Search

The concept of coupling a He-jet transfer system to the negative-ion sputter source of the Tandem accelerator at Chalk River has been tested. Negative-ion beams of the radioactive isotopes: 34mCl, 56Co, 60Cu and 103Ag were extracted from a Cs+ sputter ion source by sputtering cathodes that were covered with radioactivity-laden aerosol deposits from a gas-jet transfer system. Efficiencies of 1 to 25% were observed with about 50% of the deposit sputtered in less than one minute. A fast ion-source sample changer and a pneumatically-positioned target chamber have been installed at the MP Tandem accelerator. They are coupled by a He-jet transfer system, permitting the alternate acceleration of stable and radioactive ions.

The need for an isotopic tool capable of extending our ability to date fossil organic materials beyond the 50000 year range of 14 C has been recognized for some time. Chlorine 36, a cosmogenic nuclide with a half-life of 301000 years, appears well-suited to this role. It exchanges readily with stable chloride present in ppm quantities in soil pore water, and can be measured at extremely low levels by Accelerator Mass Spectrometry (AMS).

36Cl concentrations measured in rivers and lakes surface waters in Eastern North America are higher than those expected from natural production and fallout from the atmosphere. We tested the hypothesis that 36Cl concentrations in these systems are elevated because 36Cl from weapons test fallout is still cycling through the biosphere. Four sets of observations support this interpretation of the elevated concentrations of 36Cl. First, 36Cl:Cl atom ratios in small lakes and streams are lower than the ratios measured in precipitation. Secondly, 36Cl:Cl atom ratio in shallow groundwater (that does not contain 3H) is lower than those measured in surface waters or rain. Thirdly, 36Cl and Cl concentrations in biota are higher than groundwater and similar to precipitation. Fourthly, the inventory of Cl in the vegetation, litter and soil is large relative to the annual inputs from the atmosphere. These observations are consistent with the decadal residence time of 36Cl in the biosphere calculated using a mass balance model of stable Cl− and 36Cl distribution.

There is now a large body of evidence suggesting that chlorine does not occur in the environment entirely as the CL − ion, and that its rate of percolation to the water table is strongly influenced by the presence of vegetation. The new information is used to improve our understanding of previous measurements of the concentrations of 36 Cl in the Laurentian Great Lakes.

Measurements of 36 Cl and 129 I in inert biological matrices such as teeth and bone have the potential to provide a record of exposure to high neutron fluxes and direct exposure to these isotopes in air and food. In fossil specimens, measurements of 36 Cl and 129 I atom ratios in bone and teeth may provide a methodology to determine the age of samples that cover a range of ages from ∼ 100 k to ∼ 200 k and 15 M to 75 M years before present, respectively. To demonstrate the feasibility of exploiting these potential records, we have developed and tested a pyrolytic technique to extract the isotopes and stable analogs without introducing contamination. Concentrations of stable chlorine and iodine measured in teeth and bone range from 100 to 3000 ppm and 20 to 700 ppb, respectively and provided sufficient sample for measurement by AMS. 36 Cl and 129 I concentrations measured in samples of teeth and bone that were exposed to elevated concentrations of these two isotopes during weapons testing have elevated concentrations of the isotopes within the expected range of concentrations.

36Cl and stable Cl were extracted from solids by high temperature pyrolysis and then analyzed by Accelerator Mass Spectrometry (AMS). Cl was quantitatively extracted from rock, ore, vegetation and freshwater sediments in samples weighing from 100 mg to 2 g. 36Cl:Cl activity ratios measured following Cl extraction by pyrolysis agreed with those measured following Cl extraction by acid leaching. The simple pyrolysis extraction has the additional advantages that stable Cl− can be measured on the same sample along with other anions, the 36Cl:Cl ratio can be adjusted by diluting the known Cl− concentration in the collection solution to control the activity of 36Cl in the target and the potential interference of 36S can also be assessed prior to the AMS measurements.

We have measured lifetimes and deduced deformation parameters for rotational bands in {sup 133}Pm by the Doppler-shift attenuation method. A strongly coupled band based on the configuration with a hole in the {ital g}{sub 9/2} orbital has deformation parameter {beta}{sub 2}=0.40 (5), which is comparable to or larger than typical superdeformed bands in the {ital A}{approximately}130 region. We observe this band in {sup 133}Pm down to its {ital I}={ital K}=9/2 bandhead. Calculations with a configuration-dependent shell correction to the cranked Nilsson potential can explain the existence of such low-spin superdeformed structures in the {ital A}{approximately}130 region. These shapes, which also occur in {sup 129,131}Pr, can be observed experimentally because of relatively low-lying shell gaps for {beta}{sub 2}=0.4 near {ital Z}=58, {ital N}=72. No high-spin intruder orbitals ({upsilon}{ital i}{sub 13/2}) are occupied (over the spin range observed), however the strongly deformation-driving properties of a hole in the extruder {pi}{ital g}{sub 9/2} orbital appear to be an essential ingredient in lowering the energy of the superdeformed shape. {copyright} {ital 1996 The American Physical Society.}

In situ neutron activation of 35Cl within the rock and groundwater of geologic deposits that have elevated concentrations of uranium provides a hydrogeological tracer. We determine the production rate and mobility of 36Cl in the 1.3-billion-year-old Cigar Lake uranium ore deposit. Accelerator mass spectrometry was used to map the concentrations of 36Cl in the ore and in the groundwater that were up to 100 times greater than those encountered in unmineralized portions of the host sandstone aquifer. The residence time of this mobile anion in groundwater within the mineralized zone ranged from 14 to 280 kyr. These residence times are consistent with the hydraulic and geochemical data, suggesting significant control of Cl− and groundwater movement by the clay-rich matrix of the mineralized zone.

High-spin states have been studied in neutron-deficient $^{115}\mathrm{Xe}$ produced in the $^{60}\mathrm{Ni}$${(}^{58}$Ni,2pn\ensuremath{\gamma}) reaction. Transitions have been assigned to $^{115}\mathrm{Xe}$, for the first time, through their characteristic transition yields with respect to the total \ensuremath{\gamma}-ray sum energy and by systematic comparison to neighboring $^{113}\mathrm{Xe}$ and $^{117}\mathrm{Xe}$. \textcopyright{} 1996 The American Physical Society.

A superdeformed band has been found in $^{142}\mathrm{Eu}$. It is populated with 1.2(2)% of the total \ensuremath{\gamma}-ray intensity that decayed into $^{142}\mathrm{Eu}$ via the reaction $^{120}\mathrm{Sn}$${(}^{27}$Al,5n${)}^{142}$Eu at 152 MeV. The strength of the band is similar to that previously reported in $^{143}\mathrm{Eu}$, where the superdeformed band is populated with an intensity of 1.1(1)%. This is unexpected, since both total Routhian surface (TRS) and cranked modified oscillator (MO) calculations predict that the superdeformed band in $^{143}\mathrm{Eu}$ becomes yrast at lower spin than that in $^{142}\mathrm{Eu}$. This difference is \ensuremath{\sim}4\ensuremath{\Elzxh} in the MO calculations and \ensuremath{\sim}8\ensuremath{\Elzxh} in the TRS. Extrapolation of the ``normal''-deformed yrast states in $^{143}\mathrm{Eu}$ shows, however, that a difference of \ensuremath{\sim}8\ensuremath{\Elzxh} in spin corresponds to a change in the relative energy of the superdeformed and normal-deformed yrast lines of only \ensuremath{\sim}1 MeV.

Six rotational bands built on superdeformed intrinsic configurations have been observed in the 149 Gd nucleus with the Eurogam spectrometer. Orbital configuration assignments have been suggested on the basis of their effective alignments calculated with the Nilsson-Strutinsky cranking model. Most of the excited bands have identical partners in neighboring nuclei including one case differing by four mass units. Measurements of feeding patterns indicate that the 149 Gd yrast superdeformed band is fed over a wider range of angular momentum than other yrast superdeformed bands in this mass region whereas weaker excited bands in the same nucleus are populated in narrower spin windows.

A superdeformed band has been observed in the [ital N]=80 nucleus [sup 145]Tb which was produced with the reactions [sup 112]Sn([sup 37]Cl,2[ital p]2[ital n]) and [sup 118]Sn([sup 31]P,4[ital n]) at bombarding energies of 187 and 160 MeV, respectively. Since superdeformed bands also exist in the three lighter [ital N]=80 isotones [sup 142]Sm, [sup 143]Eu, and [sup 144]Gd, it is now possible to understand the valence-proton configurations of these bands in a systematic way. The T[sup (2)] dynamic moment of inertia in [sup 145]Tb shows no evidence for the [ital N] = 6 quasiproton crossing that is observed in [sup 144]Gd. Comparison with cranked Woods-Saxon and total Routhian surface calculations suggests that the proton configuration in [sup 145]Tb is 6[sup 1][direct product][404][sub 9/2[sup +]][sup 2] in which the quasiproton crossing is blocked. Furthermore, like [sup 143]Eu and [sup 142]Sm, there is no evidence in the T [sup (2)] for the [ital N]=6 quasineutron crossing predicted by the calculations. This may indicate that static neutron pairing correlations are quenched at the [ital N]=80 superdeformed shell closure.

States in the neutron-deficient $^{110,112}\mathrm{Te}$ isotopes have been populated following $^{58}\mathrm{Ni}$${+}^{58,60}$Ni reactions. Gamma-ray coincidence data were acquired with the Eurogam spectrometer in conjunction with the Daresbury recoil separator and with the Chalk River 8\ensuremath{\pi} spectrometer. Although the low lying yrast states are not collective in these nuclei, a long regular cascade of transitions, indicating collective behavior, was found in $^{112}\mathrm{Te}$ extending to high spin. This structure is interpreted in terms of a deformed 4-particle 2-hole ${0}_{2}^{+}$ state coupled to rotationally aligned quasiparticles, and represents the first example of a rotational ``intruder'' band in the tellurium isotopes.

Lifetimes of states in four of the oblate dipole bands in $^{197,198}\mathrm{Pb}$ have been measured with the recoil distance technique. Using all the available data on lifetimes and branching ratios, and assuming pure magnetic dipole transitions, we deduce transition probabilities. The results are consistent with weakly oblate collective structures involving high-K proton configurations coupled to rotationally aligned neutrons. Comparsions are made to the theoretical estimates of the D\"onau and Frauendorf semi-classical model and the tilted axis cranking (TAC) model.

Two weakly populated rotational bands have been established in [sup 106]Sn from the [sup 54]Fe ([sup 58]Ni,[alpha]2[ital p]) reaction at 243 MeV. One of the bands shows evidence of termination. The result is consistent with cranked Nilsson model calculations, which predict band terminations with a smooth and gradual shape change from a prolate collective to an oblate noncollective structure.

We have measured the relative concentration of 36Cl in the five major lakes of the Laurentian Great Lakes by AMS techniques at the Chalk River TASCC facility. The ratio of 36Cl to stable chloride decreases from 1300 CLU (l CLU is a ratio of 36Cl/Cl of 1 × 10−15) in Lake Superior to 300 CLU in Lake Ontario. However, the salt content increases rapidly from Superior to Ontario so that the concentration of 36Cl increases from a low of 3.3 ± 0.3 × 107 in Lake Superior to a high of 11.7 ± 1.7 × 107 atoms/1 in Lake Ontario. The results are compared to a model that includes the contribution of 36Cl from the bomb tests in the 1950s as determined from ice-core data taken in Greenland. It is found that there is too little 36Cl in Lake Superior to be consistent with current ideas on 36Cl in precipitation.

Concentrations of inorganic 36 Cl around the Chalk River Laboratories (CRL) slightly in excess of natural rates of production are a byproduct of reactor operations and radioisotope production at this site. The primary release of the 36 Cl to the atmosphere is via the 50 m main CRL stack which vents air from the reactor facilities. The atmospheric dispersion pattern for this radioisotope is correlated fairly strongly with that of tritium which is also released in low but measurable quantities at CRL. Air, rain, snow and surface water measurements are presented and evaluated. The data have been used to show that the majority of the release is deposited over a relatively small area.

We have used the 8π γ-ray spectrometer at the Chalk River TASCC facility to study superdeformed rotational bands in the chain of isotopes 145–149 Gd. The five bands already known have been extended to higher spin, while four new excited bands have been discovered. The use of very similar reactions, data-acquisition parameters and data-analysis techniques, combined with the enlarged set of data, has allowed us for the first time to approach the topic of superdeformation in the A = 150 mass region from a comprehensive point of view. Transition energies of neighbouring nuclei were compared to extract effective aligned spins of the added particle. Such a comparison requires knowledge of relative nuclear spins, which we have to assume. Nevertheless, we find that a very illustrative picture emerges making it possible to understand all nine bands in the Gd isotopes within a very simple scheme of orbital assignments.

A ridge structure consisting of stretched [ital E]2 transitions and extending from [h bar][omega][congruent]0.6 to 0.75 MeV has been found in a proton-selected [gamma]-[gamma] matrix in the reaction [sup 120]Sn([sup 37]Cl, xn)[sup 152,153]Dy at 187 MeV. The ridge spacing corresponds to a dynamic moment of inertia, [ital scrI][sup (2)], of about 130[h bar][sup 2] MeV[sup [minus]1] and suggests the existence of a hyperdeformed prolate shape with a quadrupole deformation [beta][sub 2][ge]0.9. Furthermore, a cascade of 10 discrete transitions with an average energy spacing of 30[plus minus]3 keV has been found. This result is consistent with calculations indicating conditions favorable to hyperdeformed structures.

The nucleus 108Sn has been populated via the 54Fe(58Ni, 4p) reaction channel at a beam energy of 243 MeV. The high-spin structure is dominated by three ΔI = 2 rotational sequences. These bands can be interpreted in terms of particle-hole excitations involving the proton g 7 2 , g 9 2 and h 11 2 orbitals and also aligned neutrons from the bottom of the h 11 2 shell. Lifet measurements have also been performed using the Doppler-shift attenuation method. These data have enabled quadrupole moments to be deduced for the two strongest bands. The results yield Q0 = 2.6±0.4 e·b for the positive-parity band and 3.4 ± 0.6 fse·b for one of the proposed negative-parity bands. These values yield quadrupole deformations of β2 = 0.20 and 0.26, respectively, for the two bands. The results obtained are discussed in terms of Woods-Saxon and total routhian surface calculations.

The nucleus [sup 141]Gd was investigated at high spins for the first time via the reactions [sup 112]Sn([sup 32]S,2[ital pn])[sup 141]Gd at 155 MeV and [sup 112]Sn([sup 33]S,2[ital p]2[ital n])[sup 141]Gd at 170 MeV. The methods of in-beam [gamma]-ray spectroscopy were used to establish a number of different structures. These include a band assigned as the [nu][ital i][sub 13/2][660]1/2[sup +] intruder in the Nilsson scheme. This is the heaviest nucleus thus far in which this type of intruder band has been identified, and is the first case in which the 13/2[sup +] bandhead has been clearly observed. Cranked shell model (CSM) calculations predict the occurrence of backbends due to [ital h][sub 11/2] proton and [ital h][sub 9/2] neutron band crossings. Gradual upbends are observed in the experimental alignments, which suggests the interaction strengths are stronger than those predicted by the CSM.

A rotational band based on the proton [ital h][sub 11/2] orbital has been observed to high spin (79/2[h bar]) and high rotational frequency ([h bar][omega][congruent]1.0 meV) in the nucleus [sub 51][sup 113]Sb. The measured transition quadrupole moment is [ital Q][sub 0]=4.4[plus minus]0.6 [ital e] b, consistent with an axial prolate deformation of [beta][sub 2][congruent]0.32. A large interaction strength (360[plus minus]60 keV) has been measured for the rotational alignment of [ital h][sub 11/2] neutrons which, together with a considerable delay in the crossing frequency ([Delta][h bar][omega][congruent]0.09 MeV), is construed as the first direct evidence of a large high-[ital j] proton-neutron interaction.

Using γ-γ correlation techniques, unresolved γ radiation associated with superdeformed (SD) excited states has been investigated for the nucleus 149 Gd populated via the 124 Sn( 30 Si, x n) reaction at 155 MeV bombarding energy. the intensity distrib ution has been measured as a function of rotational frequency. A “quasi-damped” region of excitation energy where the identity of individual SD bands melts is suggested by the relatively small value of the damping width Γ rot required by the fit to the experimental data.

The near-yrast spectroscopy of 127Ba has been examined in detail up to spin 39 2 with the reaction 117Sn(13C, 3n) at 60 MeV incident energy. Gamma-ray spectroscopy was performed with the 8 π spectrometer, an instrument comprising 20 Compton-suppressed HPGe detectors and 71 BGO ball elements. Evidence is presented for the identification of single-neutron quasiparticle bands corresponding to Nilsson orbitals [532] 5 2 , [523] 7 2 , [411] 1 2 and [402] 5 2 . The properties of the [523] 7 2 band suggest that the nucleus has a triaxial shape when that orbital is occupied. We draw attention to: (i) the signature splitting; (ii) staggering of B(E2) ratios; (iii) the mean Q 1 Q 2 ratio; and (iv) the comparison of the staggering in the B( M 1) B( E 2) ratios with the signature splitting. All these factors point to a γ-deformation in the neighbourhood of γ∼−20°. Based on the large B( M 1) B( E 2) ratios, the observed three-quasiparticle bands involve proton excitation. Of these bands, two have clear-cut structural assignments ν h 11 2 π( h 11 2 ) 2 and ν d 5 2 π( h 11 2 ) 2 . However, the third band presented difficulties and no assignment was consistent with all the data. The shift in crossing frequency for the π( h 11 2 ) 2 with a ν h 11 2 spectator is very large and corresponds to a shift in the routhian of about 220 keV. This we interpret to be due to a negative γ-deformation for bands involving ν h 11 2 . The opposite shift of ∼300 keV for the same crossing with a ν d 5 2 spectator cannot be explained as a shape polarization effect.

The transitional nucleus $^{86}\mathrm{Zr}$ has been studied by the $^{60}\mathrm{Ni}$${(}^{30}$Si,2p2n) reaction, with \ensuremath{\gamma}-\ensuremath{\gamma} coincidence, angular correlation, Doppler-shift-attenuation and measurements. Band structures exhibiting strongly staggered M1 transition rates were found at high spins, with large B(M1) values but no significant E2 collectivity. The unusual behavior of this nucleus is interpreted within the framework of an interacting boson model incorporating two broken boson pairs. This new model for high-spin states is seen to be remarkably successful in describing the electromagnetic decay properties in the four-quasiparticle regime.

States of 126Ba up to spin 36+ were populated in the reaction 96Zr(34S, 4n)126Ba at 155 MeV and up to spin 20+ in the reaction 116Sn(13C, 3n)126Ba at 56 MeV. Gamma-ray spectroscopy was performed with the 8π spectrometer, an instrument comprising 20 Campton-suppressed HPGe detectors and 71 BGO ball elements. A level scheme organized into fifteen rotational bands is proposed on the basis on the γγ-coincidence and γ-ray angular correlation data. The competing π h 11 2 and v h 11 2 band crossings are investigated and interpreted in terms of cranked shell model and total routhian surface calculations. Amongst the topics discussed are (i) a comparison of DCO and spin orientation techniques for determining spins and multipolarities. (ii) the addivity property of quasiparticle energies or routhians, and (iii) analysis of B(M1)/B(E2, ratios between signature partner bands. The possibility of couplings between vibrational and aligned quasiparticle structures is explored.

We have observed the yrast states of 152Dy to about spin 37 ~ and to about ]3 MeV excitation energy. Among these are 4 isomers with half-lives of 60us, 10ns, 1.4ns and ~70ps and spins of ~ 1 7 , 21, 2 7 and 31 ~. These results were obtained from very extensive y-Spectroscopic measurements using the (32S,4n) reaction, with pulsed 32S beams from the Chalk River MP Tandem. Figure I shows a plot of the energies of yrast states ~s I(I + i) which shows that for I > 14 ~ the data lie remarkably close to a straight line, indicating ~2 that E~ -~ I(l+ I). This quadratic dependence on spin of the yrast energy provides the

The AMS program at the Chalk River TASCC (Tandem Accelerator Superconducting Cyclotron) facility is directed toward 36 Cl measurements in hydrological and other samples. The automatic data acquisition/accelerator control system has been commissioned and progress is reported on particle identification with a Bragg-type detector and 36 S background control with a gas-filled beamline magnet. Evidence for large Cl memory effects in a Hiconex 860 sputter source is also presented.

Two excited superdeformed bands in $^{149}\mathrm{Gd}$ have been observed in \ensuremath{\gamma}-ray spectroscopy. Based on the behavior of the dynamical moments of inertia, we assign one band to a neutron excitation and the other to a proton excitation. The proton-excited-band \ensuremath{\gamma}-ray energies are nearly identical to those already known in $^{150}\mathrm{Tb}$. In addition, two new members of the $^{149}\mathrm{Gd}$ yrast cascade have been observed, extending the band to spin 135/2\ensuremath{\Elzxh}. The results are discussed in terms of high-N intruder orbitals in the cranked-shell model.

High-spin states in 124 Ba were populated via the 94 Zr( 34 S, 4n) 124 Ba and 110 Cd( 16 O, 2n) 124 Ba reactions at 145 MeV and 60 MeV, respectively. The yrast band has been extended to 34 ħ and seven side bands have been observed. Gamma-ray angular distributions with respect to the nuclear spin axis have been used along with directional correlation (DCO) ratios to establish spins and multipolarities. The competing π h 11 2 and ν h 11 2 crossings in this region are investigated and an interpretation of the observed crossings is proposed within the framework of the cranked shell model (CSM) and total routhian surface (TRS) calculations.

Equilibrium charge-state distributions have been measured for ∼0.2–1.0 MeV/u 63 Cu, 90 Zr, 107 Ag, 115 In, 127 I and 197 emerging from thin (10–30 μg/cm 2 ) carbon foils. In many cases, values deduced for the mean charge were found to deviate substantially (from 1 to 3 charge states) from those predicted by semi-empirical formulae. Highly asymmetric charge-state distributions were observed for 107 Ag and 115 In near 30 MeV, which are attributed to a shell effect at the boundary between the N- and M-shell electrons. The present results will be used to select the optimum charge-state combinations for heavy-ion beams from the tandem accelerator superconducting cyclotron (TASCC) at Chalk River.

The Chalk River Laboratories' Tandem Accelerator Superconducting Cyclotron (TASCC) facility comprises a 15 MV MP Tandem which can be used alone or in conjunction with a superconducting cyclotron postaccelerator. TASCC provides ions from protons to uranium and energies from a few MeV to almost 3000 MeV. The facility can thus cover the complete range of LET's required for SEE testing, with the added bonus of longer ranges to better simulate the effects of cosmic radiation. A simple irradiation chamber has been used for the study of individual devices for some time; a state-of-the-art SEE facility is being developed for testing circuit boards in vacuum or in air, This paper will describe the properties of TASCC, and the existing and planned irradiation systems.

The improved landmine detector system is a vehicle-mounted multi-sensor landmine detector which uses a thermal neutron activation (TNA) detector to confirm the presence of a mine by detecting the bulk nitrogen in its explosives. The input count rate to the TNA is very high and count rates of future systems could be an order of magnitude higher, allowing correspondingly higher sensitivity or shorter detection times for land mines. The chief bottleneck to achieving the maximum possible performance from the present TNA or a future system is the relatively slow fluorescent decay time of the NaI(Tl) scintillators which are currently used. This paper presents preliminary results from an experimental study into various fast scintillators as potential detector upgrades to the TNA confirmation detector, A number of fast, inorganic scintillators are compared with NaI(Tl) with respect to efficiency, energy resolution, available size and cost in the present application.

High-spin states have been identified in doubly odd $^{114}\mathrm{I}$ produced in the $^{60}\mathrm{Ni}$${(}^{58}$Ni,3pn\ensuremath{\gamma}) reaction. Two rotational band structures have been observed. One band exhibits no signature splitting, while the second exhibits a large signature splitting. These bands are interpreted in terms of \ensuremath{\pi}${\mathit{g}}_{9/2}$\ensuremath{\bigotimes}\ensuremath{\nu}${\mathit{h}}_{11/2}$ and \ensuremath{\pi}${\mathit{g}}_{7/2}$\ensuremath{\bigotimes}\ensuremath{\nu}${\mathit{h}}_{11/2}$ quasiparticle configurations, respectively.

States in neutron-deficient $^{110}\mathrm{Te}$ have been studied with the $^{58}\mathrm{Ni}$${+}^{58}$Ni reaction at 250 MeV. An unusual feature is that, above spin ${8}^{+}$, negative parity states become yrast. These states are interpreted in terms of a \ensuremath{\nu}[${\mathit{h}}_{11/2}$\ensuremath{\bigotimes}${\mathit{d}}_{5/2}$] structure (\ensuremath{\Delta}l=\ensuremath{\Delta}j=3). For spins above ${15}^{\mathrm{\ensuremath{-}}}$, strong (E1) dipoles are seen linking another band into the negative-parity states. This is taken as evidence for octupole correlations in $^{110}\mathrm{Te}$ at high spin.

Discrete superdeformed bands in $^{147,148}\mathrm{Gd}$ have been populated with asymmetric and symmetric fusion-evaporation reactions. The yields of the superdeformed bands compared with the total yield of the channel exhibit a large increase for symmetric reactions. We suggest an explanation for this effect in terms of the competition between neutron emission and fission at the early stage of the deexcitation process of the composite system.

129I and 36Cl were measured by accelerator mass spectrometry in 11 lichen samples (Parmelia sulcata) collected in 1990 from three regions (Novozybkov, Bragin and Ovruc) near Chernobyl. Previously measured activities of 137Cs were highest in the samples from the Novozybkov region while the measured activities of 36Cl and 129I in this study were highest in the samples from the Bragin region. The regional distribution patterns of the 36Cl and 129I show a positive correlation suggesting that these volatile radionuclides were deposited in the same manner.

The mechanism underlying Cl and I sample-to-sample interference in the new Chalk River AMS ion source has been studied and compared with the interference observed in an earlier ion source of different internal geometry. The distribution of sputtered material and its degree of migration was measured with the radioactive tracer, 82Br. The temperature dependence of the surface constitutent was measured with the elastic recoil detection (ERD) technique and the effect of sample geometry and ion source cleaning was studied with elevated (5 × 10−10) 36 Cl Cl and 129 I I samples. These measurements indicate that a hot (>350°C) aperture plate ahead of the sample can prevent the sputtering of contaminated regions near the sample. The plate itself remains relatively free of Cl or I itself since these elements or their Cs-gettered compounds are desorbed at this temperature. A small, fixed quantity of Cl or I on this surface is observed, which if sputtered by Cs+ ions, may contribute to ion source memory. Relative sample-to-sample interference for both Cl and I is about 10−3 after 20 min or 10−4 after 60 min.

A new sample magazine, sample changer and ion source have been developed and are in routine use at Chalk River. The system features a readily accessible 40-sample magazine at ground potential that is external to the ion source and high-voltage cage. The samples are held in an inert atmosphere and can be individually examined or removed; they can be exchanged en masse as a complete magazine concurrent with an AMS measurement. On-line sample changing is done with a pneumatic rabbit transfer system employing two stages of differential pumping. At Chalk River this is routinely performed across a 200 kV potential. Sample positioning is precise, and hundreds of 36Cl and 129I samples have been measured over a period of several days without interruption or alteration of ion source operating conditions.

At Chalk River, the final step in the identification of 129 I is a time-of-flight (TOF) measurement over a 2.5 m path. The start / stop signals are derived from microchannel plates that detect accelerated secondary electrons emitted from thin carbon films. This system provides a resolution of 400 ps (FWHM) for the 50 MeV iodine ions and the TOF peaks are closely Gaussian. Resolution is limited by the uniformity of the start-detector foil. Masses 128 and 130 are separeted from 129 I by about 2 ns. To improve transmission and to reduce energy straggling, the start detector has been constructed so that the beam is unobstructed by grids. In addition to TOF, the total energy ( E ) of the ions is measured with a passivated implanted planar silicon (PIPS) detector. Although 129 I standards and NaI blanks require only TOF for unambiguous identification of 129 I, many environmental samples are characterized by a plethora of peaks where both TOF and E are essential for unambiguous identification of 129 I.

Samples of porous Si (PS) were prepared from p -type 10–35 Ω-cm Si under anodisation in aqueous HF (20%) for 10 min. The composition of the PS was measured by heavy-ion elastic recoil detection analysis (HIERDA). In all samples a considerable carbon content varying from 2–20% was measured, while the oxygen and hydrogen concentrations ranged from 2–25% and 10–25%, respectively. The sample wax removing procedure was identified as a source for the high carbon content. The concentration of carbon and its influence on the photoluminescence due to different treatments of PS was investigated.

A thick foil of 238 U was bombarded with 209 Bi beams at 1130 and 1330 MeV, delivered by the TASCC facility at Chalk River Laboratories. Gamma-ray spectroscopy of states populated in multiple Coulomb excitation was performed with the 8π spectrometer, an instrument comprising 20 Compton-suppressed HPGe detectors and 71 BGO ball elements. The event trigger required that 3 BGO elements and 2 HPGe detectors fire in coincidence. The experiment achieved a high degree of sensitivity, the weakest rotational band observed had about 0.16% intensity of the ground state rotational band. Several bands were observed to high spin for the first time, including the γ-vibrational band (spin 27 + ) and the octupole bands with K = 0 (spin 31 − ), K = 1 (spin 28 − ) and K = 2 (spin 25 − ). Results for positive and negative parity vibrational bands are compared with cranked RPA calculations. Although this theory can explain some features of the data, many puzzling aspects remain to be explored.

High-spin states of the nucleus 48 Cr have been studied via particle-γ-γ spectroscopy, following the 28 Si( 28 Si,2α) reaction. A 44-element particle-detector array was used to isolate 48 Cr residues and to reduce γ-ray Doppler broadening. The collective band built upon the ground state has now been firmly established to spin 16 + , the highest possible in the isolated f 7 2 shell, and lifetimes of the four highest states have been measured from Doppler shifts. Although some of the ground-state band properties are well reproduced by recent fp -shell model calculations, a sharp reduction in E2 transition rates at the backbend is not.

The application of the elastic recoil detection technique utilizing heavy ions for the analysis of semiconductor samples is demonstrated. With this technique the depth profiles of the primary constituents as well as profiles of all impurities can be measured in one spectrum. Depending on the target material, a depth resolution down to 20 nm can be achieved. All elements except hydrogen can be detected with almost the same sensitivity, namely ∼1×1015 at/cm2 with 136 MeV I in a 30° recoil geometry. For hydrogen, the sensitivity is about four times better.

Rotational bands of 157 Ho have been populated via the 124 Sn( 37 Cl, 4n) reaction at beam energies of 155 and 165 MeV. Gamma-ray spectroscopy was performed using the 8 π spectrometer at Chalk River. Many rotational bands have been observed for the first time. A detailed level scheme is presented, containing approximately 380 transitions, and the quasiparticle structure of the various bands is discussed. Band termination has been observed in the yrast states. For strongly coupled bands, B (M1)/ B (E2) transition strength ratios are extracted and compared with previous measurements and theoretical expectations. Branching ratios for out-of-band E2 transitions are analysed to extract band mixing interaction strengths. Implications for rotational damping are considered. The interaction at the first backbend in the ground band is found to be strongly signature dependent; this is evidence for a signature-dependent triaxial shape of the nucleus.

The shape evolution of the Gd nuclei produced by the 124 Sn+ 30 Si reaction at a beam energy of 150 MeV has been studied as a function of spin by measuring the γ-ray decay of the giant dipole resonance. The nuclear deformation was extracted from the spectral shape and, independently, from the angular distribution with respect to the spin axis. Both measurements reveal that these nuclei remain prolate, with an increase in deformation as the spin increases. Sum-energy and multiplicity distributions have also been measured as a function of γ-ray energy.