R. Rinzivillo

Photofission of U and Th between 300 and 1000 MeV

Abstract Loaded nuclear emulsions have been exposed to the bremsstrahlung beam of the Frascati 1 GeV electron synchrotron to measure natural uranium and thorium fission cross-sections in the energy range 300–1000 MeV. The cross-sections “per photon” are constant within the experimental errors, and amount to 67±7 and 37±4 mb, for U and Th, respectively. These results are briefly discussed.

Photofission of Bi, W and Ag from 300 to 1000 MeV

SummaryThe fission of Bi, W and Ag induced by (300÷1000) MeV photons has been studied. Nuclear emulsions, loaded with known amounts of suitable salts of the fissionable elements were used. The cross-section per photon and the fissility were obtained.RiassuntoLa fissione del bismuto, del tungsteno e dell’argento provocata da fotoni di energia tra 300 e 1000 MeV è stata studiata mediante la tecnica delle emulsioni nucleari, caricate con sali opportuni, in quantità note. Sono stati ottenuti i valori delle sezioni d’urto pre fotone e delle rispettiva fissionabilità.

Fission of uranium, thorium and bismuth by 20 GeV protons

SummaryThe cross-sections for the proton induced fission of U, Th and Bi have been measured at 20 and 21.8 GeV. They turn out to be smaller than at 600 MeV by a factor between 3 and 4, while the ratios of the cross-sections of the various elements remain approximately constant in the same energy interval.RiassuntoÈ stata studiata la fissione dell’U, del Th e del Bi, provocata da protoni di 20 e 21.8 GeV. A queste energie le sezioni d’urto risultano notevolmente minori (di un fattore tra 3 e 4) di quelle misurate a 600 MeV; peraltro i loro rapporti si mantengono approssimativamente costanti.

Experimental results on the nuclear fission produced by 600 MeV protons

SummaryThe fission phenomena produced by protons of about 600 MeV, have been studied, in a number of nuclei (238U,232Th,209Bi,184W and natural silver), by means of loaded, low-sensitivity, nuclear emulsions. A significant improvement was obtained in the knowledge of the relative cross-sections (Table I). We have studied, for U and Th, the angular distribution, with particular regard given to the « noncollinear » tracks. No sign of anysotropy was found. For non-collinear events, a possibile correlation was sought between the angle formed by the two tracks and the angle between the line connecting the two track extremities and the direction of the incident beam. No correlation was found. These results agree only partially with those obtained by other authors.RiassuntoI fenomeni di fissione prodotti da protoni di ∼600 MeV in diversi nuclei (238U,232Th,209Bi,184W e argento naturale) sono stati studiati per mezzo di emulsioni nucleari a bassa sensibilità, caricate con i rispettivi elementi. Sono state determinate le sezioni d’urto relative (Tabella I, colonna 4), e quelle assolute (Tabella I, colonna 5), usando come normalizzazione i dati ottenuti da altri autori. Per l’U ed il Th è stata studiata la distribuzione angolare, con particolare riguardo alle tracce « non collineari ». Non è stato trovato alcun indizio di anisotropia. Per gli eventi non collineari, si è cercata l’eventuale correlazione tra l’angolo formato dalle due tracce e l’angolo tra la congiungente le due estremità delle tracce e la direzione del fascio incidente. Non si è trovata alcuna correlazione. Tali risultati concordano solo parzialmente con quelli di altri autori.

The photofission of Bi, Th and U between 300 and 1 000 MeV

Photofission cross sections for Bi/sup 209/, Th/sup 232/. and U/sup 2/ >s/sup 38/ are studied for photon enengies from 300 to 1000 Mev. Emuisions are exposed to a collimated gamma beam, and the stars which are produced are studied. A discussion of the theory of star production in terms of meson emission and reabsorption is given. The photofission and star production cross sections for the three nuclei under consideration are measured, and the fissionability of the nuclei is calculated. It is suggested that the star production cross sections are enhanced by coherent production of pi /sup 0/ mesons and pi /sup +/ pi /sup -/ pairs. It is observed that while the fissionability is dependent on the nucleus considered, it is independent of the excitation energy of that nucleus. (T.F.H.)

Some recent results concerning fission and fragmentation at high energies

Abstract An examination of our own results on the disintegration of some heavy nuclei induced by high energy protons or photons, together with those of other laboratories, leads to the conclusion that 1) there exists some essential physical difference between the fission at high energies of heavy nuclei and of medium nuclei; 2) is it not unreasonable to consider the fission of relatively light nuclei to be physically related more to the fragmentation than to the ordinary fission phenomenon.

Associated hadronic production in μ-pair events at the CERN Intersecting Storage Rings

Abstract We have measured at √s=62GeV, at the CERN Intersecting Storage Rings, the associated charged hadrons with massive μ pairs, employing a vertex detector of ∼ 4π coverage. The experiment has provided the first data on hadron multiplicity, angular distributions, and correlations in lepton pair events at ISR energies. The average associated charged multiplicity 〈nch〉 is found to increase as the invariant mass of the hadronic system Mh increases, similarly to the observed √s dependence in e+e− annihilation into hadrons at high energies. The multiplicity also increases with pT in the hemisphere opposite to the transverse momentum pT of the dimuon. Small clusters appear to be produced, whose correlation with the μ-pair can be explained in a simple Monte Carlo model incorporating the production of a weakly collimated jet of low multiplicity, recoiling against the dimuon.

Evidence for 2γ processes at the ISR

Studying the reaction Studyingpp → μ+ μ−X at the Intersecting Storage Rings (ISR),a special class of events was found for which no hadrons are seen associated with the muon pair in a large solid angle vertex detector. The origin of these events can be most naturally explained as arising from the two-photon process in pp collisions. It is emphasized that this process becomes increasingly interesting at higher hadron colliding facilities.