Microsecond Isomer at the N=20 Island of Shape Inversion Observed at FRIB.

Excited-state spectroscopy from the first experiment at the Facility for Rare Isotope Beams (FRIB) is reported. A 24(2)-µs isomer was observed with the FRIB Decay Station initiator (FDSi) through a cascade of 224- and 401-keV γ rays in coincidence with ^{32}Na nuclei. This is the only known microsecond isomer (1 µs≤T_{1/2}<1 ms) in the region. This nucleus is at the heart of the N=20 island of shape inversion and is at the crossroads of the spherical shell-model, deformed shell-model, and ab initio theories. It can be represented as the coupling of a proton hole and neutron particle to ^{32}Mg, ^{32}Mg+π^{-1}+ν^{+1}. This odd-odd coupling and isomer formation provides a sensitive measure of the underlying shape degrees of freedom of ^{32}Mg, where the onset of spherical-to-deformed shape inversion begins with a low-lying deformed 2^{+} state at 885 keV and a low-lying shape-coexisting 0_{2}^{+} state at 1058 keV. We suggest two possible explanations for the 625-keV isomer in ^{32}Na: a 6^{-} spherical shape isomer that decays by E2 or a 0^{+} deformed spin isomer that decays by M2. The present results and calculations are most consistent with the latter, indicating that the low-lying states are dominated by deformation.

Statistical (n, γ ) cross section model comparison for short-lived nuclei.

Neutron-capture cross sections of neutron-rich nuclei are calculated using a Hauser-Feshbach model when direct experimental cross sections cannot be obtained. A number of codes to perform these calculations exist, and each makes different assumptions about the underlying nuclear physics. We investigated the systematic uncertainty associated with the choice of Hauser-Feshbach code used to calculate the neutron-capture cross section of a short-lived nucleus. The neutron-capture cross section for 73 Zn (n, γ ) 74 Zn was calculated using three Hauser-Feshbach statistical model codes: TALYS, CoH, and EMPIRE. The calculation was first performed without any changes to the default settings in each code. Then an experimentally obtained nuclear level density (NLD) and γ -ray strength function ( γ SF ) were included. Finally, the nuclear structure information was made consistent across the codes. The neutron-capture cross sections obtained from the three codes are in good agreement after including the experimentally obtained NLD and γ SF , accounting for differences in the underlying nuclear reaction models, and enforcing consistent approximations for unknown nuclear data. It is possible to use consistent inputs and nuclear physics to reduce the differences in the calculated neutron-capture cross section from different Hauser-Feshbach codes. However, ensuring the treatment of the input of experimental data and other nuclear physics are similar across multiple codes requires a careful investigation. For this reason, more complete documentation of the inputs and physics chosen is important. Supplementary Information: The online version contains supplementary material available at 10.1140/epja/s10050-023-00920-0.

Crossing N=28 Toward the Neutron Drip Line: First Measurement of Half-Lives at FRIB.

New half-lives for exotic isotopes approaching the neutron drip-line in the vicinity of N∼28 for Z=12-15 were measured at the Facility for Rare Isotope Beams (FRIB) with the FRIB decay station initiator. The first experimental results are compared to the latest quasiparticle random phase approximation and shell-model calculations. Overall, the measured half-lives are consistent with the available theoretical descriptions and suggest a well-developed region of deformation below ^{48}Ca in the N=28 isotones. The erosion of the Z=14 subshell closure in Si is experimentally confirmed at N=28, and a reduction in the ^{38}Mg half-life is observed as compared with its isotopic neighbors, which does not seem to be predicted well based on the decay energy and deformation trends. This highlights the need for both additional data in this very exotic region, and for more advanced theoretical efforts.

ΔK=0 M1 Excitation Strength of the Well-Deformed Nucleus

The size of a ΔK=0 M1 excitation strength has been determined for the first time in a predominantly axially deformed even-even nucleus. It has been obtained from the observation of a rare K-mixing situation between two close-lying J^{π}=1^{+} states of the nucleus ^{164}Dy with components characterized by intrinsic projection quantum numbers K=0 and K=1. Nuclear resonance fluorescence induced by quasimonochromatic linearly polarized γ-ray beams provided evidence for K mixing of the 1^{+} states at 3159.1(3) and 3173.6(3) keV in excitation energy from their γ-decay branching ratios into the ground-state band. The ΔK=0 transition strength of B(M1;0_{1}^{+}→1_{K=0}^{+})=0.008(1)µ_{N}^{2} was inferred from a mixing analysis of their M1 transition rates into the ground-state band. It is in agreement with predictions from the quasiparticle phonon nuclear model. This determination represents first experimental information on the M1 excitation strength of a nuclear quantum state with a negative R-symmetry quantum number.

Establishing the Maximum Collectivity in Highly Deformed N=Z Nuclei.

The lifetimes of the first excited 2^{+} states in the N=Z nuclei ^{80}Zr, ^{78}Y, and ^{76}Sr have been measured using the γ-ray line shape method following population via nucleon-knockout reactions from intermediate-energy rare-isotope beams. The extracted reduced electromagnetic transition strengths yield new information on where the collectivity is maximized and provide evidence for a significant, and as yet unexplained, odd-odd vs even-even staggering in the observed values. The experimental results are analyzed in the context of state-of-the-art nuclear density-functional model calculations.

ß Decay of

The interpretation of observations of cooling neutron star crusts in quasipersistent x-ray transients is affected by predictions of the strength of neutrino cooling via crust Urca processes. The strength of crust Urca neutrino cooling depends sensitively on the electron-capture and ß-decay ground-state-to-ground-state transition strengths of neutron-rich rare isotopes. Nuclei with a mass number of A=61 are predicted to be among the most abundant in accreted crusts, and the last remaining experimentally undetermined ground-state-to-ground-state transition strength was the ß decay of ^{61}V. This Letter reports the first experimental determination of this transition strength, a ground-state branching of 8.1_{-3.1}^{+4.0}%, corresponding to a log ft value of 5.5_{-0.2}^{+0.2}. This result was achieved through the measurement of the ß-delayed γ rays using the total absorption spectrometer SuN and the measurement of the ß-delayed neutron branch using the neutron long counter system NERO at the National Superconducting Cyclotron Laboratory at Michigan State University. This method helps to mitigate the impact of the pandemonium effect in extremely neutron-rich nuclei on experimental results. The result implies that A=61 nuclei do not provide the strongest cooling in accreted neutron star crusts as expected by some predictions, but that their cooling is still larger compared to most other mass numbers. Only nuclei with mass numbers 31, 33, and 55 are predicted to be cooling more strongly. However, the theoretical predictions for the transition strengths of these nuclei are not consistently accurate enough to draw conclusions on crust cooling. With the experimental approach developed in this work, all relevant transitions are within reach to be studied in the future.

Publisher's Note: Experimental Neutron Capture Rate Constraint Far from Stability [Phys. Rev. Lett. 116, 242502 (2016)].

This corrects the article DOI: 10.1103/PhysRevLett.116.242502.

Strong Neutron-γ Competition above the Neutron Threshold in the Decay of

The ß-decay intensity of ^{70}Co was measured for the first time using the technique of total absorption spectroscopy. The large ß-decay Q value [12.3(3) MeV] offers a rare opportunity to study ß-decay properties in a broad energy range. Two surprising features were observed in the experimental results, namely, the large fragmentation of the ß intensity at high energies, as well as the strong competition between γ rays and neutrons, up to more than 2 MeV above the neutron-separation energy. The data are compared to two theoretical calculations: the shell model and the quasiparticle random phase approximation (QRPA). Both models seem to be missing a significant strength at high excitation energies. Possible interpretations of this discrepancy are discussed. The shell model is used for a detailed nuclear structure interpretation and helps to explain the observed γ-neutron competition. The comparison to the QRPA calculations is done as a means to test a model that provides global ß-decay properties for astrophysical calculations. Our work demonstrates the importance of performing detailed comparisons to experimental results, beyond the simple half-life comparisons. A realistic and robust description of the ß-decay intensity is crucial for our understanding of nuclear structure as well as of r-process nucleosynthesis.

Experimental Neutron Capture Rate Constraint Far from Stability.

Nuclear reactions where an exotic nucleus captures a neutron are critical for a wide variety of applications, from energy production and national security, to astrophysical processes, and nucleosynthesis. Neutron capture rates are well constrained near stable isotopes where experimental data are available; however, moving far from the valley of stability, uncertainties grow by orders of magnitude. This is due to the complete lack of experimental constraints, as the direct measurement of a neutron-capture reaction on a short-lived nucleus is extremely challenging. Here, we report on the first experimental extraction of a neutron capture reaction rate on ^{69}Ni, a nucleus that is five neutrons away from the last stable isotope of Ni. The implications of this measurement on nucleosynthesis around mass 70 are discussed, and the impact of similar future measurements on the understanding of the origin of the heavy elements in the cosmos is presented.

Collective structure in 94Zr and subshell effects in shape coexistence.

Based on results from a measurement of weak decay branches observed following the ß- decay of 94Y and on lifetime data from a study of 94Zr by inelastic neutron scattering, collective structure is deduced in the closed-subshell nucleus 94Zr. These results establish shape coexistence in 94Zr. The role of subshells for nuclear collectivity is suggested to be important.

Identification and functional expression of four isoforms of ATPase II, the putative aminophospholipid translocase. Effect of isoform variation on the ATPase activity and phospholipid specificity.

ATPase II, a vanadate-sensitive and phosphatidylserine-dependent Mg(2+)-ATPase, is a member of a subfamily of P-type ATPase and is presumably responsible for aminophospholipid translocation activity in eukaryotic cells. The aminophospholipid translocation activity plays an important physiological role in the maintenance of membrane phospholipid asymmetry that is observed in the plasma membrane as well as the membranes of certain cellular organelles. While the preparations of ATPase II from different sources share common fundamental properties, such as substrate specificity, inhibitor spectrum, and phospholipid dependence, they are divergent in several characteristics. These include specific ATPase activity and phospholipid selectivity. We report here the identification of four isoforms of ATPase II in bovine brain. These isoforms are formed by a combination of two major variations in their primary sequences and show that the structural variation of these isoforms has functional significance in both ATPase activity and phosholipid selectivity. Furthermore, studies with the phosphoenzyme intermediate of ATPase II and its recombinant isoforms revealed that phosphatidylserine is essential for the dephosphorylation of the intermediate. Without phosphatidylserine, ATPase II would be accumulated as phosphoenzyme in the presence of ATP, resulting in the interruption of its catalytic cycle.

Recombinant SFD isoforms activate vacuolar proton pumps.

The vacuolar proton pump of clathrin-coated vesicles is composed of two general sectors, a cytosolic, ATP hydrolytic domain (V1) and an intramembranous proton channel, V0. V1 is comprised of 8-9 subunits including polypeptides of 50 and 57 kDa, termed SFD (Sub Fifty-eight-kDa Doublet). Although SFD is essential to the activation of ATPase and proton pumping activities catalyzed by holoenzyme, its constituent polypeptides have not been separated to determine their respective roles in ATPase functions. Recent molecular characterization of these subunits revealed that they are isoforms that arise through an alternative splicing mechanism (Zhou, Z., Peng, S.-B., Crider, B.P., Slaughter, C., Xie, X.S., and Stone, D.K. (1998) J. Biol. Chem. 273, 5878-5884). To determine the functional characteristics of the 57-kDa (SFDalpha)1 and 50-kDa (SFDbeta) isoforms, we expressed these proteins in Escherichia coli. We determined that purified recombinant proteins, rSFDalpha and rSFDbeta, when reassembled with SFD-depleted holoenzyme, are functionally interchangeable in restoration of ATPase and proton pumping activities. In addition, we determined that the V-pump of chromaffin granules has only the SFDalpha isoform in its native state and that rSFDalpha and rSFDbeta are equally effective in restoring ATPase and proton pumping activities to SFD-depleted enzyme. Finally, we found that SFDalpha and SFDbeta structurally interact not only with V1, but also withV0, indicating that these activator subunits may play both structural and functional roles in coupling ATP hydrolysis to proton flow.

Identification and reconstitution of an isoform of the 116-kDa subunit of the vacuolar proton translocating ATPase.

We have identified a cDNA encoding an isoform of the 116-kDa subunit of the bovine vacuolar proton translocating ATPase. The predicted protein sequence of the new isoform, designated a2, consists of 854 amino acids with a calculated molecular mass of 98,010 Da; it has approximately 50% identity to the original isoform (a1) we described (Peng, S.-B., Crider, B. P., Xie, X.-S., and Stone, D.K. (1994) J. Biol. Chem. 269, 17262-17266). Sequence comparison indicates that the a2 isoform is the bovine homologue of a 116-kDa polypeptide identified in mouse as an immune regulatory factor (Lee, C.-K., Ghoshal, K., and Beaman, K.D. (1990) Mol. Immunol. 27, 1137-1144). The bovine a1 and a2 isoforms share strikingly similar structures with hydrophilic amino-terminal halves that are composed of more than 30% charged residues and hydrophobic carboxyl-terminal halves that contain 6-8 transmembrane regions. Northern blot analysis demonstrates that isoform a2 is highly expressed in lung, kidney, and spleen. To determine the possible role of the a2 isoform in vacuolar proton pump function, we purified from bovine lung a vacuolar pump proton channel (VO) containing isoform a2. This VO conducts bafilomycin-sensitive proton flow after reconstitution and acid activation, and supports proton pumping activity after assembly with the catalytic sector (V1) of vacuolar-type proton translocating ATPase (V-ATPase) and sub-58-kDa doublet, a 50-57-kDa polypeptide heterodimer required for V-ATPase function. These data indicate that the a2 isoform of the 116-kDa polypeptide functions as part of the proton channel of V-ATPases.

Molecular characterization of the 50- and 57-kDa subunits of the bovine vacuolar proton pump.

The vacuolar type proton-translocating ATPase of clathrin-coated vesicles is composed of two large domains: an extramembranous catalytic sector and a transmembranous proton channel. In addition, two polypeptides of 50 and 57 kDa have been found to co-purify with the pump. These proteins, termed SFD (sub-fifty-eight-kDa dimer) activate ATPase activity of the enzyme and couple ATPase activity to proton flow (Xie, X.-S., Crider, B.P., Ma, Y.-M., and Stone, D. K. (1994) J. Biol. Chem. 269, 28509-25815). It has also been reported that the clathrin-coated vesicle proton pump contains AP50, a 50-kDa component of the AP-2 complex responsible for the assembly of clathrin-coated pits, and that AP50 is essential for function of the proton pump (Liu, Q., Feng, Y., and Forgac, M. (1994) J. Biol. Chem. 269, 31592-31597). We demonstrate through the use of anti-AP50 antibody, identical to that of the latter study, that hydroxylapatite chromatography removes AP50 from impure proton pump preparations and that purified proton pump, devoid of AP50, is fully functional. To determine the true molecular identity of SFD, both the 50- and 57-kDa polypeptides were directly sequenced. A polymerase chain reaction-based strategy was used to screen a bovine brain cDNA library, yielding independent full-length clones (SFD-4A and SFD-21); these were identical in their open reading frames and encoded a protein with a predicted mass of 54,187 Da. The SFD-21 clone was then used in a reverse transcription-polymerase chain reaction-based strategy to isolate a related, but distinct, transcript present in bovine brain mRNA. The nucleotide and predicted amino acid sequences of this isolate are identical to SFD-21 except that the isolate contains a 54-base pair insert in the open reading frame, resulting in a protein with a predicted mass of 55,933 Da. Both clones had 16% identity to VMA13 of Saccharomyces cerevisiae. No sequence homology between the SFD clones and AP50 was detectable. Anti-peptide antibodies were generated against an epitope common to the two proteins and to the unique 18-amino acid insert of the larger protein. The former reacted with both components of native SFD, whereas the latter reacted only with the 57-kDa component. We term the 57- and 50-kDa polypeptides SFDalpha and SFDbeta, respectively.

Subunit G of the vacuolar proton pump. Molecular characterization and functional expression.

The vacuolar type proton pump of clathrin-coated vesicles has a multisubunit ATP hydrolytic center that is peripheral to the membrane. Polypeptides present in this domain include the well characterized subunits A, B, C, D, E, and F; SFD, a dimer composed of 50- and 57-kDa polypeptides; and polypeptides termed G and H. Of these, subunits A, B, C, and E have been shown to be necessary but not sufficient for significant ATPase activity; in addition, either polypeptide G or H is also required for ATP hydrolysis (Xie, X.-S. (1996) J. Biol. Chem. 271, 30980-30985). In this study, the polypeptides G and H were purified and directly sequenced. Subsequent molecular analysis has revealed that these proteins are isoforms, which we designate G1 and G2. The cDNAs encoding the rat and bovine brain and chicken osteoclast forms of G1 have been cloned. The open reading frames of the rat and bovine clones encode hydrophilic proteins of 118 amino acids that differ at only five residues; bovine G1 has 36% identity with VMA10, a component of the proton channel of yeast. Northern blot analysis revealed a 1. 0-kilobase pair transcript encoding G1 in bovine brain, kidney, heart, and spleen. The cDNA encoding bovine polypeptide H was cloned and sequenced, revealing this protein to be 64% identical to G1, constituting isoform G2. In Northern blot analysis, a single 1. 7-kilobase pair transcript hybridized with a probe to G2 in brain, but not in heart, kidney, or spleen. An antibody against a bovine G1-specific domain reacts with V pump from bovine brain, kidney, and chromaffin granule, whereas an anti-G2 antibody reacts only with proton pump from brain. The bovine forms of G1 and G2 were subsequently expressed in Escherichia coli and Sf9 cells, respectively, and purified to homogeneity. Reconstitution of ATP hydrolysis was achieved by combination of recombinant subunits A, B, C, and E with either recombinant G1 or G2, demonstrating the role of these isoforms in pump function.

Identification of a 14-kDa subunit associated with the catalytic sector of clathrin-coated vesicle H+-ATPase.

The clathrin-coated vesicle H+-ATPase is composed of a peripheral catalytic sector (VC) and an integral membrane proton channel (VB), both of which are multiple subunit complexes. This study was conducted to determine if subunit F, previously identified in vacuolar proton pumps of tobacco hornworm and yeast, was present in mammalian pumps. Using a polymerase chain reaction-based strategy, we have isolated and sequenced cDNA clones from bovine and rat brain cDNA libraries. A full-length clone from rat brain encodes a 119-amino acid polypeptide with a predicted molecular mass of 13, 370 Da and with approximately 72 and 49% identity to subunit F of tobacco hornworm and yeast, respectively. Southern and Northern blot analyses indicate that the protein is encoded by a single gene. An anti-peptide antibody, directed against deduced protein sequence, was affinity-purified and shown to react with a 14-kDa polypeptide that is present in a highly purified pump prepared from clathrin-coated vesicles and also isolated VC. When stripped clathrin-coated vacuolars and purified chromaffin granule membranes were treated with KI in the presence of ATP, the 14-kDa subunit was released from both membranes, further indicating that it is part of the peripheral catalytic sector. In addition, direct sequencing of this 14-kDa component of the coated vacuolar proton pump confirmed its identity as a subunit F homologue.

Reconstitution of the recombinant 70-kDa subunit of the clathrin-coated vesicle H+ ATPase.

Vacuolar-type proton pumps are complex heterooligomers. When dissociated into subcomplexes and subunits, the partial reactions of ATP hydrolysis and transmembranous proton flow can be assigned to isolated domains. Data suggest that the molecular site of ATP hydrolysis resides within the 70-kDa subunit but that ATPase activity likely requires at least three additional subunits of 58, 40, and 33 kDa (Xie, X.-S., and Stone, D. K. (1988) J. Biol. Chem. 263, 9859-9867). We have now cloned and sequenced the 70-kDa subunit from bovine brain and have expressed the protein in insect Sf9 (Spodoptera frugiperda) cells with a recombinant baculovirus. When purified, the protein has no significant ATPase activity but can be photoaffinity labeled with [alpha 32P]ATP and UV irradiation with an apparent Kd of 35 microM. When reconstituted with biochemically prepared 58-, 40-, and 33-kDa polypeptides, the recombinant 70-kDa subunit restores Ca(2+)-activated ATP hydrolysis to a specific activity of 0.6 mumol P(i).mg protein-1.min-1, thus demonstrating that ATP hydrolysis in vacuolar-type proton pumps is dependent upon both the 70-kDa subunit as well as multi-subunit interactions.

Role of a 50-57-kDa polypeptide heterodimer in the function of the clathrin-coated vesicle proton pump.

The vacuolar-type proton-translocating ATPase of clathrin-coated vesicles is composed of an integral membrane proton channel (VB) and a peripheral catalytic sector (VC). Native enzyme can catalyze the hydrolysis of both MgATP and CaATP and support proton pumping when reconstituted into liposomes. In contrast, isolated VC catalyzes only Ca(2+)-activated ATP hydrolysis and cannot support proton pumping when reconstituted into liposomes (Xie, X.-S., and Stone, D. K. (1988) J. Biol. Chem. 263, 9859-9867). We now report that solubilized isolated VC can be reassembled with purified VB to restore properties of native enzyme, including Mg(2+)-activated ATP hydrolysis and proton-pumping capability. Investigation of this reassembly revealed that a heterodimer, composed of polypeptides of 50 and 57 kDa, stimulates Ca(2+)-activated ATPase activity of isolated VC 2-fold and Mg(2+)-activated ATPase activity catalyzed by the reassembled pump 9-fold. Moreover, this heterodimer stimulated proton transport by the reassembled pump > 20-fold. When separated from the proton pump, the dimer has no detectable kinase activity. Maximal stimulation occurs at a molar ratio of heterodimer to reassembled pump of 3, implying a structural, nonenzymatic mechanism. These data indicate that the 50-kDa and/or the 57-kDa polypeptide likely plays an essential and potentially regulatory role in the function of the proton-translocating ATPase of clathrin-coated vesicles.

Bafilomycin inhibits proton flow through the H+ channel of vacuolar proton pumps.

Vacuolar-type proton-translocating ATPases are complex heterooligomers that are characterized by a specific inhibition by bafilomycin A1. These enzymes have a peripheral ATP hydrolytic domain as well as a transmembranous sector. The transmembranous sector has been isolated by glycerol gradient centrifugation, and this subcomplex is composed of polypeptides of 116, 39, and 17 kDa. Both this sector and native holoenzyme were reconstituted into potassium-loaded (150 mM KCl) liposomes prepared from pure lipids. When diluted into potassium-free buffer, a valinomycin-induced membrane potential did not drive proton uptake, as assessed by acridine orange quenching. In contrast, pretreatment of both the reconstituted proton pump and isolated transmembranous sector at pH 4.2 activated a latent proton conductance. Bafilomycin A1 (1 nM) inhibited ATP-energized proton pumping catalyzed by the proton pump, as well as membrane potential-driven proton flow through both the acid-activated proton pump and the isolated proton pore. Thus bafilomycin A1 inhibits vacuolar proton pumps by blocking proton conduction through the proton pore, which we term VB.

Alternative mRNA splicing generates tissue-specific isoforms of 116-kDa polypeptide of vacuolar proton pump.

The cDNA encoding the 116-kDa polypeptide of the bovine brain vacuolar-type proton translocating ATPases has been cloned and sequenced. One of five clones differed from all others in that it contained an 18-base pair deletion within the coding region, whereas it was identical to the other clones in overlapping coding and noncoding regions, indicating that this heterogeneity arises through an alternative splicing mechanism. By conventional Northern analysis, only one 4.1-kilobase mRNA was identified in bovine brain, heart, kidney, liver, and spleen. However, a polymerase chain reaction-based analysis revealed two species of mRNA with a tissue-specific distribution. Type I, containing the 18-base pair insert, was found in brain, whereas the truncated (Type II) form was found in all tissues examined. Similar tissue distributions of rat mRNA were observed. The deletion site accounting for this variability occurs within a predicted protease sensitivity motif (PEST site), suggesting that differences in the biological half-life of the two 116-kDa isoforms may exist.