High Energy Induced Fission of Bismuth and Lead

101

The repartition of nuclear charge in fission has a narrower dispersion than almost any other property connected with the fission process. T o a crude approximation, the distribution of nuclear charge between light and heavy partners L and H leads to the most probable charges (ZP)L and (Zp)= displaced from the respective charges ZA of 8-stability by the same amount for the two fragments (Glendenin rule of equal charge displacement ECD, 1946). The existence of shell offsets in the Z,i vs. A function for different neutron-and proton-shell regions must be considered. All available data for thermal fission U235 (nt~,,F) are examined critically. The data show sudden offset-like drifts (fine structure) that may well be associated with shell properties of the products before the "neck" has dissolved. It is shown that these data eliminate naive equal charge displacement ECD, also an older competitive prescription of constant charge ratio CCR for the products, and an empirical Russian prescription (Apalin et al., 1960). The data are also examined in the light of the postulate that fission gives minimum nuclear plus coulombic potential energy (Present 1947, Fong 1955, Swiatecki-Blann 1960, and it is shown that the present mass formulas give too much uncertainty three to four &decays from stability t o give a useful test, but that shell effects in masses must be retained. Data from charged-particle fission with energy deposit up to 40-50 Mev are in reasonable accord with the low-energy data on correcting for composition and neutron boil-off. It is concluded from experiment that Zp is a single-valued function of A, known to about f 0.15 unit for low-energy fission and f 0.25 unit for medium-energy fission, and that the fine structure very probably present is a n indication of intrinsic nuclear chemistry.The early thinking on the wartime Manhattan Project about nuclear charge distribution in low-energy fission, before any yield data were available, was based on the concept (1, 2) of constant charge ratio CCR. Professor Wheeler had suggested (4) a search for positron enlitters t o see how wide the charge dispersion might be: the results of crude searches in the products of U235 (n,h,F) were, of course, negative (5). Way and Wigner (7) postulated t h a t the charge would distribute t o give a minimum nuclear potential energy, and Present (8) postulated a minimum for the sun1 of nuclear potential energy plus coulombic energy (equal to final kinetic energy). Present took considerable pains to handle the coulombic term for polarizable spheres with non-uniform proton density.The identification of shielded nuclides4 36-hour BrS2, 19-day Rbs6, and 13-day Cs136 among the fission products gave the first experimental leverage on charge distribution. The first detailed treatment of the problem (5) established the classical rule, or prescription (3, 6) of equal charge displacement E C D ; namely that the nuclear charge distribution between light and heavy partners L and H leads to most probable charges Zp displaced from stability...

mentioning

confidence: 99%

SEARCH FOR CORRELATIONS OF MOST PROBABLE NUCLEAR CHARGE Z_P OF PRIMARY FISSION FRAGMENTS WITH COMPOSITION AND EXCITATION ENERGY

Coryell¹,

Kaplan²,

Fink³

1961

101

“…This process implies a rearrangement of nucleons prior to or during the fission act, and is commonly assumed to be a lowenergy process. T h e unchanged-charge-distribution mode of fission (first postulated by Goecltermann and Perlman (9) to occur in the fission of bismuth with 190-Mev deuterons) produces fragments whose most probable neutron-to-proton ratio is the same as that of the fissioning nucleus. This implies that the nucleonic rearrangement indicated in ECD-type fission does not take place, and UCD is considered characteristic of "fast" or "high-energy" fission (9).…”

mentioning

confidence: 99%

“…These parameters include the critical Z2/A value a t which the sharp increase in the I',/(I',+r,) function occurs, the energy dependence of I',/(I',+I',) (assumed to be one of two limiting forms) and the mode of distribution of nuclear charge in fission. Experimental evidence has been cited in the literature for two charge distribution processes, namely the equal charge displacement (ECD) process (7, 8 , 19) and the unchanged charge distribution (UCD) process (9). The former, observed to account well for independent yield data for the products from the fission of U235 a t thermal energies and a t 14 Mev (29), is based on the postulate that the most probable nuclear charges of compleme~ltary fragments will be equally displaced from the charge value corresponding t o stability for their respective masses.…”

mentioning

confidence: 99%

Fission of Excited Heavy Nuclei

Pate¹

1958

ABS'TRAC'T The results of p~~blished calculations on the prompt il~rclear cascade and rluclear evaporation processes are combined with some assumptions regarding the 11uclear fission process and fission widths to calculate the average neutron-to-proton ratios of fission products from Th + %Me\. protons, Th + 87-PvIev protons, and U + 450-Mev protons. Cornparison of the results with experimental data indicates that the fission charge distribution mode characterized by the equal charge displacement hypothesis, hitherto considered a low energy phenomenon, persists in uranium and thoriu~n LIP to the highest energy studied. 'There is also some evidence that fission occurs in excited heavy nuclei before complete de-excitation by particle emission, in agreement with other experimental worli. A. IN'T'RODUC'TIOXParticle evaporation (30) and fission predominate during the de-excitation of heavy nuclei excited to energies above a few million electron volts. These two processes are expected to compete a t each step during the de-excitation in the sense of the following scheme (25): particle particle particle ~~x / I I A "~( E I~I ) 4in which a nucleus of mass A excited to energy E evaporates particles to form successively nuclei of masses A', A", etc. and energies E', E" . . . with fission as an alternative to each particle evaporation step. The decrease in excitation energy per step, E-E', El-El', etc., equals the binding energy of the particle evaporated plus, in each case, the (variable) kinetic energy it carries off. The fragments from the fission of a nucleus of a given excitation energy E will themselves share in some way an excitation energy close to E, and may de-excite by further particle emission. The relative probability that a nucleus in a given energy state de-excites via a specific reaction path is commonly described in terms of a partial "width" of the state for that reaction. Thus the competition between fission and particle emission from a given nucleus a t a given energy may be expressed as a ratio r,/I'ev,D, where is the fission width and rev,, is a sum of particle emission widthsThe ratio is expected to be a function of nuclear charge, mass, and excitation energy.Fission-evaporation competition has been the subject of a number of previous studies, among the more recent being investigations with transuranium nuclei excited t o energies up to 40 Mev (27, 28), and with thorium bombarded with protons of energies up t o 100 Mev (3). 'Afanuscript received Jzrly 22, 1958.

“…(12) or unchanged charge division (u.c.d.) (13). Compound nucleus formation was assumed in this energy range.…”

Section: Zp Valuesmentioning

confidence: 99%

Nuclear charge dispersion in the fission of ²³³U by protons of medium energy

Tomita¹,

Yaffe²

1969

Independent yields and excitation functions have been determined for the formation of cesium isotopes with masses 129 < A < 138 in the fission of 233U with protons of energies between 20 and 80 MeV. The energies at which the various excitation functions reach their maxima have been compared with those obtained for the same isotopes from fissioning systems with targets of different neutron-toproton (N/Z) ratios. The neutron-deficient species have a higher probability of being produced at lower energies from targets with lower N/Z ratios. Charge dispersion curves have been obtained and these show the familiar broadening and displacement of Z,, the most probable charge, towards stability with increase in bombardment energy. Values of Z,, -Z, have been compared with those obtained from other fissioning systems, and predictions made for values of Z, in thc fission of systems yet unstudied, e.g. 235U and Z 3 9 P~. In addition Z, values were calculated for 233U fission, assuming compound nucleus formation up to a proton energy of 50 MeV, assuming pre-and post-fission neutron evaporation on the equal charge displacement and unchanged charge division postulates. Experimental values were found to lie between the two predictions.