Angular momentum generation in nuclear fission

When a heavy atomic nucleus splits (fission), the resulting fragments are observed to emerge spinning¹; this phenomenon has been a mystery in nuclear physics for over 40 years^2,3. The internal generation of typically six or seven units of angular momentum in each fragment is particularly puzzling for systems that start with zero, or almost zero, spin. There are currently no experimental observations that enable decisive discrimination between the many competing theories for the mechanism that generates the angular momentum^4-12. Nevertheless, the consensus is that excitation of collective vibrational modes generates the intrinsic spin before the nucleus splits (pre-scission). Here we show that there is no significant correlation between the spins of the fragment partners, which leads us to conclude that angular momentum in fission is actually generated after the nucleus splits (post-scission). We present comprehensive data showing that the average spin is strongly mass-dependent, varying in saw-tooth distributions. We observe no notable dependence of fragment spin on the mass or charge of the partner nucleus, confirming the uncorrelated post-scission nature of the spin mechanism. To explain these observations, we propose that the collective motion of nucleons in the ruptured neck of the fissioning system generates two independent torques, analogous to the snapping of an elastic band. A parameterization based on occupation of angular momentum states according to statistical theory describes the full range of experimental data well. This insight into the role of spin in nuclear fission is not only important for the fundamental understanding and theoretical description of fission, but also has consequences for the γ-ray heating problem in nuclear reactors^13,14, for the study of the structure of neutron-rich isotopes^15,16, and for the synthesis and stability of super-heavy elements^17,18.

Shape Coexistence at Zero Spin in ^{64}Ni Driven by the Monopole Tensor Interaction

The low-spin structure of the semimagic ^{64}Ni nucleus has been considerably expanded: combining four experiments, several 0^{+} and 2^{+} excited states were identified below 4.5 MeV, and their properties established. The Monte Carlo shell model accounts for the results and unveils an unexpectedly complex landscape of coexisting shapes: a prolate 0^{+} excitation is located at a surprisingly high energy (3463 keV), with a collective 2^{+} state 286 keV above it, the first such observation in Ni isotopes. The evolution in excitation energy of the prolate minimum across the neutron N=40 subshell gap highlights the impact of the monopole interaction and its variation in strength with N.

Inhibition of DNA and RNA synthesis in rat liver nuclei by oncogenic and non-oncogenic beta-blockers

The beta-blocker DL-1-(2-nitro-3-methylphenoxy)-3-tert-butylaminopropan-2-ol (ZAMI 1305), hepatocarcinogenic to the female rat, and the non-oncogenic beta-blockers DL-1-(2-nitro-5-methylphenoxy)-3-tert-butylaminopropan-2-ol (ZAMI 1327), DL-propranolol, and DL-atenolol were studied for their capacity to interfere with hepatic DNA and RNA synthesis. These moieties inhibit DNA and RNA synthesis, in a dose-dependent fashion, when added in vitro to nuclei isolated from the liver of male or female rats. The inhibition is due to a decrease of the initial rate of synthesis and of the total amount of labeled precursor incorporated into the growing chains. When administered in vivo both the oncogenic ZAMI 1305 and its non-oncogenic isomer ZAMI 1327 inhibit hepatic DNA and RNA synthesis in female rats, as evaluated by the determination of nucleic acids synthesis in liver nuclei isolated from female rats 5 and 15 min after the injection of the drug. No influence on hepatic DNA and RNA synthesis is observed when the molecules are administered to male rats. The in vivo administration of DL-propranolol causes an increase of hepatic DNA and RNA synthesis in male rats, while it is uneffective in female rats.

Early liver alterations induced by the sex-dependent hepatocarcinogen beta-blocker ZAMI 1305

Liver alterations occurring after 1, 6 or 10 days of treatment with the hepatocarcinogen beta-blocker DL-1-(2-nitro-3-methyl-phenoxy)-3-tert-butyl-amino-propan-2-ol (ZAMI 1305) were studied in male and female Wistar rats. In agreement with its sex-dependent oncogenicity, ZAMI 1305 administration causes DNA damage in the liver of the female but not of the male rat, with the only exception of 2 out of 4 males treated for 6 days. In female rat, the amount of DNA damage increases from 1 to 6 days of treatment, being unchanged at 10 days; a small portion of DNA is however damaged. ZAMI 1305 administration to female rat induces also: (i) an increase of the relative liver weight, of the DNA and RNA synthesizing activity; (ii) a decrease of the number of hepatocytes in mitosis; (iii) a minimal oval cell hyperplasia. When the same parameters were studied in ZAMI 1305-treated male rats, they were unaffected or changed to a less extent in respect to female rats.

Tumor-initiating activity of the beta-blocker ZAMI 1305 in the liver of the female Wistar rat

A 1-week treatment with the hepatocarcinogen beta-blocker DL-1-(2-nitro-3-methyl-phenoxy)-3-tert-butylamino-propan-2-ol (ZAMI 1305) induces the appearance of preneoplastic liver lesions--oval cell hyperplasia, basophilic and gamma-glutamyltranspeptidase positive (GGT+) foci--in female Wistar rats, as evidenced by the Solt and Farber short-term test of carcinogenesis. ZAMI 1305-treatment also induces liver DNA damage, as evaluated by alkaline sucrose gradient analysis. The data suggest that the oncogenic B-blocker ZAMI 1305 has initiating activity in the liver of the female Wistar rat.

Age-dependent, seasonal and daily variations of the DNA damaging capacity of the hepatocarcinogen ZAMI 1305 in female rat liver

A single injection of the sex-dependent hepatocarcinogen DL-1-(2-nitro-3-methyl-phenoxy)-3-tert-butylamino-propan-2-ol (ZAMI 1305) caused age-related DNA damage, as evaluated by alkaline sucrose gradient analysis, in the liver of female Wistar rats. DNA damage reached a maximum at 4-6 weeks of age, about the onset of sexual maturity, and decreased thereafter. In young rats (5-8 weeks of age), the amount of ZAMI 1305-induced DNA damage showed seasonal and daily differences, being higher when the molecule was administered in winter in respect to summer and in the evening in respect to the morning. In older rats (15-22 weeks of age), no seasonal and daily variations were observed.

Optical isomers of the hepatocarcinogenic beta-blocker ZAMI 1305: influence on nucleic acids synthesis and DNA integrity

The influence on nucleic acids synthesis and DNA integrity of the D-isomer and of the DL-racemic form of the oncogenic beta-blocker 1-(2-nitro-3-methyl-phenoxy)-3-tert-butylamino-propan-2-ol (ZAMI 1305) and of the non-oncogenic beta-blocker propranolol was tested in vitro and in vivo. Both D- and DL-ZAMI 1305, when added in vitro to nuclei isolated from rat liver, cause inhibition of DNA and RNA synthesis and DNA fragmentation, as evaluated by alkaline sucrose gradient analysis, in a similar dose-dependent fashion. D- and DL-ZAMI 1305 also inhibit to a similar extent the activity of DNA polymerase alpha and beta from regenerating rat liver. When administered in vivo to female rats both D and DL-ZAMI 1305 cause a dose-dependent fragmentation of liver DNA. The D-isomer and DL-racemic form of the non-oncogenic beta-blocker propranolol inhibit DNA and RNA synthesis and cause DNA fragmentation when added in vitro to isolated liver nuclei, being instead without effect when administered in vivo.

Thyroid and chemical hepatocarcinogenesis: further insights from the hepatocarcinogen ZAMI 1305

The beta-blocker DL-1-(2-nitro-3-methyl-phenoxy)-3-tert-butylaminopropan-2-ol (ZAMI 1305), oncogenic to the liver of the female but not of the male Wistar rat, was used to investigate some aspects of the relationship between liver and thyroid during chemical hepatocarcinogenesis. Thyroidectomy (TDX) strongly reduces the amount of hepatic DNA damage induced by a single administration of ZAMI 1305 in the female Wistar rat. One week of treatment with triiodothyronine (T3) completely restores the susceptibility of the liver of thyroidectomized animals to the genotoxic activity of the molecule. The amount of hepatic DNA damage in intact females varies with the age of the animal, being maximal in rats of 4-8 weeks of age, when T3 serum concentration are also maximal. An increase of relative thyroid weight, coupled with histological hyperplasia of the gland, is observed in female Wistar rats treated for 6 months with ZAMI 1305. Minimal changes of the thyroid are observed in ZAMI 1305-treated male rats. The increase of relative thyroid weight in female rats appears to be related to the severity of preneoplastic and neoplastic liver changes. These findings and several suggestions from the literature lead us to propose a model for the interaction between liver and thyroid during chemical hepatocarcinogenesis.

In vitro and in vivo DNA damage of male and female rat liver nuclei by oncogenic and nononcogenic beta blockers

The beta blocker DL-1-(2-nitro-3-methyl-phenoxy)-3-tert-butylamino-propan-2-ol (ZAMI 1305), hepatocarcinogenic to the female rat, and the nononcogenic beta blockers DL-1-(2-nitro-5-methyl-phenoxy)-3-tert-butylamino-propan-2-ol (ZAMI 1327), DL-propranolol, and DL-atenolol were tested for their capacity to damage liver DNA in vitro and in vivo. As revealed by alkaline sucrose gradient analysis, all the beta blockers tested, with the exception of DL-atenolol, caused a dose-dependent DNA fragmentation when they were added in vitro to nuclei isolated from livers of both male and female Wistar rats. Analysis of the DNA sedimentation patterns in neutral sucrose gradients demonstrated the absence of DNA fragmentation, thus indicating that the drugs did not induce double-strand DNA breaks. When the beta blockers were administered in vivo, liver DNA damage was observed only in female Wistar rats treated with ZAMI 1305. A single injection of ZAMI 1305 caused the onset of two distinct episodes of DNA damage. The first episode occurred within 5 minutes, and the damage was repaired within 1 hour after the injection; the second, apparently spontaneous, episode occurred 14 hours after the injection, and the damage was more pronounced than that seen in the first episode and took a much longer time to subside. However, a second injection of ZAMI 1305 into female Wistar rats 8 hours after the first injection did not induce the immediate short-lived episode of DNA damage but, like the first injection, it caused late DNA damage that peaked about 16 hours after drug administration.