Neutron-deficient Products Research Articles

Projectile fragmentation of112SnatElab=1A GeV

We have measured production cross sections of projectilelike fragments in the reaction of neutron-deficient 112Sn with a 9Be target at an energy of Elab=1A GeV. Results have been obtained for neutron-deficient products along the N=Z line down to the subnanobarn range. We find very good agreement with the recently revised empirical parametrization EPAX. In addition, a few one- and two-proton pickup cross sections have been measured. The former ones are well reproduced by intranuclear-cascade calculations.

Proton drip line nuclei aroundZ=30to 40

Neutron deficient collision products with $Z=30$ to 40 at high excitations are investigated. The proton drip line is significantly altered due to temperature and rotation. The effects of thermal and rotational excitations on proton separation energy are investigated. The level density parameter and the nuclear level density of the isotopes of these f p shell nuclei are also calculated.

Yield and recoil properties of iodine isotopes from the interaction of 240 MeV 12C with 238U.

The independent yields, recoil properties, and forward-to-backward ratios (F/B) of iodine isotopes from the interaction of 240 MeV $^{12}\mathrm{C}$ with $^{238}\mathrm{U}$ have been measured radiochemically by the thick-target/thick-catcher method. The isotopic yield distribution curve has been constructed and is found to consist of two overlapping Gaussians, peaking at A=126.5 and 133.8 with width parameters of 2.29 and 2.04 mass units, respectively. All the measured iodine isotopes had ranges of 7.7\ifmmode\pm\else\textpm\fi{}0.4 mg/${\mathrm{cm}}^{2}$. The neutron-deficient products have F/B of 1.76\ifmmode\pm\else\textpm\fi{}0.14, but the neutron-excessive products have F/B of only 1.09\ifmmode\pm\else\textpm\fi{}0.06. The yield curve was analyzed with the liquid drop model and the recoil curve was analyzed by the standard two-step vector model; the results show that the neutron-deficient products are formed from nonequilibrium processes and the neutron-excessive products are formed from the normal low-energy fission process.

Angular distributions and differential ranges of Ba products from the interactions ofU238with 0.8-400-GeV protons

Angular distributions and differential ranges at ${90}^{0}$ to the beam of $^{128}\mathrm{Ba}$, $^{131}\mathrm{Ba}$, $^{135}\mathrm{Ba}^{m}$, and $^{140}\mathrm{Ba}$ emitted in the interaction of $^{238}\mathrm{U}$ with 0.8-400 GeV protons have been measured. The angular distributions of $^{128}\mathrm{Ba}$ and $^{131}\mathrm{Ba}$ change from forward-peaked to sideward-peaked between 3 and 11.5 GeV, while those of $^{135}\mathrm{Ba}^{m}$ and $^{140}\mathrm{Ba}$ are sideward-peaked and independent of bombarding energy in the above regime. Momentum distributions of all products are Gaussian at 0.8 GeV with a mean value increasing from 120 ${(\mathrm{M}\mathrm{e}\mathrm{V}\phantom{\rule{0ex}{0ex}}\mathit{A})}^{1/2}$ for $^{128}\mathrm{Ba}$ to 139 ${(\mathrm{M}\mathrm{e}\mathrm{V}\phantom{\rule{0ex}{0ex}}\mathit{A})}^{1/2}$ for $^{140}\mathrm{Ba}$. The momenta of the neutron-deficient products decrease by nearly a factor of 2 with increasing proton energy and the distributions broaden and become asymmetric. On the other hand, the momentum distributions of $^{135}\mathrm{Ba}^{m}$ and $^{140}\mathrm{Ba}$ are independent of proton energy. The results are analyzed with the two-step model and compared with the kinematics of the coherent interaction model. The implications of the results and comparisons for the reaction mechanism are considered.NUCLEAR REACTIONS $^{238}\mathrm{U}(p,X)^{128}\mathrm{Ba}$, $^{131}\mathrm{Ba}$, $^{135}\mathrm{Ba}^{m}$, $^{140}\mathrm{Ba}$. ${T}_{p}=0.8\ensuremath{-}400$ GeV. Measured angular distributions and differential ranges at 90\ifmmode^\circ\else\textdegree\fi{} to the beam, deduced two-step model parameters.

Three-dimensional charge dispersion curves from interactions of 11-29 GeV protons with uranium

Experimental nuclear charge dispersion curves from interactions of 11--29 Gev protons with /sup 238/U have been used in the construction of three-dimensional charge dispersion curves. They show the yield variation with mass number A. Neutron-deficient products are distributed over the entire mass range with a peak at A near 87, while the yield of neutron-excessive products is distributed only in the relatively narrow mass region between A=70 and A=150 and has a maximum around A=115. An isobaric yield curve has been obtained by summing up each of the charge dispersion curves and shows a peak, rather than the flat top, in the mass region A=80 to 140 reported previously. The mass yield curves of neutron-excessive and neutron-deficient products are obtained by a decomposition of the charge dispersion curve with two Gaussians, and the mechanism of formation is suggested.

Formation of isobaric nuclides withA=131in the interaction ofU238with 0.8-11.5 GeV protons

Cross sections and thick-target recoil properties of $^{131}\mathrm{Ce}$, $^{131}\mathrm{La}$, $^{131}\mathrm{Ba}$, and several neighboring Ba nuclides formed in the interaction of $^{238}\mathrm{U}$ with 0.8-11.5 GeV protons have been determined. The results reveal a dichotomy between $^{131}\mathrm{Ba}$ and the more neutron-rich products on the one hand, and the more neutron-deficient ones on the other. The latter have rising excitation functions, forward-to-backward ratios (F/B) that peak at 3 GeV, and ranges that decrease abruptly just below this energy. The former have decreasing excitation functions as well as F/B and ranges that decrease slowly and featurelessly with increasing proton energy. The energy dependence of F/B as well as the changes in the angular distributions of the very neutron deficient products observed between 3 and 11.5 GeV are explained in terms of a change in the nature of near-central proton-nucleus interactions. At the lower energies the interaction consists of a series of nucleon-nucleon collisions while at high energies the proton interacts collectively with all the nucleons lying in its path. The dropoff in the mean ranges of these same nuclides and the accompanying broadening of the spectra indicate a transition between fission and deep spallation. The possible connection between these two effects is discussed. Several experiments to test the proposed model are suggested.NUCLEAR REACTIONS Measured $\ensuremath{\sigma}$, range, and F/B of $^{131}\mathrm{Ce}$, $^{131}\mathrm{La}$, $^{131}\mathrm{Ba}$, $^{128}\mathrm{Ba}$, $^{133}\mathrm{Ba}^{m}$, $^{135}\mathrm{Ba}^{m}$, and $^{140}\mathrm{Ba}$ formed in interaction of $^{238}\mathrm{U}$ with 0.8-11.5 GeV protons. Proposed explanation for change in properties of very neutron deficient products.

Pion-induced spallation of copper across the (3,3) resonance

Cross sections for about two dozen radioactive spallation products have been measured for 50-, 100-, 190-, and 350-MeV ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$ and for 200- and 350-MeV proton interactions with copper. Sums of measured pion-induced cross sections at 50 and 350 MeV are about the same as for protons of 200 and 590 MeV, respectively. However, at 100 and 190 MeV, nearer to the (3,3) pion-nucleon resonance, these pion-induced cross section sums are 50 to 75% greater than those for protons of similar total energy. Ratios of the ${\ensuremath{\pi}}^{\ensuremath{-}}$ to ${\ensuremath{\pi}}^{+}$ cross sections are greater than unity for neutron-rich products and less than unity for neutron-deficient products at all four energies, indicating that pion absorption is a substantial reaction mode, even at 190 and 350 MeV. At 100 MeV, and especially at 50 MeV, absorption appears to dominate over inelastic scattering and the $\frac{{\ensuremath{\sigma}}_{\ensuremath{\pi}}^{\ensuremath{-}}}{{\ensuremath{\sigma}}_{\ensuremath{\pi}}^{+}}$ ratios are noticeably changed from those at the higher energies. The intranuclear cascade plus evaporation codes reproduce the proton cross sections fairly well. The pion version of the intranuclear cascade code reproduces the pion results quite well at 350 MeV, but it underestimates them significantly at 50 and 100 MeV.NUCLEAR REACTIONS Cu (${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$, spallation) and Cu ($p$, spallation), ${E}_{{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}}=50, 100, 190, 350$ MeV, ${E}_{p}=200, 350$ MeV. Production cross sections for radioactive residual nuclei. Comparison with intranuclear cascade plus evaporation calculations.

Spallation of Cu by 3.9-GeVN14ions and 3.9-GeV protons

Relative yields of 35 products extending from $^{3}\mathrm{H}$ to $^{76}\mathrm{Br}$ have been measurd for the interaction of 3.9-GeV (278-MeV/amu) $^{14}\mathrm{N}$ ions with copper. For purposes of comparison, cross sections of 54 nuclides produced by the irradiation of Cu with 3.9-GeV protons are also reported. Although the over-all patterns of yields for $^{14}\mathrm{N}$ ions and protons are qualitatively similar, there are significant differences. In the mass region $37\ensuremath{\le}A\ensuremath{\le}64$, the mass yield curve for $^{14}\mathrm{N}$ ions decreases more rapidly with decreasing $A$ than does the proton curve. No difference could be detected in the shapes of the charge dispersion curves. However, the data indicate a small shift ($l0.1Z$ unit) favoring neutron-deficient products near the target in the case of $^{14}\mathrm{N}$ ions. The mass yield curves appear to have similar shapes from $A\ensuremath{\approx}40$ down to $A\ensuremath{\approx}24$, but formation of the still lighter products, $^{7}\mathrm{Be}$ and particularly $^{3}\mathrm{H}$, is favored in the $^{14}\mathrm{N}$ irradiation. An observed enhancement of products such as $^{62}\mathrm{Zn}$, $^{66}\mathrm{Ga}$, and $^{69}\mathrm{Ge}$ is interpreted as arising largely from secondary reactions rather than from primary processes which add charge or mass from the $^{14}\mathrm{N}$ to the target nuclei. Some discussion of results from this and other experiments with high-energy protons and heavy ions with complex nuclei is presented in terms of the concepts of limiting fragmentation and factorization.NUCLEAR REACTIONS Cu($^{14}\mathrm{N}$, spallation), $E=3.9$ GeV; measured relative $\ensuremath{\sigma}(A,Z)$, 35 products $^{3}\mathrm{H}$-$^{76}\mathrm{Br}$. $\mathrm{Cu}(p, \mathrm{spallation})$, $E=3.9$ GeV; measured $\ensuremath{\sigma}(A,Z)$, 54 products $^{3}\mathrm{H}$-$^{69}\mathrm{Ge}$. Natural targets, Ge(Li), $\ensuremath{\beta}$ counting, radiochemistry.

Spallation of copper by high-energy alpha particles

28 radionuclides produced by spallation of natural copper by 720 MeV $\ensuremath{\alpha}$ particles were investigated by direct $\ensuremath{\gamma}$ counting of irradiated targets. Although the isobaric yield distributions and mass yield curve are similar in many respects to those found in proton spallation, a suppression of yields of neutron deficient products is observed. Several interpretations of this latter phenomenon are critically discussed.NUCLEAR REACTIONS $\mathrm{Cu}(\ensuremath{\alpha}, \mathrm{spallation})$, $E=410, 720$ MeV; measured formation cross sections 28 radionuclides, direct $\ensuremath{\gamma}$ counting, natural target.

Studies of the Recoil Properties of Products of the Interaction ofU238with 11.5-GeV Protons

The thick-target thick-catcher recoil properties of 23 rare-earth nuclides, produced in the interaction of 11.5-GeV protons with uranium, were measured in the mass range 135 to 165. The ratio of the range of a product from multi-GeV protons to that from 450-MeV protons, called the "relative range", was calculated for all nuclides studied in the mass region 82-165. For neutron-excess products, the relative range was around 0.95 and dropped sharply over two $Z$ units to 0.48 for very neutron-deficient products. This relative range behavior was used to analyze isobaric charge dispersion curves into three individual charge dispersion curves corresponding to two fission mechanisms (a low-excitation energy process, fission I, and a high-excitation energy process, fission II) and a nonfission (spallation) mechanism. The fractional isobaric cross section for the spallation process decreases with decreasing mass number from 0.98 at mass 170 to about 0.14 at mass 111 and then increases with decreasing mass number to 0.49 at mass 74. The products are neutron deficient, of low range, and with forward-to-backward recoil ratios (F/B) close to unity. The range values increase almost linearly with $\ensuremath{\Delta}A$ (mass difference from the target nucleus) from 2.2 ${\mathrm{m}\mathrm{g}/\mathrm{c}\mathrm{m}}^{2}$ U at $\ensuremath{\Delta}A$ of 70 to 5.6 ${\mathrm{m}\mathrm{g}/\mathrm{c}\mathrm{m}}^{2}$ U at $\ensuremath{\Delta}A$ of 160. The value of ${v}_{\ensuremath{\parallel}}$, the velocity imparted to the target nucleus in the beam direction as a result of the intranuclear cascade, decreases from 0.04 ${(\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{a}\mathrm{m}\mathrm{u})}^{1/2}$ at $\ensuremath{\Delta}A$ of 70 to 0.01 ${(\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{a}\mathrm{m}\mathrm{u})}^{1/2}$ at $\ensuremath{\Delta}A$ of 100 and then increases to 0.02 ${(\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{a}\mathrm{m}\mathrm{u})}^{1/2}$ at $\ensuremath{\Delta}A$ of 160. The fractional isobaric cross section for the fission II process is roughly constant with a maximum value of 0.17 for mass number 131. The relative range values for these products are about 0.9 and the F/B values are also close to unity.

Recoil Properties of Rare-Earth Nuclides Produced by the Interaction of 28-GeV Protons with Uranium and Gold

The thick-target-thick-catcher technique was used to determine the recoil properties of some rare-earth nuclides produced by the interaction of 28-GeV protons with uranium and gold. Mean momenta derived from this and other experiments are discussed in terms of fission and spallationlike processes. It is concluded that, while fission can account for neutron-rich products from uranium, the momenta of neutron-deficient rare-earth nuclides from both uranium and gold are too low to arise from any conventional fission process. In the mass region below the rare earths, many neutron-deficient isotopes appear to be formed by mixed mechanisms. Only the most neutron-deficient products have momenta as low as those predicted for a spallationlike process. Mean cascade deposition energies, ${E}^{*}$, derived from the forward to backward ratios average 145 MeV for the neutron-deficient rare earths from uranium and 199 MeV for these products from gold. Such low values for gold and the even lower values for uranium cannot be understood in terms of a model for the spallationlike process involving a nucleonic cascade followed by conventional evaporation. Either there is a break-down of the two-step model, e.g. as might be expected if fragment emission is involved, or it is not valid to extrapolate to 28 GeV the momentum-transfer excitation-energy relationship inferred from Monte Carlo calculations at much lower energies.

Charge distribution and recoil properties in the fission of 208Pb by 11·5 GeV protons

Cross-sections and thick-target recoil properties have been determined for a number of products in the mass region 100–113 from the interaction of 208Pb with 11·5 GeV protons. The recoil ranges of the neutron deficient products are approximately a factor of two smaller than those of the neutron excessive products are approximately a factor of two smaller than those of result a charge dispersion curve consisting of two gaussians, peaking at N Z = 1·24 and 1·34 , was constructed to fit the measured cross-sections of the A = 111 isobars. The resulting curve is single-peaked but has a pronounced shoulder on the neutron excessive side. The curve for the neutron excessive products is very similar to that obtained in the fission of 208Pb by 450 MeV protons except for a reduction in the value of the integrated cross-section. The values of the average deposition energy, average total kinetic energy of the primary fragments, and average separation distance at scission obtained for these products from their recoil properties are nearly equal to the corresponding values at 450 MeV. Integration of the charge dispersion curve for the neutron deficient products indicates a 82 per cent contribution to the total isobaric cross section at A = 111. Comparison of the properties of the neutron deficient products with similar results previously obtained for a 238U target as well as with evaporation calculations indicates that only a fragmentation mechanism can explain all the experimental results.

Energy Dependence of the Recoil Properties of Products from the Interaction ofU238with 0.45-11.5-GeV Protons

The energy dependence of the integral recoil properties and formation cross sections of a number of barium and strontium nuclides produced in the interaction of ${\mathrm{U}}^{238}$ with protons has been studied between 0.45 and 11.5 GeV. The ranges and forward-to-backward ratios ($\frac{F}{B}$) of the neutron-excessive products are nearly independent of energy, and the results indicate that these products are formed in binary following interactions with low deposition energies. The ranges of the neutron-deficient products are practically constant up to 1 GeV, decrease by nearly a factor of 2 between 1 and 5 GeV, and decrease very slightly thereafter. The $\frac{F}{B}$ values exhibit a pronounced peak in the vicinity of the midpoint of this interval. The results up to 1 GeV are consistent with binary fission. At this energy a different mechanism becomes noticeable and by 5 GeV this process predominates. Comparisons are made with the results expected for spallation, of a moderately light nucleus, and fragmentation. The latter is the only process which can account for the various experimental results. It is concluded that fragmentation is the main mechanism for the formation of neutron-deficient products lying in the fission region for incident energies above 5 GeV.

Charge distribution and recoil properties in the fission of 238U by 11·5 GeV protons

Cross-sections and thick-target recoil properties have been determined for a number of products in the mass region 100–115 from the interaction of 238U with 11·5 GeV protons. The recoil ranges of the neutron deficient products are nearly a factor of two smaller than those of the neutron excessive products suggesting a difference in reaction mechanisms. On the basis of this finding a charge dispersion curve consisting of two separate gaussians, peaking at N Z = 1·26 and 1·40, was constructed to fit the measured cross-sections. The resulting curve is single-peaked but has a pronounced shoulder on the neutron deficient side. The isobaric yield at A = 111 is 35·0 ± 3·5 mb and the curve associated with the neutron excessive products contributes 80 per cent of the total yield. This curve is very similar to that obtained in the fission of 238U by 450 MeV protons except for a reduction in the fission cross-section by a factor of 2·4. The values of the average deposition energy, average total kinetic energy of the primary fragments, and average separation distance at scission obtained for the neutron excessive fragments from their recoil properties are nearly equal to the corresponding values at 450 MeV. Many of the properties of the neutron deficient products are consistent with either spallation or fission of a nucleus in the heavy rare earth region but neither mechanism can explain all the results.

Charge dispersion studies in the fission of 239Pu by protons of medium energy

The independent formation cross sections of 129Cs, 130Cs, 131Cs, 132Cs, 134Cs m , 134Cs m+ g , 136Cs, and 138Cs and the cumulative yield of 138Xe produced in the fission of 239Pu induced by protons of energies 20–85 MeV have been determined by radiochemical methods. The excitation functions of the neutron-deficient products increase with increasing bombarding energy, whereas those of the neutron-rich products show maxima in this energy range. The cross sections have been used to construct charge dispersion curves. As the bombarding energy increases, the charge dispersion curve becomes broader and the most probable charge, Z p , shifts towards β-stability. The full-widths at half-maximum of the dispersion curves, and the displacement of Z p from β-stability ( Z A − Z p ) have been compared with the published data for other targets, and a correlation with the neutron-to-proton ratio in the target has been established. Calculations of Z p have been made, based on the unchanged charge distribution and equal charge displacement hypotheses. The results, although not definitive, favor the former hypothesis. The total number of neutrons, ν T , emitted in fission has been estimated.

Rare earth yields in the interaction of 28-GeV protons with uranium, bismuth and gold

Cross sections for the formation of a number of lanthanides by the interaction of 28-GeV protons with uranium, bismuth, and gold were measured by a radiochemical method. Yield-vs.-mass curves and some information concerning the charge dispersion curves were deduced from the independent and cumulative cross sections. The yield-vs.-mass curve for uranium shows a rapid decrease with increasing mass in the region from A = 131 to A ≈ 150. The total isobaric yields appear to reach a valley or flat region at A ≈ 155. Examination of neutron deficient ( N Z ⩽ 1·38 ) and neutron excess products ( N Z > 1·38 ) indicates the observed decrease in the total yields is due primarily to a rapid decrease in yields of the neutron excess products. The charge dispersion curve for products from uranium in the rare earth region is double peaked. The valley separating the neutron excess and neutron deficient peaks is deeper in the europium region than that reported previously in the cesium region. Total isobaric yields in the mass 134–155 region from bismuth and gold targets are approximately constant at ≈11 mb, a higher value than those reported for products in the mass 72–131 region from lead targets. The charge dispersion curves for rare earth products from Bi and Au show single peaks on the neutron deficient side of stability. The results are discussed in terms of fission and spallation as possible mechanism leading to products in the rare earth region.

Yields and recoil properties of scandium isotopes produced in the interaction of 590 MeV and 18 GeV protons with uranium

Absolute cross-sections and some thick target recoil properties have been measured for the production of 43–49Sc in uranium bombarded with protons of 590 MeV and 18 GeV, using radiochemical and electromagnetic isotope separation techniques. The yield of the neutron-deficient isotopes 43Sc and 44Sc increased by about a factor of 10 relative to 49Sc when the proton energy was increased from 590 MeV to 18 GeV. The average kinetic energy of the products calculated from the recoil results was 79 ± 4 MeV for both neutron-rich and neutron-deficient products at 590 MeV proton energy. At 18 GeV, the average kinetic energy of the neutron-rich products was 52·7 ± 1·0 MeV, whereas the kinetic energy of the most neutron-deficient product observed, 43Sc, had decreased to only 34·6 ± 1·0 MeV. New determinations of the half-lives of seven scandium isotopes were made on isotopically separated samples.

Fission ofU238by 2.2-GeV Protons

Angular distributions and differential range curves at angles of 15\ifmmode^\circ\else\textdegree\fi{}, 90\ifmmode^\circ\else\textdegree\fi{}, and 165\ifmmode^\circ\else\textdegree\fi{} have been obtained by radiochemical methods for several representative products which recoil out of thin uranium targets irradiated with 2.2-GeV protons. Mean momenta observed at 90\ifmmode^\circ\else\textdegree\fi{} to the beam direction are 133, 125, and 125 ${(\mathrm{MeV}\mathrm{amu})}^{\mathrm{\textonehalf{}}}$ for the neutron-excess isotopes ${\mathrm{Ba}}^{140}$, ${\mathrm{Mo}}^{99}$, and ${\mathrm{Sr}}^{91}$, but only 96 and 104 ${(\mathrm{MeV}\mathrm{amu})}^{\mathrm{\textonehalf{}}}$ for the neutron-deficient products ${\mathrm{Ba}}^{131}$ and ${\mathrm{Pd}}^{103}$. The neutron-deficient isotopes have broader momentum distributions and more forward-peaked angular distributions than do the neutron-excess products. Results are discussed in terms of a two-step vector model, i.e., a fast nucleonic cascade followed by fission or another slower de-excitation step. The observed spectra and angular distributions for all the isotopes are consistent with such a model. Observed mean velocities are compared to those predicted by a liquid-drop theory of fission. It is concluded that products in the mass-90-to-140 range are produced primarily by a conventional fission mechanism. However, some contribution of a spallationlike process appears necessary to account for low-velocity fragments in the case of the neutron-deficient products (particularly ${\mathrm{Ba}}^{131}$).

Direct heavy ion induced excitation

Observation of the radionuclides 127Cs through 132Cs resulting from the reaction 133Cs + 14N at 140 MeV indicates that the more neutron deficient products are formed as a result of a direct interaction mechanism followed by neutron emission. Kinematic arguments based on the recoil momenta and angular distributions indicate that the formation of a highly excited intermediate state is required to account for the formation of 127Cs through 130Cs.

Ultra-high vacuum techniques

Cross sections for the formation of a number of lanthanides by the interaction of 28-GeV protons with uranium, bismuth, and gold were measured by a radiochemical method. Yield-vs.-mass curves and some information concerning the charge dispersion curves were deduced from the independent and cumulative cross sections. The yield-vs.-mass curve for uranium shows a rapid decrease with increasing mass in the region from A = 131 to A ≈ 150. The total isobaric yields appear to reach a valley or flat region at A ≈ 155. Examination of neutron deficient (NZ ⩽ 1·38) and neutron excess products (NZ > 1·38) indicates the observed decrease in the total yields is due primarily to a rapid decrease in yields of the neutron excess products. The charge dispersion curve for products from uranium in the rare earth region is double peaked. The valley separating the neutron excess and neutron deficient peaks is deeper in the europium region than that reported previously in the cesium region. Total isobaric yields in the mass 134–155 region from bismuth and gold targets are approximately constant at ≈11 mb, a higher value than those reported for products in the mass 72–131 region from lead targets. The charge dispersion curves for rare earth products from Bi and Au show single peaks on the neutron deficient side of stability. The results are discussed in terms of fission and spallation as possible mechanism leading to products in the rare earth region.