-Verlag, Vol. ll;1990.Lowry, K.A., Johnson, T.L. Modification to Iterative Recurssion Unfolding Algorithms andComputer Codes to Find More Appropriate Neutron Spectra. U.S. Naval ResearchLaboratory, Report NRL-5340, Washigton, D.C.; 1984.Routti, J.T., Sandeberg, J.V. General Purpose Unfolding LOUHT78 with Linear and Non-LinearRegularization. Comp. Phy. Commun. 21:119-144;1980.Sanna, R.S. Thirty One Group Response Matrices for the Multisphere Neutron Spectrometerover the Energy Range Thermal to 400 MeV. USAEC, HASL-267; 1973.Thomas, R.H., Stevenson, G.R. Radiological Safety Aspects of the Operation of ProtonAccelerators. Technical Report Series No 283, IAEA, Vienna; 1988.Torres, M.M.C., Neutron Radiation Fields Outside Shielding at the Fermilab Tevatron, Ph.DDissertation, University of Michigan, Unpublished; 1996.3"Beamt30Wooden Plank95r 52183Figure 1. Bonner Sphere Layout. Shown is the layout of the Bonner sphere spectrometer used at NWA. The pion beam istraveling into the page. The extent of the iron plate shielding is indicated by the gray outline. The positions of the spheres,beam and the wooden plank support structure are shown. The dimensions indicated are in centimeters. The origin of thecoordinate system is at the center of the beam position.10-110'2\-oI~10 -10-5-—__ArA' -o--1 t 1 I l l t l 1 1 I M|tli 1 t 1 Dlt l l 1 1•- -A- "-A- -o--i i mil i i i i mil" * O" * - * " - < * • A*"-A- "O" 0-o-i i i Mini i t r i ti ill i i i m750 GeV700 GeV75 GeV25 G7V,I ,,,i , ,10'5 lO'4 10'3 10'2 10"1 1 10 10Neutron Energy (MeV)Figure 2. Unfolded Neutron Spectra For Incident Pions on 40.6 cm of Iron. Shownare the neutron spectra obtained using BUNKI for pions at 5 different energiesincident on 40.6 cm (16 in, 2.4 X) of iron. For each energy, the fluence per unitlethargy per pion is plotted as a function of neutron energy.25GeV50GeV75GeV100 GeV150 GeV10100 150 200Thickness in Fe (cm)250 30025 GeV50 GeV75 GeV100 GeV150 GeV10Thickness in Fe (cm)Figure 3. Fluence and Dose Equivalent vs. Thickness of Iron. Shown are fluenceand dose equivalent plotted as a function of thickness in the longitudinal direc-tion for incident pions at five energies. The fluence and dose equivalent de-crease as a function of thickness at a specific energy.20101040 60 80 100 120Incident Pion Energy (GeV)60 80 100 120Incident Pion Energy (GeV)40.6cm, 2.4 X61cm, 3.6 X81.3cm, 4.8 X121.9cm, 7.3 X142.2cm, 8 5 X162.6cm, 9.7 X182.9cm, 10.9 X243.8cm, 14.5 X264.2cm, 15.7 X40.6cm, 2.4X61cm, 3.6 X81.3cm, 4.8 X121.9cm, 7.3 X142.2cm, 8.5 X162.6cm, 9.7 X182.9cm, 10.9 X243.8cm, 14.5 X264.2cm, 15.7 XFigure 4. Fluence and Dose Equivalent vs. Incident Pion Energy. Shown are fluence anddose equivalent plotted as a function of incident pion energy in the longitudinal direc-tion The fluence and dose equivalent increase as a function of incident energy at aspecific thickness.

Experimental Results of Neutron Fluence Outside an Iron Shield in the Forward Direction

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Experimental Results of Neutron Fluence Outside an Iron Shield in the Forward Direction

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