H. G. de Carvalho

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.

Photonuclear reactions in carbon at energies between 300 MeV and 1000 MeV

SummaryIn the energy range from 300 MeV up to 1000 MeV, cross-sections for the reactions12C(γ, n)11C and12C(γ, x)7Be have been measured using bremsstrahlung beams of the Frascati electronsynchrotron by means of the activation method. The (γ, n) cross-section measured, (1.18 ± 0.20) mb, is in agreement with the predicted from the photomeson model. The12C(γ, x)7Be cross-section found was (8±4) · 10−29 cm2.RiassuntoSono state misurate, mediante il metodo dell’attività residua, le sezioni d’urto per le reazioni12C(γ, n)11C e12C(γ, x)7Be nell’intervallo di energie compreso tra 300 e 1000 MeV usando fasci di bremsstrahlung dell’elettrosincrotrone di Frascati. Il valore di (1.18 ± 0.20) mb misurato per la reazione12C(γ, n)11C è in accordo con quello calcolabile dal modello fotomesonico. La sezione d’urto trovata per la reazione12C(γ, x)7Be è pari a (8 ± 4) · 10−29 cm2.РеэюмеВ области знергий от 300 до 1000 МзВ были иэмерены поперечные сечения для реакций12C(γ, n)11C и12C(γ, x)7Ве, испольэуя пучки тормоэного иэлучения от злектросинхротрона Фраскати, посредством метода активации. Иэмеренное поперечное сечение (γ, n), (1.18±0.20) мбарн, согласуется с предскаэанным иэ фотомеэонной модели. Найденное поперечное сечение12C(γ, x)7Ве равно (8±4)·10−29см2.

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.)

(γ, n) reactions in the energy range 1.0–5.5 GeV

Abstract The (γ, n) cross sections per equivalent quantum of 12 C, 103 Rh, 127 I, 197 Au and 238 U have been measured with a bremsstrahlung beam of maximum energies in the range 1–5.5 GeV. Within the experimental errors of the activation method, no contribution of photons above 1 GeV was found. This result disagrees with some previous measurements at energies below 1 GeV but agrees with the predicted values obtained from simple theoretical estimates.

238U fission induced by low-energy monochromatic γ-rays: Cross Sections Between 5 and 8 MeV

Abstract The photofission cross-section of 238 U has been measured at six energies between 5.43 and 7.72 MeV, using nuclear emulsions and monochromatic γ-rays. The existence of minima in this energy region, suggested by previous workers, has been confirmed.

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.

Photodisintegration of light and medium-weight nuclei at intermediate energies I—Photoproduction of 18F, 22Na, and 24Na from targets with masses up to 40

Abstract Mean cross sections for the photoproduction of 18 F, 22 Na, and 24 Na have been determined from 23 Na, 27 Al, 28 Si, 31 P, 32 S, 35,37 Cl, 39 K, and 40 Ca targets using bremsstrahlung beams in the energy range 0·3–1 GeV. The results have been compared with a modified Rudstams CDMD formula and found to be consistent with the cross section dependence predicted on the basis of conventional spallation processes for 24 Na only. For 18 F and 22 Na, some deviations have been observed from the spallation patterns when target mass numbers exceed 31 and 37, respectively, thus suggesting other mechanisms different from spallation being effective in their production, such as fission and fragmentation.