Search

Multi-quasiparticle high-K states, several of which are isomeric, were observed in Hf-180 with the Gammasphere array. We determined the lifetimes in the ns-μs range using centroid-shift and decay measurements within a mu s coincidence time window. The configurations of high-K states involve two and four quasiparticles, with states up to Kπ = (18-) established. High-K excitations are found to be progressively more favored with increasing excitation energy. The K quantum number is quite robust up to the highest spins observed, as evidenced by the large values of the reduced hindrance for isomeric decays. Furthermore, rotational bands built on three high-K states are identified, and the measured branching ratios in these sequences enable the assignment of underlying configurations. Multi-quasiparticle calculations using the Lipkin-Nogami approach for pairing, with blocking included, reproduce the observed high-K energies quite well.

Gas catchers allow the transformation of radioactive recoils from various sources into a good optical quality low-energy radioactive beam that is then available for experiments at low-energy or for further acceleration. The CARIBU project uses such a large gas catcher to create beams of neutron-rich isotopes from a Californium source for post-acceleration through the ATLAS superconducting linac to open new research opportunities for nuclear structure physics and astrophysics. The RF gas catcher developed at Argonne has now demonstrated operation at the high intensity required for this application.

High-spin terminating bands in heavy nuclei were first identified in nuclei around Er-158(90). While examples of terminating states have been identified in a number of erbium isotopes, almost nothing is known about the states lying beyond band termination. In the present work, the high-spin structure of Er-156,Er-157,Er-158 has been studied using the Gammasphere spectrometer. The subject of triaxial superdeformation and 'wobbling' modes in Lu nuclei has rightly attracted a great deal of attention. Very recently four strongly or superdeformed (SD) sequences have been observed in Hf-174, and cranking calculations using the Ultimate Cranker code predict that such structures may have significant triaxial deformation. We have performed two experiments in an attempt to verify the possible triaxial nature of these bands. A lifetime measurement was performed to confirm the large (and similar) deformation of the bands. In addition, a high-statistics, thin-target experiment took place to search for linking transitions between the SD bands, possible wobbling modes, and new SD band structures.

Conventionally, reaction rates relevant to nova nucleosynthesis are determined by performing the relevant proton capture reactions directly for stable species, or as has become possible more recently in inverse kinematics using short-lived accelerated radioactive beams with recoil separators. A secondary approach is to compile information on the properties of levels in the Gamow window using transfer reactions. We present a complementary technique where the states of interest are populated in a heavy-ion fusion reaction and their gamma decay studied with a state-of-the-art array of high-purity germanium detectors. The advantages of this approach, including the ability to determine resonance energies with high precision and the possibility of determining spins and parities from gamma-ray angular distributions are discussed. Two specific examples related to the 22Na(p, γ) and 30P(p, γ) reactions are presented.

High-spin terminating bands in heavy nuclei were first identified in nuclei around Er-158(90). While examples of special terminating states have been identified in a number of erbium isotopes, almost nothing is known about the states lying beyond band termination. In the present work the high-spin structure of Er-157 has been studied using the Gammasphere spectrometer. The subject of triaxial superdeformation and 'wobbling' modes in Lu nuclei has rightly attracted a great deal of attention. Very recently, four strongly or superdeformed (SD) sequences have been observed in Hf-174 and ultimate cranker calculations predict, such structures may have significant triaxial deformation. We have performed two experiments in an attempt to verify the possible triaxial nature of these bands. A lifetime measurement was performed to confirm the large (and similar) deformation of the bands. In addition, a high-statistics, thin-target experiment was run to search for linking transitions between the SD bands and possible wobbling modes.

The triaxial strongly deformed (TSD) bands in 167Lu were populated by the 123Sb(48Ca, 4n) reaction with a beam energy of 203 MeV. Gamma rays, requiring five fold or more in prompt coincidence, were detected with the Gammasphere spectrometer. Of particular interests are TSD bands 1 and 2 which have previously been interpreted as zero phonon and one phonon wobbling bands, respectively. Using the Doppler shift attenuation method (DSAM), a preliminary transition quadrupole moment of 6.9+0.3−0.3 eb was extracted for the TSD1 band. Data analysis continues for TSD2 which is considerably more weakly populated.

A study of high spin states in the odd-neutron isotones 239 Pu and 237 U is reported. Striking differences were found in the high-spin properties of rotational bands built on the 1 /2 + [631] ground states in these two nuclei. These differences mirror those observed in the even–even Pu and U immediate neighbors and appear to be related to the strength of octupole correlations.

Understanding the processes which create and destroy 22Na is important for diagnosing classical nova outbursts. Conventional 22Na(p,γ) studies are complicated by the need to employ radioactive targets. In contrast, we have formed the particle-unbound states of interest through the heavy-ion fusion reaction, 12C(12C,n)23Mg and used the Gamma-sphere array to investigate their radiative decay branches. Detailed spectroscopy was possible and the 22Na(p,γ) reaction rate has been re-evaluated. Hydrodynamical calculations incorporating the revised rate indicate a reduction in the yield of 22Na by a factor of four from previous estimates implying a reduction by a factor of two for the maximum detectability distance for 22Na γ rays from novae.

Two Gammasphere experiments have been performed in order to establish the possible triaxial nature of strongly deformed (SD) bands in 174Hf. A lifetime measurement, using the Doppler-shift attenuation method, confirmed the large deformation of the four previously observed bands in this nucleus with transition quadrupole moments ranging from 12.6 to 13.8 b. These values are significantly larger than those predicted for triaxial minima by ultimate cranker (UC) calculations. A thin-target, high-statistics experiment was also carried out to search for linking transitions between the SD bands. No such transitions, which represent an experimental signature for wobbling modes, were observed. Four additional SD bands were found in 174Hf together with a single SD band in 173Hf. These results indicate that the strongly deformed sequences of N ≈ 102 Hf isotopes behave differently than the triaxial strongly deformed (TSD) bands found in Lu nuclei near N = 92 . The interpretation of these bands in terms of possible stable triaxial deformation is confronted with the experimental findings and UC predictions.

The (8)B(alpha,p)(11)C reaction is part of the network that can bypass the triple a process leading to the production of (12)C. We have measured the astrophysical reaction rate for this reaction by studying the inverse (11)C(p,alpha)(8)B process. The radioactive (11)C beam was produced via the p((11)B,(11)C)n reaction using the in-flight facility at the ATLAS accelerator. The astrophysical reaction rate obtained from the excitation function measured in the energy range E(x)=8.8-10 MeV was found to be a factor of 10-50 higher than previous estimates.

We have studied the elastic scattering of the exotic 17 F nuclei from 208 Pb at Coulomb barrier energies. For the measurement we used a new detector array with pixel structure and a large solid angle. Optical potential analysis shows that 17 F has an imaginary potential much less deep than 19 F. Information on the exclusive 17 F breakup cross section has been deduced too.

Isomer decays, β–γ and electron–γ spectroscopy, α–γ fine structure studies, and Coulomb excitation are cases of nuclear structure experiments where the ideal detector is a compact, highly segmented, very efficient germanium (Ge) detector box. In order to develop such a structure, we are working on the R&D of large, segmented, high-purity planar Ge strip detectors (HPGeDSSD) which form the walls of such a box. We have developed a 92 mm×92 mm×20 mm HPGeDSSD, which has 16×16 orthogonal strips of 5 mm width. We are in the process of designing a focal plane detector for the Argonne fragment mass analyzer which consists of a five-sided box, each side having a HPGeDSSD backed by a large segmented clover HPGe detector. MCNP simulations indicate this detector would have an efficiency of ∼60% for 122 keV gamma rays and ∼15% for 1.33 MeV radiation, which is ideal for studying the decays of nuclei far from stability that are usually produced with very low cross-sections.

Sealed 252 Cf sources in the microCurie to milliCurie range are routinely used in industry and research. At present, no reliable analytical method is available for precise determination of the activity. Very strong sources can be analyzed by neutron counting but the error is large. We propose to use γ-ray spectroscopy for such analysis. In particular, high-energy γ rays (above 1 MeV ) of fission fragments in equilibrium with the source are very convenient because they have little absorption in materials surrounding the source. It is found that the 1435.8-keV γ-ray of 138 Cs and the 1596.5-keV γ-ray of 140 La are well isolated and sufficiently above the background under these peaks. We have measured the γ/α ratio for the 252 Cf with an uncertainty of better than 5%. The experiment involved the preparation of several thin 252 Cf sources, alpha pulse height analysis, determination of α decay rate, and the measurement of γ singles spectrum with a well-shielded germanium spectrometer. The measured ratio and the γ-ray spectroscopy of unknown samples can provide the activity of 252 Cf in these samples.

The 15 O(α,γ) 19 Ne reaction is an important pathway from the hot CNO cycle to the rp process that leads to the synthesis of heavier nuclei. At lower temperatures, the reaction rate is dominated by the transition to the 4.033 MeV 3/2 + state in 19 Ne, the first level above the 15 O(α,γ) threshold at 3.534 MeV. We have performed an indirect measurement of the ratio Λ α Λ γ by populating the 3/2 + state via the 3 He( 20 Ne,α) 19 Ne reaction in inverse kinematics. The α particles were detected in a double-sided Si strip detector telescope while the heavy reaction products, 19 Ne and 15 O, were identified in the focal plane of a magnetic spectrograph. For the 4.033 MeV state in 19 Ne an upper limit Λ α Λ γ of ≤ 7×10 −4 has been obtained.

Two simple model problems are presented that can be very useful for assessing the accuracy of numerical algorithms designed to solve for aerosol dynamics in stagnation-flow reactors via the moment transport method. These model problems have been simplified to the point that analytical solutions useful for algorithm testing are possible, although complete aerosol solutions are not obtained here. Model problem solutions are presented over a range of applicable parameters.

The strength distribution of the primary γ rays in the decay from superdeformed (SD) states is investigated by applying the maximum likelihood method. For the 194 Hg nucleus, 41 primary transitions are identified above 2600 keV. It is concluded that they represent the strongest 10% of the transitions selected stochastically from a Porter-Thomas distribution. This would support the scenario of a statistical decay of SD states via coupling to a compound state at normal deformation. However, the occurrence of several very strong “one-step linking” transitions is found to have a very small probability. Based on the absence of strong primary transitions from SD states in adjacent nuclei, the situation in 194 Hg is viewed as a very lucky incidence.

The strength distribution of the γ rays in the decay-out from superdeformed (SD) states is investigated by applying the maximum likelihood method, with special emphasis on the influence of the lower threshold given by experimental conditions. Clear graphical solutions are found, and a careful estimation of the dispersion in the values of the number of degrees of freedom and of the average strength of the most likely χ 2 distribution is carried out. For the 194 Hg nucleus, 41 primary transitions from the decay-out of SD states are identified above 2600 keV. It is concluded that they represent the strongest 10% of the transitions selected stochastically from a Porter-Thomas distribution. This would support the scenario of a statistical decay of SD states via coupling to a compound state at normal deformation. However, the occurrence of several very strong direct one-step transitions as previously observed in 194 Hg has a very small probability of the order of 10 −4 . This may indicate special selection rules governing the decay. However, based on the absence of strong primary transitions from SD states in adjacent nuclei, the situation in 194 Hg is viewed as a very lucky incidence.

The low intensity of the RIBS presently available at the first generation production facilities (10(5)-10(6) pps) and the necessity to reconstruct the event kinematics in RIB measurements require detection systems having both a large solid-angle coverage and a high granularity. The EXODET (EXOtic DETector) apparatus has been accomplished to respond to these requirements and the first experiment has been successfully performed studying the F-17 scattering on Pb-208 at 90.4 MeV.

An experiment designed to populate two-phonon vibrational states in {sup 208}Pb by Coulomb excitation was performed with a {sup 136}Xe beam at a bombarding energy of 650 MeV. The {gamma} rays from the decay of the excited states were measured with Gammasphere and scattered particles were detected in the compact heavy-ion counter CHICO. We have not been able to observe any state close to the expected harmonic energy of 5.2 MeV. However, we were able to extract the B(E3,3{sub 1}{sup {minus}}{r_arrow}6{sub 1}{sup +}) value for the lowest known 6{sup +} state at 4.424 MeV based on measured {gamma}-ray intensities. About 20{percent} of the expected total E3 strength can be found in this state, suggesting a large fragmentation of this second octupole phonon state in {sup 208}Pb. Upper limits for the B(E3) strength were determined for higher-lying, but unseen, 6{sup +} states ranging from 15{percent} of the harmonic value at 5.2 MeV to 100{percent} at 6.0 MeV. {copyright} {ital 1998} {ital The American Physical Society}

Five superdeformed (SD) bands have been observed in ${}^{149}\mathrm{Tb},$ using the Gammasphere spectrometer. The measurement was performed with a backed target to obtain lifetime information from a Doppler-shift attenuation method analysis. It is proposed that the yrast SD band corresponds to the ${}^{148}\mathrm{Gd}(\mathrm{yrast})\ensuremath{\bigotimes}\ensuremath{\pi}{6}^{3}$ intruder configuration. The first excited band (band 2) is found to be identical to the ${}^{150}\mathrm{Tb}$ yrast SD band. Band 3 is proposed to correspond to the $\ensuremath{\pi}[301]1/2(\ensuremath{\alpha}=\ensuremath{-}1/2)\ensuremath{\rightarrow}[651]3/2(\ensuremath{\alpha}=\ensuremath{-}1/2)$ proton-hole excitation coupled to the $\ensuremath{\pi}{6}^{4}\ensuremath{\nu}{7}^{1}(\ensuremath{\nu}[651]{1/2)}^{\ensuremath{-}1}$ intruder configuration. Bands 4 and 5, the weakest SD bands seen in the present work, show similarities with band 3 and are most likely associated with a similar intruder configuration.

Quadrupole moments of the six known superdeformed (SD) bands of {sup 193}Hg and the yrast SD band of {sup 192}Hg have been determined by a Doppler-shift-attenuation-method measurement utilizing the gammasphere array. The quadrupole moments of all {sup 193}Hg SD bands were found to be similar, suggesting the active single-particle orbitals in the mass-190 region exhibit only small shape-driving effects. Additionally, there is evidence for an unexpected difference in the quadrupole moments of SD bands in {sup 192}Hg and {sup 193}Hg. {copyright} {ital 1998} {ital The American Physical Society}

High-spin states in {sub 94}{sup 240}Pu and {sub 96}{sup 248}Cm have been populated with Coulomb excitation at beam energies slightly above the Coulomb barrier. In each nucleus a new side band has been identified in {gamma}-{gamma} coincidence measurements and has been interpreted as the lowest-lying octupole excitation. The alignments and Routhians of these bands, as well as the branching ratios of E1 decays to the ground-state bands, are in good agreement with results from cranked RPA calculations. {copyright} {ital 1998} {ital The American Physical Society}

Superdeformation studies in {sup 189,191,192}Tl at Gammasphere are discussed. New results on {sup 191}Tl are the observation of interband transitions between the signature partner E2 bands and the measurement of an average quadrupole moment (Q{sub 0} = 18 {+-} 1 eb) for these superdeformed structures. These data enable them to derive absolute M1 strengths and confirm that the 81st proton occupies the [642]5/2 orbital. Tentative data for one superdeformed sequence in {sup 189}Tl are consistent with the prediction of a decrease in quadrupole deformation for the lightest nuclei at the limit of this island of superdeformation.

Lifetime measurements with the Doppler shift attenuation method have been performed for the two superdeformed (SD) bands of 142Sm. The inferred transition quadrupole moments, Q0, are 11.7±0.1 eb for the yrast SD band and 13.2+0.8−0.7 eb for the excited SD band. These results are consistent with expectations from mean-field calculations, and indicate that the proposed additivity of contributions of individual orbitals to Q0 is valid for SD nuclei over an extended mass range in the vicinity of 152Dy.

A new pixel-structure detector array with a large solid angle coverage has been used for the first time to study the elastic scattering of exotic F-17 nuclei from a Pb-208 target at 90.4 MeV. The experimental data have been analyzed in the framework of the optical model potential and the real and imaginary strong absorption radii have been evaluated. These quantities have been compared with those obtained for the system F-19+Pb-208 at the same energy in the center of mass frame. The F-17+Pb-208 reaction cross section is more similar to those of the systems O-16,O-17+Pb-208 rather than to the one of the system F-19+Pb-208 at similar energies: this indicates that in the energy range around the Coulomb barrier the breakup channel is still weak. The exclusive breakup cross section F-17-->O-16+p has been measured for the first time at energy below the Coulomb barrier.

A new detector apparatus has been designed and developed to be used in experiments performed with radioactive ion beams. It consists of 16 highly segmented silicon strip detectors arranged in two‐layer telescopes and subtending a large solid angle (about 70% of 4π sr). An innovative readout system for the position information that uses highly integrated electronics (ASIC chips) has been implemented. A first successful experiment has been performed at the Argonne National Laboratory (USA) to study the 17F scattering by a 208Pb target at 90.4 MeV of incident energy. The 17F angular distribution has been analyzed and the optical model potential best‐fit parameters determined. The same analysis performed on 17F data taken at higher incident energy, in completely different experimental conditions, gives consistent results. The comparison with experiments performed with stable beams (19F, 16O, 17O) indicates a behavior for the 17F more similar to that of the Oxygen isotopes than to the 19F one. Despite of the short data collection time, also the cross section for the 17F →16O + p break‐up process has been estimated.

An excited superdeformed band in {sup 194}Hg , observed to decay directly to both normal-deformed and superdeformed yrast states, is proposed to be a K{sup {pi}}=2{sup {minus}} octupole vibrational band, based on its excitation energies, spins, and likely parity. The transition energies are identical to those of the yrast superdeformed band in {sup 192}Hg , but originate from levels with different spins and parities. The evolution of transition energies with spin suggests that cancellations between pairing and particle alignment are partly responsible for the identical transition energies. {copyright} {ital 1997} {ital The American Physical Society}

The Doppler-shift attenuation method was used to measure lifetimes of superdeformed (SD) states for both the yrast and the first excited superdeformed band of 190Hg. Intrinsic quadrupole moments Q o were extracted. For the first time, the dipole transition rates have been extracted for the inter-band transitions which connect the excited SD band to the yrast states in the second minimum. The results support the interpretation of the excited SD band as a rotational band built on an octupole vibration.

One-step decay transitions linking the superdeformed (SD) bands 1 and 3 in194Hg to yrast levels are discussed. Inter-band transitions between bands 1 and 3 have also been identified. For the first time, the spin, parity and excitation energy have been determined for two SD bands in the same nucleus. The low excitation energy of the excited band supports the view that it is based on an octupole excitation. It is believed that Porter-Thomas fluctuations play a major role in determining the strength of the one-step transitions as suggested by the fact that only one other SD band has been linked in theA=190 mass region (194Pb) at the present time. When Porter-Thomas fluctuations prevent the observation of one-step or two-step linking transitions, as e.g. in the case of192Hg, the analysis of the quasi-continuous part of the decay-out spectrum provides an alternative method for the determination of the excitation energy and spin of an SD band. This method is discussed in detail.

High-precision lifetime measurements have been performed in superdeformed (SD) bands of {sup 192,194}Hg with the Doppler-shift attenuation method. Intrinsic quadrupole moments Q{sub 0} were extracted for three SD bands in {sup 194}Hg and for the yrast SD band in {sup 192}Hg. Within experimental uncertainties, all four SD bands have equal Q{sub 0} values. These results provide constraints on differences in Q{sub 0} values between the {open_quotes}identical{close_quotes} SD bands {sup 194}Hg(3) and {sup 192}Hg(1). {copyright} {ital 1997} {ital The American Physical Society}

The ${}^{29}$Si$(p,\ensuremath{\gamma})$ reaction has been studied in the range ${E}_{p}=1.00$--1.75 MeV. Three previously unknown states in ${}^{30}$P were identified, and one state previously assigned to ${}^{30}$P was identified as a state in ${}^{14}$N. Gamma-ray strengths were determined for the three new levels, and branching ratios were measured for 17 resonances. Revised ${J}^{\ensuremath{\pi}};T$ assignments were made for nine of these states.

High-precision lifetime measurements have been performed in superdeformed (SD) bands of 151,152 Dy and 151 Tb with the Doppler-shift attenuation method. Differences found in the quadrupole moments Q 0 of the yrast SD bands are understood in terms of shape polarization effects by the high- N intruder orbitals. Q 0 moments are also presented for three excited SD bands in 151 Dy. Recent calculations can account for most of the observations.

Background: Obtaining reaction rates for nuclear astrophysics applications is often limited by the availability of radioactive beams. Indirect techniques to establish reaction rates often rely heavily on the properties of excited states inferred from mirror symmetry arguments. Mirror energy differences can depend sensitively on nuclear structure effects.Purpose: The present work sets out to establish a detailed comparison of mirror symmetry in the $A=23$, $T=1/2$ mirror nuclei ${}^{23}$Na and ${}^{23}$Mg both to high spin, and high excitation energy, including beyond the proton threshold. These data can be used to benchmark state-of-the-art shell-model calculations of these nuclei.Methods: Excited states in ${}^{23}$Na and ${}^{23}$Mg were populated using the ${}^{12}$C(${}^{12}$C,$p$) and ${}^{12}$C(${}^{12}$C,$n$) reactions at beam energies of 16 and 22 MeV, and their resulting $\ensuremath{\gamma}$ decay was measured with Gammasphere.Results: Level schemes for ${}^{23}$Na and ${}^{23}$Mg have been considerably extended; highly excited structures have been found in ${}^{23}$Na, as well as their counterparts in ${}^{23}$Mg for previously known rotational structures in ${}^{23}$Na. Mirror symmetry has been investigated up to an excitation energy of 8 MeV and spin-parity of 13/2${}^{+}$. Excited states in the region above the proton threshold have been studied in both nuclei.Conclusions: A detailed exploration of mirror symmetry has been performed which heavily constrains expectations as to how mirror energy differences should evolve for different structures. Agreement with shell-model calculations provides confidence in using such estimations where real data are absent.

The $^{29}\mathrm{Si}$(p,\ensuremath{\gamma}) reaction has been studied for 30 resonances in the range ${\mathit{E}}_{\mathit{p}}$=1.75--2.51 MeV. Branching ratios have been measured for 28 of these resonances. The separation energy ${\mathit{S}}_{\mathit{p}}$ for $^{30}\mathrm{P}$ has been determined to be 5594.5\ifmmode\pm\else\textpm\fi{}0.5 keV. Improved spin, parity, and isospin assignments have been made to a number of the resonant states. A new level has been identified in $^{30}\mathrm{P}$ at ${\mathit{E}}_{\mathit{x}}$=6006 keV, and its \ensuremath{\gamma}-ray branching ratios have been measured. \textcopyright{} 1996 The American Physical Society.

We present here strong evidence that recently published values for the relative intensities of γ-ray lines in the 66 Ga(β + + EC) 66 Zn decay are incorrect at the higher energies (∼ 30% too low at 4.8 MeV). In particular, we find that our current results are consistent with a set of correction factors which were first suggested 20 years ago, but have gone largely ignored until now. Our validation of these little known correction factors will have bearing on experiments which use the 66 Ga radioisotope to extrapolate absolute detector efficiencies to higher energies. In particular, we discuss the conclusions of a recent D(p, γ ) 3 He experiment which will be strongly affected by our current results. The astrophysical S -factor data derived from this D(p, γ ) 3 He experiment are now seen to be systematically too low by ∼ 30%.

Discrete {gamma} rays directly connecting states of a superdeformed (SD) band in {sup 194}Hg to the yrast states have been discovered. Thus, the excitation energies and spins of all members of the lowest SD band are established for the first time, together with their likely parity. The SD band decays from its 10{sup +} and 12{sup +} states, which lie 4204.8 and 4407.4 keV above the normal-deformed yrast states of the same spins. {copyright} {ital 1996 The American Physical Society.}

We report on Coulomb dissociation experiments utilizing a 208 Pb-beam (641 MeV/nucleon) impinging on C, Sn, Ho, Pb and U targets. Excitation energy distributions for 208 Pb are extracted from an exclusive measurement of the projectile decay products. The measured cross section is dominated for heavy targets by Coulomb excitation of giant resonance states in lead. A thorough analysis yields evidence for the excitation of the double-phonon giant dipole resonance. In contrast to earlier results obtained for other nuclei the extracted cross section for 208 Pb exceeds that of theoretical predictions on the basis of a harmonic oscillator scenario only slightly by about a factor 1.3.

A new superdeformed band has been identified in {sup 191}Hg bringing the total number of bands observed in this nucleus to four. The new band has properties similar to those of a superdeformed band reported recently in {sup 193}Hg. Both bands are believed to be built on the unfavored signature of the {ital j}{sub 15/2} intruder configuration. Comparisons between the data and cranked Woods-Saxon calculations highlight the strengths and weaknesses of theory in describing high-{ital N} orbitals at large deformation.

One approach to studying chaos in quantum systems utilizes the statistical behavior of eigenvalues. Such analyses require data of very high quality, since both completeness and purity are essential. The design of a Compton-suppressed γ-ray spectrometer for the purpose of establishing a nearly complete level scheme in 30 P via the 29 Si(p, γ) reaction is described. Design criteria and implementation are discussed, and early results from the system are presented.

The population intensity of some SD bands in the mass 150 region were observed to depend on the mass symmetry of the entrance channel in the fusion reaction. The authors raised the possibility that the population of SD bands had a memory of the entrance channel. To check this interesting possibility, we made measurements of the population intensities of superdeformed (SD) bands in the {sup 160}Gd({sup 36}S,5n){sup 191}Hg and {sup 130}Te({sup 64}Ni,3n){sup 191}Hg reactions. To ensure that any observed effect was not due to a simple angular momentum difference in the entrance channels, we also measured the average entry points and spin distributions of normal and SD states in {sup 191}Hg in the two reactions. The entry points and spin distributions for {sup 191}Hg are the same and, indeed, so are the SD intensities in the two reactions. Hence, no entrance-channel effect is observed in the population of the SD band in {sup 191}Hg, in contrast with data for SD bands in the mass 150 regions. We suggest that the effect observed previously in the mass 150 region is due to an angular momentum effect. A letter reporting our results was submitted for publication.

We performed an experiment to search for the two-phonon octupole vibrational state in {sup 208}Pb. Thick targets of {sup 208}Pb, {sup 209}Bi, {sup 58,64}Ni, and {sup 160}Gd were bombarded with 1305 MeV beams of were bombard {sup 208}Pb supplied by ATLAS. Gamma rays were detected using the Argonne-Notre Dame BGO gamma-ray facility, consisting of 12 Compton-suppressed germanium detectors surrounding an array of 50 BGO scintillators. We identified some 30 known gamma rays from {sup 208}Pb in the spectra gated by the 5{sup -} {yields} 3{sup -} and 3{sup -} {yields} 0{sup +} transitions in {sup 208}Pb. In addition, after unfolding these spectra for Compton response, we observed broad coincident structures in the energy region expected for the 2-phonon states. Furthermore, we confirmed the placement of a 2485 keV line observed previously in {sup 207}Pb and find no evidence consistent with the placement of this line in {sup 208}Pb. We are currently in the process of investigating the origin of the broadened lines observed in the spectra, extracting the excitation probability of states in {sup 208}Pb, and determining the relative probability of mutual excitation and neutron transfer in this reaction. An additional experiment is also being performed to collect much highermore » statistics germanium-germanium coincidence data for the thick {sup 208}Pb target.« less

The region of nuclei with mass {approximately} 190 was studied extensively over the last few years following the discovery of superdeformation in {sup 190}Hg. More recently, considerable interest in the neutron-deficient Pb isotopes developed with the discover of a number of bands at high spin connected by dipole transitions in both even {sup 192-200}Pb and odd {sup 197-201}Pb nuclei. The majority of the dipole bands are regular in character (i.e. transition energies increase smoothly with spin) while the remaining bands are referred to as irregular in character, due to the fact that the transition energies do not increase smoothly with spin. The properties of the dipole bands were interpreted in terms of high-K, moderately-deformed oblate states built on configurations involving high-J, shape-driving quasiproton excitations coupled to rotation-aligned quasineutrons. It was suggested that the difference between the regular and irregular dipole sequences is related to the deformation where the irregular sequences are thought to be less collective than their regular counterparts.

The neutrino spectrum from the decay of 8 B is a crucial ingredient in interpreting recent data from solar neutrino detectors. The β + decay of 8 B proceeds to a broad state in 8 Be, and the shape of the neutrino spectrum may be obtained from a measurement of the α spectrum following the β + decay. A new technique has been used at the ATLAS accelerator to measure this spectrum by implanting 8 B particles into the mid-plane of a 91 μm thick Si detector. The advantage of this method is that both α particles are detected and systematic effects due to energy loss in catcher foils and dead layers of the detector are eliminated. To calibrate the detector, α's from the decay of 20 Na ions produced and implanted with the same technique were used.

The neutrino spectrum from the decay of 8 B is a crucial ingredient in interpreting recent data from solar neutrino detectors. Since 8 B decays to a broad state in 8 Be, the shape of the neutrino spectrum is best obtained from a measurement of the α spectrum following the β + decay. A new technique has been used at the ATLAS accelerator to measure this spectrum by implanting 8 B particles into the mid-plane of a 91 μm thick Si detector. The advantage of this method is that both α particles are detected and systematic effects due to energy loss in catcher foils and dead layers of the detector are eliminated. To calibrate the detector, α's from the decay of 20 Na ions produced and implanted with the same technique were used.

A description is given of the design and performance of a diagnostic-accessible, perpendicular-flow, single-wafer deposition reactor for use with 50 mm wafers. The reactor chamber design is based on a simple flow tube, with diagnostic access achieved by replacing sections of the reactor chamber wall with recessed diagnostic ports. Reactor chamber performance is evaluated for the purpose of performing optical measurements during atomic layer deposition (ALD). Computational fluid dynamics simulations predict that the when used with windows the diagnostic port design produces minimal perturbations to the gas flow under typical deposition conditions, as compared to a design without diagnostic ports. Temperature measurements of the inside surface of a window installed in a diagnostic port suggest that for reactor chamber operation at 110 °C, under typical deposition conditions, the inside surface window temperature is approximately equal to or greater than the surrounding reactor chamber temperature, thereby minimizing possible species condensation on the window surface. As a consequence of using recessed diagnostic ports, an increase in the amplitude of optical intensity fluctuations was generally observed when performing measurements at elevated chamber temperatures. These intensity fluctuations could be readily reduced by enclosing the optical path to the exterior side of the windows. The performance of two straight-forward methods to reduce these intensity fluctuations is presented. The results outlined above demonstrate that this reactor design can be operated with short gas residence times and with all reactor surfaces at elevated temperatures, making it useful for simulating a wide range of gas flow conditions with relevance to microelectronics-related ALD processes.

Bismuth germanate (BGO) Compton-suppresion shields have been constructed to surround the Ge detectors of the GAMMASPHERE array. A shield consists of six hexagonal tapered BGO elements, each coupled to two 1-in. × 1-in. photomultiplier tubes. In addition, a cylindrical BGO detector is placed behind the Ge detector to intercept the forward scattered γ-rays. One hundred ten such shields are planned for the GAMMASPHERE array. Procedures for measuring the performance of these shields have been developed. Large (70%) Ge detectors when used with these shields give a peak-to-total ratio of better than 0.60. To date 85 shields have been tested and approved for use in GAMMASPHERE.

The low-lying level structure of the neutron-deficient isotope [sup 202]Rn has been studied for the first time, using the [sup 181]Ta([sup 27]Al,6[ital n]) and [sup 192]Pt([sup 16]O,6[ital n]) reactions. Gamma-ray transitions between excited states in [sup 202]Rn were identified by mass tagging using a recoil mass separator, the Argonne Fragment Mass Analyzer, and by observation of coincident x rays. Transitions in [sup 203]Rn were also identified. The level scheme deduced from these data is consistent with the systematics of light radon isotopes below the [ital N]=126 shell closure and with theoretical calculations indicating that the ground-state shape should not be strongly deformed at [ital N]=116.

The level structures of the 189 Hg and 190 Hg nuclei have been extended considerably from earlier studies by using the 160 Gd( 34 S, 4n and 5n) reactions in conjunction with an array of twelve Compton suppressed Ge detectors. Eight band structures have been delineated in 189 Hg and ten in 190 Hg. Most bands can be understood within the framework of cranked shell model calculations assuming an oblate collective shape and specific quasiparticle configurations are proposed. There is also some evidence for the onset of triaxiality. In both nuclei level structures which appear to be non-collective in character are interpreted in a band termination picture.

The level structure of the odd-odd nucleus 224Ac has been studied by measuring radiations associated with the α-decay of the 22 h 228Pa isotope. The 228Pa activity was produced by the irradiation of Th metal foils with 45 MeV protons. The Pa fraction was subsequently chemically isolated. Alpha singles as well as αγ-, αe-, αγγ-coincidence measurements were performed with thin sources, and a partial level scheme was deduced for the 224Ac. Although the spins of the observed levels have not been uniquely determined, the data establish positive- and negative-parity bands which exhibit characteristics of parity doublets. In particular, two bands at 354 and 360 keV appear to be members of a parity doublet. The similarity in the hindrance factors of α-decays to the 354 and 360 keV levels, which have opposite parities, indicates that the wavefunctions of the two states are very similar, which is a signature for octupole deformation.