Energy spectra for p, d, t, 3He, 4He, and 6He from the reaction 12C+197Au at 35 MeV/nucleon are presented. A common intermediate rapidity source is identified using a moving source fit to the spectra that yields cross sections which are compared to analogous data at other bombarding energies and to several different models. The excitation function of the composite to proton ratios is compared with quantum statistical, hydrodynamic, and thermal models

Crawley, G. M.

Curtin, M. W.

Gelbke, Claus-Konrad

Harwood, L. M.

Hasselquist, B.

Jacak, Barbara V.

Koenig, Z. K.

Lynch, W. G.

Panagiotou, A. D.

Scott, D. K.

Stöcker, Horst

Tsang, M. B.

Westfall, Gary D.

English

Lynch, William G.

Panagiotou, Apostolos D.

Hochschulschriftenserver - Universität Frankfurt am Main

PHYSICAL REVIEW C  VOLUME 29, NUMBER 3  MARCH 1984 Light particle spectra from 35 MeV/nucleon  12C-induced  reactions on '97Au G. D. Westfall, Z. M. Koenig, B. V. Jacak, L. H. Harwood, G.  M. Crawley, M. W. Curtin, C. K. Gelbke, B. Hasselquist, W. G. Lynch, A. D. Panagiotou?  D. K. Scott, H. Stöcker, and M. B. Tsang National Superconducting  Cyclotron Laboratory, Michigan State Uniuersity, East Lansing, Michigan 48824 (Received 21 October 1983) Energy spectra for p,  d, t, 'He,  4~e,  and 6~e  from the reaction  '2C+197~~  at 35 MeV/nucleon are presented.  A common intermediate rapidity source is identified using a moving source fit to the spectra that yields cross sections which are compared to analogous data at other bombarding ener- gies  and  to several different models.  The excitation function of  the composite to proton ratios is compared with quantum statistical, hydrodynamic, and thermal models. A  recent  systematic  studyl of  inclusive data on light particle  emission  from  asymmetric nucleus-nucleus  col- lisions  at  various  bombarding  energies1-4  has  revealed that  light  fragments emitted  at angles  2 50" can  be  at- tributed to a common, intermediate rapidity source.  Data are scarce, however, in the intermediate energy region of 20 <  Elab  < 100 MeV/nucleon.  This first set of  measure- ments  using beams  from the K500 Superconducting Cy- clotron at MSU was designed to supplement our previous systematics  and  to  fill  the gap  existing  at intermediate beam  energies.  We  measured  angular  distributions  and energy spectra for light  fragments produced  in the reac- tion of 35 MeV/nucleon 12c  + 197~~.  We extracted cross sections, temperatures, and velocities for p, d, t, 3~e,  4~e, and  6~e  fragments  using  single  moving  source  fits  as described in Ref.  1.  The present data combined with the previously observed  excitation  function for composite to proton ratios are compared to quantum ~tatistical,~  hydro- dynamical,6  and thermal m~dels.',~ The 35  MeV/nucleon  12c4+  beam  from the K500 Su- perconducting  Cyclotron  averaged  about  3  particle  nA. The target was a 9.6 mg/cm2 self-supporting natural gold foil.  Particles were detected with two telescopes each con- sisting of a 0.4 mm thick Si AE counter and a 10 cm thick Na1 E counter.  Each telescope had  a solid angle of  3.0 msr.  The  energy  spectra  were  corrected  for  reaction losses.  The overall normalization was done using a shield- ed  Faraday  cup coupled to a charge integrator and was reproducible to within 5%. The double differential  cross  sections of  p,  d,  t, 3~e, 4~e,  and 6~e  fragments are shown in Fig. 1 for the labo- ratory angles 40°, 60",  75", 90",  105", 120", and  140".  The depicted errors are statistical.  A shoulder is visible in the p,  d, and t  spectra at 40" near the beam  energy per nu- cleon.  A nearly isotropic distribution is observed for these three fragments below 20 MeV. The large angle and high energy parts of  the fragment spectra have been attributed to a hot, intermediate rapidi- ty source.'  We extract information concerning fragments from this source in the same way  as was done in Ref.  1. The solid  lines  in  Fig.  1 correspond  to  single  moving source fits in which a relativistic Boltzmann distribution in  the  rest  frame of  the  moving  source  was  assumed. This distribution is characterized by  a temperature, T, and a normalization  factor, (70, identified with the production cross section.  A  10 MeV  Coulomb shift was applied for Z =  1 fragments and  18 MeV  was  used  for Z =2 frag- 2 10  ...,l~.I!,,,Z.1 Deuterons 0  50  100  150  0  50  100  150 E (MeV) FIG. 1.  Energy spectra of  p, d, t, 3He, 4He, and 6~e  from the reaction  of  35  MeV/nucleon  12c  + Au.  The  angles  measured are 40"  (squares), 60"  (crosses),  75"  (diamonds), 90" (triangles), 105" (inverted triangles), 120" (double triangles), and 140" (circles) in  the laboratory.  The errors depicted are statistical. The solid lines correspond to moviny source fits as described in the text. 29  861  -  01984 The American Physical Society 862  G. D. WESTFALL et  al.  -  29 TABLE I.  Extracted moving source parameters for 35 MeV/ nucleon I2c  +  Au. Temperature  Cross  section  Velocity T  uo  ß Particle  (MeV)  (mb)  (C) ments.',4  The laboratory  spectra were  calculated  assum- ing the source is moving with a velocity, ß,  in the labora- tory.  The three parameters  T, 00, and ß were determined using  a  least  squares  fit8  The  extracted  temperature, cross sections, and velocities are shown in Table I.  The velocities and temperatures  smoothly follow the previous- FIG. 2.  Extracted composite/p  cross  section ratios using  a moving source parametrization as in the text compared with the cross  sections  from  the  reactions  0 + Au  (Ref. 4)  (circles), Ne + Au  (Ref. 1) (squares), Ne +  U  (Ref. 2) (diamonds), and Ne +  Pb (Ref.  3) (triangles)  plotted versus laboratory energy per nucleon above the Coulomb barrier.  The current data are shown as double triangles.  The theoretical curves are described in  the text. ly obtained systematics.  It has been shown in Ref. 2 that the large angle spectra  of  light  fragments are associated with high multiplicity.  Thus inclusive data can be biased toward near central collisions by  studying the large angle spectra. The cross section ratios  d/p,  t/p,  and 4~e/p  are com- pared in Fig. 2 with the corresponding quantities obtained from 160- and 20~e-induced  reactions on heavy targets.Ip4 The extracted d/p ratio increases with increasing beam en- ergy  up  to  about  200  MeV/nucleon,  where it  decreases again.  This effect was predicted earlier7 using a model in- corporating  chemical equilibrium  where the d/p  ratio is strongly affected by the decay of  particle unstable, excited nuclei.  On  the  other  hand,  the  t/p  ratio  seems  to  be almost  constant  (t/p =  0.2)  between  8  and  400 MeV/nucleon,  with some indication  of  structure between 30  and  200  MeV/nucleon,  and  decreases  at  lower  and higher energies.  The 4~e/p  ratio decreases monotonically with  increasing beam  energy, becoming very  small above 100 MeV/nucleon.  The solid lines in Fig. 2 correspond to hydrodynamic (HD)  calculations including a quantum sta- tistical  treatment of  the breakup6 performed at a density of  0.7  of  normal  nuclear  density, po=O. 15 fm-3.  The dashed  lines represent  a  quantum  statistical (QS) model calculation incorporating in-medium corrections and criti- cal  phenomena5 performed  at  a  density  of  po/3.  The dash-double-dot  line  represents  a  fireball  WB)  calcula- tion7 at a freezeout density of  0.8~~.  The QS model is re- lated to the bombarding energy through the observed tem- Derature, while the HD and  FB calculations  are carried out  at the impact  Parameter  with  the maximum  weight, b„.  The QS and HD calculations are for symmetric sys- tems,  while the  FB model takes  the number  of  protons and neutrons from the fireball geometry at b„.  All three models  are  shown only for the beam  energies (tempera- tures)  where  the underlying  assumptions  of  each  model are reasonable. At energies above  100 MeV/nucleon  the HD, QS, and FB calculations'~5-7  agree qualitatively with the observed composite/p  ratios.  The maxima near 200 MeV/nucleon for both d/p  and t/p  ratios are predicted by  the HD and QS models, while for the FB model the d/p ratio peaks at too  low  an energy  and  the t/p  ratio predictions are too high at the lower beam energies.  All three models predict the strong  decrease in the d/p,  t/p,  and 4~e/p  ratios at high incident energies which is due to the fact that at high temperatures  the emitted particles are predominantly nu- cleons.  The results for the HD model are not shown for the 4~e/p  ratio because they  are very  similar to the FB predictions. At  energies  below  50  MeV/nucleon,  the  QS  model predicts a second maximum in the d/p and t/p ratios near Elab  =  30  MeV/nucleon.  The resulting  minima  are pre- dicted to occur at an incident energy corresponding to a temperature of  about  20  MeV  which is the critical tem- perature  of  a  liquid-vapor  phase  transition.  The  large qualitative  disagreement  of  the theoretical  and  observed d/p  ratio  is  attributed5 to inadequacies in  the  deuteron wave function used in the calculation.  On the other hand. the observed excitation function for the t/p  ratio exhibits some structure between 30 and 200 MeV/nucleon, in qual- 22  LIGHT PARTICLE SPECTRA FROM 35 MeV/NUCLEON I2c-  . . ,  863 itative agreement  with the predictions  of  the QS model. However, the departure from completely smooth behavior could be due to the fact that several different systems are being considered here, 0 +  Au (Ref. 4), Ne +  Au (Ref. 11, Ne +  U (Ref. 2),  and Ne +  Pb (Ref. 3). In Summary, we  have measured  light  particle  spectra from the reaction  of  35  MeV/nucleon  12c  + 19'~u  and have  fit  them  with  a  moving  source  model  to extract source velocities, temperatures,  and production cross sec- tions.  These parameters  extend  previous  systematics  of 160-  and  20~e-induced  reactions  on  heavy  ta~-~ets.'-~ From a theoretical point of view there are two clearly dis- tinct regions  of  the composite to proton cross section ra- tios.  The region  above Elab=  100 MeV/nucleon  can be understood in terms of HD and QS models incorporating thermal  and  chemical  equilibrium.  Below  100 MeV/nucleon  the ratios can be understood in terms of the QS model except for the d/p  ratios.  On the other hand, the available data seem  to change smoothly between  the two energy  regimes, which may be  indicative of  a  com- mon mechanism at the high and low energies.  Structure in the t/p  ratio for incident  energies between  8 and 400 MeV/nucleon  resembles the QS calculation that incorpo- rates  the effects of  a  liquid-vapor  phase  transition.  Be- cause the present results incorporate data from several dif- ferent systems, the measurement of additional light parti- cle  spectra  for  intermediate  energies,  355Elab  2 100 MeV/nucleon,  is  necessary  in  order  to  allow  the  con- clusive confirmation or rejection  of  the existence of  this structure. We acknowledge the Operations Group at National Su- perconducting  Cyclotron Laboratory (NSCL) for their ef- forts in producing the first beams from the K500 cyclo- tron.  This material  is based  on work  supported by  the National  Science Foundation under  Grant  No.  PHY80- 17605-01. * Permanent  address:  Physics  Department,  University  of Athens, Athens, Greece. 'G.  D. Westfall, B.  V.  Jacak, N. Anantaraman, M. W. Curtin, G. M. Crawley, C. K. Gelbke, B.  Hasselquist, W. G. Lynch, D. K. Scott, B.  M. Tsang, M. J.  Murphy, T. J. M. Symons, R. Legrain, and T. J. Majors, Phys. Lett. m,  118 (1982). 2J. Gosset, H. H. Gutbrod, W. G. Meyer, A. M. Poskanzer, A. Sandoval, R. Stock, and G. D. Westfall, Phys. Rev. C fi,  629 (1977); A. Sandoval, H. H. Gutbrod, W. G. Meyer, R. Stock, Ch. Lukner, A. M. Poskanzer, J. Gosset, J.-C. Jourdain, C. H. King, Nguyen Van Sen, G. D. Westfall, and K. L. Wolf, ibid. 21, 1321 (1980).  - 3S.  Nagamiya,  M.-C.  Lemaire, E.  Moeller,  S.  Schnetzer, G. Shapiro, H. Steiner, and I. Tanihata, Phys. Rev. C 3,  971 (1981). 4T. C. Awes, G. Poggi, S. Saini, C. K. Gelbke, R. Legrain, and G. D. Westfall, Phys. Lett. M,  417 (1981). 5H. Schulz, L.  Münchow, G.  Röpke,  and  M.  Schmidt, Phys. Lett. W,  12 (1982). 6H. Stöcker, Lawrence Berkeley Laboratory Report LBL-12302, 1981 (unpublished);  H. Stöcker, G. Buchwald, G. Graebner, P. Subramanian, J. A. Maruhn, W. Greiner, B.  V. Jacak, and G. D. Westfall, Nucl. Phys. M,  63c (1983). 7J. Gosset, J. I. Kapusta, and G. D. Westfall, Phys. Rev. C '8, 844 (1978);  A. Mekjian, ibid. H,  1051 (1978). 8The contributions from evaporation from target and projectile residual nuclei were a concern and we therefore included only a subset of  the data in the fits.  The applied cuts suppressed the 40" spectra and the low energy, isotropic components.  For example, the proton spectra exhibit a nearly isotropic com- ponent between  10 and 20 MeV  and show a bump in the 40" spectrum near  the beam  energy  per  nucleon.  We  attribute these features to target and projectile evaporation, respective- ly. 

Light particle spectra from 35 MeV/nucleon 12C-induced reactions on 197Au

Light particle spectra from 35 MeV/nucleon 12C-induced reactions on 197Au

Abstract

Similar works

Full text

Available Versions

Hochschulschriftenserver - Universität Frankfurt am Main