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The Purdue Rare Isotope Measurement Laboratory (PRIME Lab) is a dedicated research and service facility for AMS that provides the scientific community with timely, reliable and high quality chemical processing (∼600 samples/year) and AMS measurements (∼3000 samples/year) of 10 Be, 14 C, 26 Al, 36 Cl, 41 Ca and 129 I. The AMS system is based on an upgraded FN (7 MV) tandem accelerator that has recently been modified to improve performance. The precision is 1% for 14 C and it is 3–5% for the other nuclides for radioisotope/stable isotope ratios at the 10 −12 levels. System background for 10 Be, 14 C, 26 Al, 36 Cl and 41 Ca is 1–10×10 −15 while for 129 I the natural abundance limits it to 20×10 −15 . Research is being carried out in Earth, planetary, and biomedical sciences. Geoscience applications include determination of exposure ages of glacial moraines, volcanic eruptions, river terraces, and fault scarps. Burial histories of sand are being determined to decipher the timing of human expansion and climatic history. Environmental applications are tracing the release of radioactivity from nuclear fuel reprocessing plants, water tracing, and neutron dosimetry. The applications using meteoric nuclides are oil field brines, sediment subduction, radiocarbon dating, and groundwater 36 Cl mapping. Radionuclide concentrations are also determined in meteorites and tektites for deciphering space and terrestrial exposure histories.

The nuclear structure of ${}^{99}$Tc was studied using the ${}^{98}$Mo${(}^{3}$He,$\mathrm{pn}\ensuremath{\gamma})$ reaction, which has populated most states in the nucleus below 2 MeV excitation energy. The proton exit channel was isolated from competing reaction channels by operating $\ensuremath{\gamma}$-ray detectors in coincidence with a large-solid-angle proton detector. The experiments included $\ensuremath{\gamma}$-ray excitation functions, $\ensuremath{\gamma}$-ray angular distributions, and $\ensuremath{\gamma}$-$\ensuremath{\gamma}$ coincidences. The results were interpreted using a particle-rotor model. The systematics of increasing deformations in odd-$A$ Tc nuclei are discussed.

The operation, status, performance, and upgrade plans for the Purdue Rare Isotope Measurement Laboratory (PRIME Lab) are described. The AMS system is in routine operation for all of the commonly-used AMS nuclides. Chemical preparation is being performed for all nuclides measured in many different matrices. Construction of a new injector and terminal stripper system is in progress; a fast-isotope-switching system is in the final design stage.

The nuclear structure of {sup 101}Tc was studied using the {sup 100}Mo({sup 3}He,pn{gamma}) reaction, which has populated most states in the nucleus below 2 MeV excitation energy. The proton exit channel was isolated from competing reaction channels by operating {gamma}-ray detectors in coincidence with a large-solid-angle proton detector. The experiments included {gamma}-ray excitation functions, {gamma}-ray angular distributions, and {gamma}-{gamma} coincidences. The results were interpreted using a particle-rotor model. {copyright} {ital 1997} {ital The American Physical Society}

The nuclear structure of {sup 97}Tc was studied using the {sup 96}Mo({sup 3}He,{ital pn}{gamma}) reaction, which has populated most states in the nucleus below 2 MeV excitation energy. The proton exit channel was isolated from competing reaction channels by operating {gamma}-ray detectors in coincidence with a large-solid-angle proton detector. The experiments included {gamma}-ray excitation functions, {gamma}-ray angular distributions, and {gamma}-{gamma} coincidences. The results were interpreted using a particle-rotor model. {copyright} {ital 1996 The American Physical Society.}

A new facility for accelerator mass spectrometry has been established at Purdue University. First results have been obtained for 10Be and 36C1, and several internal research projects have been initiated. Plans are to become a national AMS facility to serve the Earth and planetary science communities for the full range of cosmogenic radionuclides.

Even-A Pd nuclei were studied using the reactions /sup 92/,/sup 94/,/sup 96/Zr(/sup 13/C,3n..gamma..)/sup 102/,/sup 104/,/sup 106/Pd. The experiments included ..gamma..-ray yield as a function of energy, ..gamma..-ray angular distributions, ..gamma..-ray linear polarizations, and ..gamma..-..gamma.. coincidence measurements. A new treatment of ..gamma..-..gamma.. directional correlation from oriented nuclei, which combines data from all appropriate coincidence pairs to reduce uncertainties, has aided in determining multipolarities for 49 contaminated transitions. Extensive decay schemes are presented which include many high-angular-momentum, positive- and negative-parity states. In addition to the ground-state band, there appear to be three collective bands built on excited two-quasiparticle states. The measurements favor a slightly deformed rotor description of these nuclei over an interpretation in terms of a vibrational (interacting boson) model. (AIP)

Levels in 108Cd were studied using the reaction 96 Zr ( 16 O , 4nγ) 108 Cd with a bombarding energy of 56 MeV. The experiments included γ-ray yields as a function of bombarding energy, γ-ray angular distributions, γ-ray linear polarizations and γ-γ coincidence measurements. The decay scheme which includes 48 γ-ray transitions is very similar to those reported for even-even Pd isotopes. In addition to the ground-state band, there appear to be four collective bands built on two-quasiparticle states. The results are in excellent agreement with a slightly deformed rotor description of the 108Cd nucleus.

Levels in /sup 106/Ag have been studied using heavy-ion reactions. The experiments included ..gamma..-ray yields as a function of bombarding energy, ..gamma..-ray angular distributions, ..gamma..-ray linear polarizations, and three-detector ..gamma..-..gamma.. coincidence measurements. The decay scheme includes four collective bands with low-lying bandheads. The energies, ..gamma..-ray mixing ratios, and branching ratios of states in these bands are shown to be in good agreement with the corresponding quantities calculated using a two-particles-plus-rotor model at a small, symmetric deformation (delta=0.12). The Coriolis and recoil effects are explicitly included and a variable moment of inertia is used. The calculation also shows that the four bands have predominant configurations ..nu..(d/sub 5/2/)x..pi..(g/sub 9/2/), ..nu..(g/sub 7/2/)x..pi..(g/sub 9/2/), ..nu..(h/sub 11/2/)x..pi..(p/sub 1/2/), and ..nu..(h/sub 11/2/)x..pi..(g/sub 9/2/), respectively. The ..delta..I=1 and ..delta..I=2 level sequences in the bands can be understood on the basis of the positions of the neutron and proton Fermi surfaces in the respective Nilsson bases. Another band observed experimentally on a 10/sup -/ bandhead at 2.4416 MeV is thought to have a four-quasiparticle structure.

The structure of /sup 111/Cd was studied using the /sup 110/Pd(/sup 3/He,2n..gamma..) /sup 111/Cd reaction, which populated a number of new nonyrast states. The experiments included ..gamma..-ray excitation functions, ..gamma..-ray angular distributions, and ..gamma..-..gamma.. coincidences. Even though /sup 111/Cd is only two protons away from the closed Z = 50 shell, there is evidence that it is slightly deformed. A symmetric particle-plus-rotor model has been used to interpret the results, and shows that rotational features are present.

Several non-yrast states of both parities in $^{101}\mathrm{Pd}$ have been observed using three-detector $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence measurements following the reaction $^{92}\mathrm{Zr}(^{12}\mathrm{C}, 3n)^{101}\mathrm{Pd}$. Energy levels, $\ensuremath{\gamma}$-ray mixing ratios, and branching ratios are deduced from the data and compared with the results of a rotational model calculation. Five of the newly discovered states are identified as non-yrast members of rotational bands based on the $\frac{5}{{2}^{+}}$ and $\frac{11}{{2}^{\ensuremath{-}}}$ states.NUCLEAR STRUCTURE $^{92}\mathrm{Zr}(^{12}\mathrm{C}, 3n)^{101}\mathrm{Pd}$ at 48 MeV; measured ${I}_{\ensuremath{\gamma}}(\ensuremath{\theta})$, 3-detector $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coin, $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ DCOQ. $^{101}\mathrm{Pd}$ deduced levels, $J$, $\ensuremath{\pi}$, $\ensuremath{\gamma}$ mixing ratios. Rotational model calculations, Coriolis, recoil, calculated levels, mixing ratios, branching ratios, half-lives.

A rotational interpretation in the strong coupling limit is applied to transitional nuclei in the mass-70, -100, -120 regions. The calculations use a symmetric rotor with relatively small deformation and a variable moment of inertia is included explicitly. It is shown that this simple model, which is essentially similar to that used for strongly deformed nuclei, is in good agreement with a wide range of phenomena observed in transitional nuclei. Energy levels, branching ratios, and mixing ratios are included in the comparison to experimental data.

The structure of /sup 98/Ru was studied using the /sup 97/Mo(/sup 3/He,2n..gamma..) /sup 98/Ru reaction, which populated many non-yrast states that had not been observed previously. The experiments included ..gamma..-ray excitation functions, angular distributions, linear polarizations, and ..gamma..-..gamma.. coincidences. It is shown that low-lying non-yrast states in this transitional nucleus can be described in a rotational framework without using a vibrational degree of freedom. The interpretation used a model with two quasiparticles coupled to a symmetric rotor. A surface-delta residual interaction was included.

The energy-level structure of /sup 99/Ru was studied using the /sup 98/Mo(/sup 3/He, 2n..gamma..)/sup 99/Ru reaction at a bombarding energy of 13 MeV, in an attempt to populate an extensive set of non-yrast states. The experiments included ..gamma..-ray excitation functions, ..gamma..-..gamma.. coincidence measurements, ..gamma..-ray angular distributions, and ..gamma..-ray linear polarization measurements. A large number of non-yrast states were observed, with angular momenta ranging from (1/2) to (17/2). The comparison of experimental energies and electromagnetic transition properties to the predictions of a particle-rotor model is presented. This interpretation suggests that several complete particle-core multiplets have been populated. The population of non-yrast states is attributable to the properties of the reaction rather than any details of the nuclear structure involved.

A proton induced X-ray emission (PIXE) method has been developed for determination of the concentrations of elements (with Z > 12) in geological samples. In this procedure proton bombardment is utilized to induce characteristic X-rays in all elements present in samples to be analyzed. The method involves observation and counting of these X-rays thereby providing a quantitative measurement of the concentration of each element (with Z > 12) in the samples. Complete analysis can be obtained in about 30 min, only mg amounts of material are required, samples are not destroyed, the concentrations of as many as 75 elements are obtained in one operation and the procedure is fully automated. Concentrations of a few ppm, in some favorable cases 1 ppb, can be quantitatively determined. Advantages and limitations of the method are discussed.

States excited by the $^{99}\mathrm{Ru}(\ensuremath{\alpha},2n\ensuremath{\gamma})^{101}\mathrm{Pd}$, $^{88}\mathrm{Sr}(^{16}\mathrm{O},3n\ensuremath{\gamma})^{101}\mathrm{Pd}$, and $^{92}\mathrm{Zr}(^{12}\mathrm{C},3n\ensuremath{\gamma})^{101}\mathrm{Pd}$ reactions have been studied by measuring the $\ensuremath{\gamma}$-ray yield as a function of incident projectile energy, $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidences, $\ensuremath{\gamma}$-ray angular distributions, and $\ensuremath{\gamma}$-ray linear polarizations. Three intense $\ensuremath{\gamma}$-ray cascades were observed. These cascades are produced by quasirotational bands built on the 5/${2}^{+}$, 7/${2}^{+}$, and 11/${2}^{\ensuremath{-}}$ intrinsic states of the nucleus. A decay scheme is given which includes states with energy up to 6488 keV and angular momentum up to 35/2. The quasirotational bands seen in $^{101}\mathrm{Pd}$ are similar to the bands in the adjacent even-even nuclei which suggests that this odd-$A$ nucleus can be described as a particle decoupled from an even-even core. A cascade of seven $\ensuremath{\gamma}$ rays in $^{102}\mathrm{Rh}$ was also observed following the $^{92}\mathrm{Zr}(^{12}\mathrm{C},p n)^{102}\mathrm{Rh}$ reaction.[NUCLEAR REACTION $^{99}\mathrm{Ru}(\ensuremath{\alpha},2n\ensuremath{\gamma})$, $E=16\ensuremath{-}24$ MeV; $^{92}\mathrm{Zr}(^{12}\mathrm{C},3n\ensuremath{\gamma})$, $E=45\ensuremath{-}56$ MeV; $^{88}\mathrm{Sr}(^{16}\mathrm{O},3n\ensuremath{\gamma})$, $E=56\ensuremath{-}64$ MeV; measured $\ensuremath{\sigma}(E)\ensuremath{\gamma}$, $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coin, $\ensuremath{\sigma}(\ensuremath{\theta})\ensuremath{\gamma}$, linear $P$; deduced decay scheme, $J$, $\ensuremath{\pi}$.]

The structure of $^{111}\mathrm{Ag}$ was studied using the $^{110}\mathrm{Pd}$${(}^{3}$He, pn\ensuremath{\gamma}) $^{111}\mathrm{Ag}$ reaction, which has populated an essentially complete set of low-energy states. The proton exit channel was isolated from competing reaction channels by operating \ensuremath{\gamma}-ray detectors in coincidence with a large-solid-angle proton detector. The experiments included \ensuremath{\gamma}-ray excitation functions, \ensuremath{\gamma}-ray angular distributions, and \ensuremath{\gamma}-\ensuremath{\gamma} coincidences. Four rotational bands have been identified. A symmetric particle-rotor model has been used to interpret these bands, and the model has identified other rotational features as well. A tentative interpretation of several nonrotational states is presented, suggesting that the mixing between rotational and nonrotational states is minimal.

$\ensuremath{\gamma}$-ray linear polarization measurements have been used to locate negative parity states in the even-odd isotopes $^{99,101,103}\mathrm{Pd}$. A Compton polarimeter based on two Ge(Li) coaxial detectors was used. Collective bands ($I={\frac{11}{2}}^{\ensuremath{-}}, {\frac{15}{2}}^{\ensuremath{-}}, {\frac{19}{2}}^{\ensuremath{-}}, \dots{}$) built on ${\frac{11}{2}}^{\ensuremath{-}}$ states were observed in $^{101}\mathrm{Pd}$ and $^{103}\mathrm{Pd}$. Many negative parity states were also observed in $^{99}\mathrm{Pd}$, but they do not appear to be part of a similar collective band. These polarization measurements also confirm many angular momentum assignments which previously had been made using systematic arguments.NUCLEAR REACTIONS $^{90,92,94}\mathrm{Zr}(^{12}\mathrm{C}, 3n)$, $E=48\ensuremath{-}56$ MeV; measured linear polarization of $\ensuremath{\gamma}$ rays; parities and multipole mixing ratios in $^{99,101,103}\mathrm{Pd}$ transitions deduced.

The linear polarization of ..gamma.. rays from the deexcitation of Cd isotopes A = 105 through 109 following (/sup 16/O,xn) reactions were measured using a Ge(Li) Compton polarimeter. The linear polarizations are used with ..gamma..-ray angular distributions to make unique-parity assignments and to remove ambiguity in angular-momentum values. With these measurements it is possible to identify many states in /sup 105hyphen109/Cd which are similar to states in Pd nuclei that have been interpreted as having one, two, or three quasiparticles coupled to a slightly deformed rotating core.

Levels in $^{105,107}\mathrm{Ag}$ have been studied using heavy-ion reactions. The experiments included $\ensuremath{\gamma}$-ray yields as a function of bombarding energy, $\ensuremath{\gamma}$-ray angular distributions, and three-detector $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence measurements. The positive-parity band based on the unique-parity ${g}_{\frac{9}{2}}$ orbital in both nuclei exhibits a $\ensuremath{\Delta}I=1$ character, unlike its counterpart ${h}_{\frac{11}{2}}$ band in Pd nuclei. The energy levels, $\ensuremath{\gamma}$-ray mixing ratios, branching ratios, and lifetimes in this band as well as in the ground-state negative-parity band are shown to be in good agreement with calculations using a particle-plus-rotor model at a small, symmetric deformation ($\ensuremath{\delta}=0.12$). The Coriolis and recoil effects are explicitly included and a variable moment of inertia is used. The low-lying "anomalous" $\frac{7}{{2}^{+}}$ state is also readily reproduced by this model. The calculation also shows that the $\ensuremath{\Delta}I=1$ nature of the positive-parity band results primarily from the fact that the Fermi surface is far from the $K=\frac{1}{2}$ state, whereas the transition properties are governed by the Coriolis mixing of the strong-coupled bands. Two bands built on the $\frac{17}{{2}^{\ensuremath{-}}}$ and $\frac{21}{{2}^{+}}$ states with high bandhead energies are thought to have three-quasiparticle configurations.NUCLEAR STRUCTURE $^{92}\mathrm{Zr}$($^{16}\mathrm{O}$,$p2n$)$^{105}\mathrm{Ag}$ at 60 MeV, $^{96}\mathrm{Zr}$($^{14}\mathrm{N}$, $3n$)$^{107}\mathrm{Ag}$ at 49 MeV: measured ${I}_{\ensuremath{\gamma}}(E(^{16}\mathrm{O}))$, ${I}_{\ensuremath{\gamma}}(E(^{14}\mathrm{N}))$, ${I}_{\ensuremath{\gamma}}(\ensuremath{\theta})$, $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coin $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ DCOQ. $^{105,107}\mathrm{Ag}$ deduced levels, $J$, $\ensuremath{\pi}$, $\ensuremath{\gamma}$ mixing ratios. Rotational model calculations, Coriolis, calculated levels, mixing ratios, branching ratios, lifetimes. Ge(Li) detectors.

The energy level structure of 103 Pd was studied using the reactions 100 Ru(α, nγ) 103 Pd and 94 Zr( 12 C, 3nγ) 103 Pd. The experiments included the measurement of γ-ray yield as a function of incident projectile energy, γ-γ coincidence measurements, γ-ray angular distributions, and γ-ray linear polarization measurements. Two ΔI = 2 bands built on the 7 2 + and 11 1 − intrinsic states were observed. The ( 12 C, 3n) reaction was found to be much more effective than the (α, n) reaction in exciting these bands. A decay scheme is presented showing energies up to 6049 keV and spins up to 35 2 . Additional information relating to the coupling between the odd particle and the core is presented. A cascade of six γ-rays in 103 Rh was observed following the reaction 94 Zr( 12 C, p2nγ) 103 Rh.

The excellent sensitivity and broad range of the PIXE technique suggests that it can be an attractive method for the multielemental analysis of water. The basic nature of water samples however presents several challenges to the analist, in particular sample preparation, efficient data collection, and data analysis. The Purdue University PIXE system has been developed to overcome these difficulties. The system has been used to analyze a large number of samples where the concentration ratio of abundant to trace elements is as large as 106. Detection limits obtained for most elements are in the range 0.1 to 5 ppb.

Previously observed positive and negative parity states of /sup 105/Ag have been interpreted using a particle-rotor model. Experimental energies and transition properties have been compared to those predicted by the calculation. The results show that this simple model with very few parameters can explain successfully most of the structure of /sup 105/Ag. Most members of the multiplets resulting from the coupling of R -- 0, 2, and 4 excitations of the core to g/sub 9/2/, f/sub 5/2/, p/sub 3/2/, and p/sub 1/2/ orbitals have been identified.

$^{97}\mathrm{Mo}$ nuclei were produced by the $^{96}\mathrm{Zr}$${(}^{3}$He,2n\ensuremath{\gamma}${)}^{97}$Mo reaction at 11 MeV. Excitation functions, angular distributions, linear polarizations, and gamma-gamma coincidences were measured and used to construct a $^{97}\mathrm{Mo}$ level scheme. By taking advantage of the relatively high Q value of $^{3}\mathrm{He}$ reactions, several new non-yrast states were populated and observed. A standard particle-plus-rotor calculation was performed and used to interpret the experimental level scheme. Special emphasis was placed on identifying complete angular momentum multiplets arising from particle-core coupling.

A new application of the delayed coincidence technique for measuring short nuclear lifetimes has been developed which is basically similar to the well-known prompt comparison method. Systematic errors can be reduced and greater accuracy obtained for nuclei which exhibit both a prompt and a delayed event. A transistorized time-to-amplitude converter was used. The mean life of the second excited state of ${\mathrm{Ca}}^{42}$ [(4.8\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ sec] and the first excited state of ${\mathrm{Sc}}^{47}$ (5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}12}$ sec) were measured. Also the mean life of the first excited state of ${\mathrm{Hg}}^{198}$ [(3.5\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}11}$ sec] was determined by the usual self-comparison method.

The angular dependence of the circular polarization of the 1.6-MeV $\ensuremath{\gamma}$ radiation following the 2.2-MeV $\ensuremath{\beta}$ transition of ${\mathrm{La}}^{140}$ has been measured. The analysis of these data, together with $\ensuremath{\beta}\ensuremath{-}\ensuremath{\gamma}$ directional-correlation and spectrum-shape data, was made using exact electron wave functions and taking into consideration finite-nuclear-size effects. The nuclear beta matrix elements determined in this manner show that the deviations of the 2.2-MeV $\ensuremath{\beta}$ transition from the Coulomb approximation is caused by a cancellation effect of the vector-type matrix elements and not by a selection-rule effect. The experimentally determined ratio of the $\ensuremath{\int}i\ensuremath{\alpha}$ and the $\ensuremath{\int}\mathrm{r}$ matrix elements does not agree well with the value predicted by Fujita on the basis of the conserved-vector-current theory. The discrepancy may be due to the presence of higher order effects in the 2.2-MeV $\ensuremath{\beta}$ transition of ${\mathrm{La}}^{140}$.

The energy-dependent $\ensuremath{\beta}\ensuremath{-}\ensuremath{\gamma}$ circular polarization was measured in order to set better limits on the matrix-element ratio $\ensuremath{\Lambda}=\ensuremath{\int}\frac{\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\alpha}}}{(\ensuremath{\int}\frac{i\stackrel{\ensuremath{\rightarrow}}{\mathrm{r}}}{\ensuremath{\rho}})}$ for the 696-keV first-forbidden $\ensuremath{\beta}$ transition of ${\mathrm{Rb}}^{86}$. The results show that $\ensuremath{\Lambda}$ is consistent with the Fujita-Eichler relation, so the contributions of the off-diagonal matrix elements of the Coulomb Hamiltonian to $\ensuremath{\Lambda}$ are small for this transition. The nuclear matrix elements are in agreement with previous results, but the limits of error have been reduced.

The time‐to‐amplitude converter is designed for time measurements in the 10−7 to 10−11 sec range. The time resolution of the system is defined as the full width at half maximum of the amplitude distribution produced by the time converter in response to prompt coincidence events. The best time resolution obtained was 4.7×10−10 sec. The time converter and all of the accessory circuits are described.

The sum‐delay amplifier and the gate generator are in separate single‐width NIM modules. The amplifier has five input channels which can be switched in or out. The delay can be switch selected to be 0, 1, or 2 μsec. The gate generator is very flexible. It will accept almost any type of input signal and produce an output pulse which can be adjusted to meet most gating requirements. One frequent use of the instrument is to observe the performance of an electronic system while an experiment is being performed.

Mutants for Saccharomycopsis lipolytica temperature sensitive for alkaline extracellular protease production, but not for growth, were isolated. Thirty-three isolates were temperature sensitive for protease production, and one (xpr-32) produced a temperature-sensitive protease. Genetic analysis indicated that xpr-32 was located in gene XPR2, and allele xpr2-7 was found to also produce a temperature-sensitive protease. None of five independently isolated xpr2 mutations affects the production of extracellular ribonucleases and acid protease(s). Diploids with zero, one, or two active alleles of the XPR2 locus were constructed, and the XPR2 locus was shown to exhibit a gene dosage effect on alkaline extracellular protease synthesis (enzyme activity/cell protein). These results suggest that the XPR2 gene is the structural gene for the alkaline extracellular protease of S. lipolytica.

First forbidden beta decay is interesting to study because a large number of nuclear matrix elements can be observed in a single transition. The large number of unknowns also makes it difficult to analyze the experimental data. Nevertheless, at the present time the experimental values of nuclear — matrix elements for several first forbidden transitions are more accurate than the nuclear model calculations which are used to understand the matrix elements. Thus these nuclear matrix elements provide a stringent test for the accuracy of nuclear model calculations.