Fragmentation cross sections of Fe26+, Si14+ and C6+ ions of 0.3 ÷ 10 A GeV on polyethylene, CR39 and aluminum targets S. Cecchini1, T. Chiarusi1, G. Giacomelli1, M. Giorgini1, A. Kumar1,4, G. Mandrioli1, S. Manzoor1,2,3, A. R. Margiotta1, E. Medinaceli1, L. Patrizii1, V. Popa1,5, I. E. Qureshi2,3, G. Sirri1, M. Spurio1 and V. Togo1 1. Phys. Dept. of the University of Bologna and INFN, Sezione di Bologna, Viale C. Berti Pichat 6/2, I-40127 Bologna, Italy 8 2. PD, PINSTECH, P.O. Nilore, Islamabad, Pakistan 0 0 3. COMSATS Institute of Information Technology 30, H/8-1, Islamabad, Pakistan 2 4. Dept. Of Physics, Sant Longowal Institute of Eng. and Tech., Longowal 148 106, India 5. Institute of Space Sciences, Bucharest R-077125, Romania n a J 1 2 Abstract. We present new measurements of the total and partial fragmentation cross sections in the energy range 0.3÷10 A GeV of 56Fe, 28Si and 12C beams on polyethylene, CR39 and aluminum ] x targets. The exposureswere made at BNL, USA and HIMAC, Japan. The CR39nuclear trackdetectors e were used to identify the incident and survived beams and their fragments. The total fragmentation - l cross sections for all targets are almost energy independent while they depend on the target mass. The c measured partial fragmentation cross sections are also discussed. u n [ 1 Introduction 1 v The interaction and propagation of intermediate and high energy heavy ions in matter is a subject of 5 interestinthefieldsofastrophysics,radio-biologyandradiationprotection[1]. Anaccuratedescriptionof 9 1 thefragmentationofheavyionsisimportanttounderstandtheeffectsofthehighZ componentofCosmic 3 Rays (CRs) on humans in space [2] and for shielding in space and in accelerator environments. More 1. recently the interaction and transport of light energetic ions in tissue-like matter became of particular 0 interest in medicine and for hadron therapy of cancer [3]. 8 When a heavy ion impinges on a target, it undergoes fragmentation processes depending on the 0 impact parameter between the colliding nuclei. The target fragments carry little momentum. At high : v energies, the projectile fragments travel at nearly the same velocity as the beam ions and have only a i small deflection. X The availability of heavy ion beams at the CERN SPS, at BNL (USA) and at the HIMAC (Japan) r a facilities made possible to investigate the projectile fragmentation on different targets and for different projectile energies. Several authors [4-10] have successfully used Nuclear Track Detectors (NTD’s) for systematic measurements of nuclear fragmentation cross sections. The present study is focused on Fe, Si and C ion interactions in CH2, CR39 (C12H18O7)n and Al targets. We used CR39 detectors, which are sensitive for a wide range of charges down to Z = 6e in the relativistic energy region [4, 11]. NTD’s have been used to search for exotic particles like Magnetic Monopoles and Nuclearites [12, 13], to study cosmic ray composition [14] and for environmental studies [15]. 1 2 Experimental Procedure Stacks composed of several CR39 NTD’s, of size 11.5×11.5 cm2, and of different targets were exposed to 0.3, 1, 3, 5 and 10 A GeV Fe26+, 1, 3, 5 A GeV Si14+ ions at BNL, 0.41 A GeV Fe26+, 0.29 A GeV C6+ ions atHIMAC. For these exposureswe used the geometry sketchedin Fig. 1: three and four CR39 sheets, ∼ 0.7 mm thick, were placed before and after the target, respectively. The exposures were done at normal incidence, with a density of ∼ 2000 ions/cm2. After exposures the CR39 foils were etched in ◦ 6N NaOH aqueous solution at 70 C for 30 h (in two steps 15h+15h)in a thermostatic water bath with constant stirring of the solution. After etching, the beam ions and their fragments manifest in the CR39 NTD’s as etch pit cones on both sides of each detector foil. Figure 1: Sketchof thetarget-detector configuration used for theexposures to different ion beams. The base areas of the etch-pit cones (“tracks”), their eccentricity and central brightness were mea- sured with an automatic image analyzer system [16] which also provides their absolute coordinates. A trackingprocedure wasused to reconstructthe pathof beam ions throughthe front faces ofthe detector upstream(withrespecttothetarget)foils;asimilartrackingprocedurewasperformedthroughthethree measuredfrontfacesofdownstreamCR39detectors. Theaveragetrackbaseareawascomputedforeach reconstructed ion path by requiring the existence of signals in at least two out of three sheets of the detectors. In Fig. 2a,b the average base area distributions for 1 A GeV Si14+ and 1 A GeV Fe26+ beam ions and their fragments after the CH2 targets are shown. 3 Total fragmentation cross sections Thenumbersofincidentandsurvivedbeamionsweredeterminedconsideringthemeanareadistributions of the beam peaks before and after the target and evaluating the integralof the gaussianfit of the beam peaks. The total charge changing cross sections were determined with the survival fraction of ions using the following relation A ln(N /N ) T in out σ = (1) tot ρ t N Av where AT is the nuclear mass of the target (average nuclear mass in case of polymers: ACH2 = 4.7, ACR39 = 7.4); Nin and Nout are the numbers of incident ions before and after the target, re- spectively; ρ (g/cm3) is the target density; t (cm) is the thickness of the target and N is Avogadro Av number. 2 Figure 2: Distributions of the average base areas for tracks present in at least 2 out of 3 measured CR39 sheets located after the CH2 target. The data concern (a) 1 A GeV Si14+ and (b) 1 A GeV Fe26+ ions. Each peak has a gaussian shape with σ ∼ 0.2e. Notice that the peaks with Z even are generally higher than theclose by peaks with Z odd. Systematic uncertainties in σ were estimated to be smaller than 10%: contributions arise from the tot measurementsofthe densityandthicknessofthe targets,fromthe separationofthe beampeakfromthe ∆Z = Z −Z = −1 fragments (Fig. 2), from fragmentation in the CR39 foils and from the fragment beam tracking procedure. The measured total charge changing cross sections are given in the 4th column of Table 1. Fig. 3a shows the total cross sections of Fe26+ projectiles at various beam energies on the CH2 and Al targets. Our results for Si14+ and C6+ projectiles are given in Table 2 and are plotted vs energy in Fig. 3b. Thetotalcrosssectionsarealmostenergyindependent,inagreementwiththedatafromotherauthors [6, 7, 8, 9]. Various theoretical models/formulae for the total fragmentation cross sections were proposed and fitted to the experimental data with different geometrical radii and overlapping parameters [5]. In Fig. 3 our data are compared with the semi-empirical formula [17] for nuclear cross sections (solid lines) σtot =πr02 (A1P/3+A1T/3−b0)2 (2) where r0 = 1.31 fm, b0 = 1.0, AP and AT are the projectile and target mass numbers, respectively. Various authors used different values for the overlapparameter b0 within the interval 0.74÷1.3 [5-10]. Figs. 4a,b show the total fragmentation cross sections vs target mass number A for Fe26+, Si14+ T and C6+ beams of various energies. The solid lines are the predictions of Eq. 2, to which we added the electromagnetic dissociation contribution, σ =αZδ, with α=1.57 fm2 and δ =1.9 [last ref. of [4]]. EMD T The totalfragmentationcross sections increasewith increasingtargetmass number. Partof the increase is due to the effect of electromagnetic dissociation. The data from other authors [6, 7, 9, 10] are plotted for comparison and show good agreement with our data, within the experimental uncertainties. 3 Energy Target AT σtot (mb) (A GeV) 10 CH2 4.7 1147 ± 97 10 CR39 7.4 1105 ± 360 5 CH2 4.7 1041 ± 130 5 CR39 7.4 1170 ± 470 3 CH2 4.7 904 ± 140 3 CR39 7.4 1166 ± 67 1 CH2 4.7 1105 ± 60 1 CR39 7.4 1113 ± 176 1 Al 27 1870 ± 131 0.41 CH2 4.7 948 ± 54 0.41 CR39 7.4 1285 ± 245 0.41 Al 27 1950 ± 126 0.30 CH2 4.7 949 ± 61 0.30 CR39 7.4 1174 ± 192 0.30 Al 27 2008 ± 144 Table 1: Measuredtotalfragmentationcrosssections,with statisticalstandarddeviations,for Fe26+ ions of different energies (col. 1) on different targets (col. 2). Si14+ ions C6+ ions Energy Target σtot (mb) Energy Target σtot (mb) (A GeV) (A GeV) 5 CH2 757 ± 168 0.29 CH2 460 ± 53 3 Al 1533 ± 133 0.29 CR39 513 ± 52 1 CR39 1113 ± 176 0.29 Al 1155 ± 108 1 H 483 ± 76 1 CH2 694 ± 70 1 C 1117 ± 62 1 Al 1397 ± 138 Table 2: Measured total fragmentation cross sections σ for Si14+ ions of different energies (col.1) on tot different targets (col. 2) and for 0.29 A GeV C6+ ions on different targets (col. 5). Errors are statistical standard deviations. 4 Figure 3: Total fragmentation cross sections for (a) Fe ions of different energies in CH2 and Al targetsand(b)forSiionsinCH2,CR39andAltargets. Forcomparison themeasuredcrosssections from refs. [6, 7,8, 9] are also shown, together with thepredictions from Eq. 2. 4 Partial fragmentation charge changing cross sections If the thickness of the target is small compared to the mean free path of the fragments in that material, the partial fragmentation cross sections can be calculated using the simple relation 1 N σ(Z ,Z )≃ f (3) i f Kt N i where σ(Z ,Z ) is the partial fragmentation cross section of an ion Z into the fragment Z , K is the i f i f number of target nuclei per cm3, t is the thickness of the target, N is the number of survived ions after i the target and N is the number of fragments produced with charge Z . This expression may be valid f f also for a thick target, assuming that the number of fragments before the target is zero. For the Fe ions, we observed that fragments are present even before the targets. In this case the partial charge change cross sections have been computed via the relation 1 Nf Nf σ∆Z = Kt Noupt − Nipn (4) s in! where Nf and Nf are the numbers of fragments of each charge before and after the target, and Np in out in and Np are the numbers of incident and survived projectile ions. s Thedistributions,afterthe CH2 targets,ofthe fragmentsfor1AGeVSi14+ and1AGeVFe26+ ions are shown in Figs. 2a,b. The relative partial fragmentation cross sections for ∆Z =−1,−2,−3, .., −18 are given in Table 3. The quoted errors are statistical standard deviations; systematic uncertainties are estimatedto beabout10%. Aclearodd-eveneffectisvisibleinFig. 2: the crosssectionsforthe Z−even fragments are generally larger than those for the Z−odd fragments close by. 5 Figure 4: Dependence of the total fragmentation cross sections on the target mass (a) for Fe ions and(b)forSiandCions. Forcomparisonthemeasuredcrosssectionsfromrefs. [6,7,9,10]arealso shown. The solid lines are from Eq. 2 corrected by the σEMD term. 5 Conclusions The total fragmentation cross sections for 56Fe, 28Si and 12C ion beams of 0.3÷10 A GeV energies on polyethylene, CR39 and aluminum targets were measured using CR39 NTD’s [18]. The total cross sections for all the targets and energies used in the present work do not show any observable energy dependence. There is a dependence on target mass; the highest cross sections are observed for Al targets and this is mainly due to the contribution of electromagnetic dissociation. The presentdataof totalfragmentationcrosssections arein agreementwith similarexperimentaldata inthe literature [4-10]. The presence of well separated fragment peaks, see Fig. 2, allowed the determination of the partial fragmentationcrosssections. Onthe averagethepartialcrosssectionsdecreaseasthe chargechange∆Z increases. The data in Fig. 2 and the partial crosssections in Table 3 indicate a clear Z odd-eveneffect. ThemeasuredcrosssectiondataindicatethatpassiveNTD’s,specificallyCR39,canbeusedeffectively forstudiesofthetotalandpartialchargechangingcrosssections,alsoincomparisonwithactivedetectors. Acknowledgments This work was in part financed by the MIUR PRIN 2004 Program(ex 40%), Prot. 2004021217. We thank the technical staff of BNL and HIMAC for their kind cooperation during the beam expo- sures. We acknowledge the contribution of our technical staff, in particular of A. Casoni, M. Errico, R. Giacomelli,G. Grandiand C.Valieri. We thank INFN and ICTP forproviding fellowshipsand grantsto non-Italian citizens. References [1] C. X. Chen et al., Phys. Rev. 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