88-Inch Cyclotron Research Articles

Magnetic Moments of States in 110Sn.

The semi-magic Sn isotopes with Z = 50 are the subject of extensive experimental and theoretical studies. The measured B(E2) values to the 21+ states for the neutron-deficient side of the isotope chain suggest enhanced collectivity when fewer particles are available if the proton shell is not broken. Magnetic moments which are sensitive to proton and neutron contributions to the wave functions of the states could provide critical and relevant information. Magnetic moments were previously measured only for the even stable and a few neutron-rich unstable Sn isotopes. A measurement of the g factors of excited states in 110Sn using the transient field technique was performed at the 88-Inch Cyclotron at the LBNL in Berkeley. The 110Sn nuclei were produced via an α-particle transfer to 106Cd.

Nondestructive synchronous beam current monitor.

A fast current transformer is mounted after the deflectors of the Berkeley 88-Inch Cyclotron. The measured signal is amplified and connected to the input of a lock-in amplifier. The lock-in amplifier performs a synchronous detection of the signal at the cyclotron second harmonic frequency. The magnitude of the signal detected is calibrated against a Faraday cup and corresponds to the beam intensity. It has exceptional resolution, long term stability, and can measure the beam current leaving the cyclotron as low as 1 nA.

Measurement of the 240Am production cross section via proton irradiation of 242Pu

Abstract A new nuclear reaction for the production of 240Am was experimentally investigated. Targets of 150 – 500 μg/cm2 242Pu on 2 μm Ti were produced through molecular deposition. Five irradiations, in which 242Pu, natTi, and natNi targets were jointly activated with protons from the Lawrence Berkeley National Laboratory 88-Inch Cyclotron produced 240Am, 48V, and 57Ni, respectively. The radioactive decay of these nuclides was monitored using high-purity Ge gamma ray detectors in the weeks following irradiation. A maximum 242Pu(p, 3n)240Am nuclear reaction cross section was measured to be 45 ± 13 mb with 23 MeV protons. While this value is lower than theoretical predictions, it is high enough to be the most viable nuclear reaction for the large-scale production of 240Am.

Production of high intensity 48Ca for the 88-Inch Cyclotron and other updates

Recently the Versatile ECR for NUclear Science (VENUS) ion source was engaged in a 60-day long campaign to deliver high intensity (48)Ca(11+) beam to the 88-Inch Cyclotron. As the first long term use of VENUS for multi-week heavy-element research, new methods were developed to maximize oven to target efficiency. First, the tuning parameters of VENUS for injection into the cyclotron proved to be very different than those used to tune VENUS for maximum beam output of the desired charge state immediately following its bending magnet. Second, helium with no oxygen support gas was used to maximize the efficiency. The performance of VENUS and its low temperature oven used to produce the stable requested 75 eμA of (48)Ca(11+) beam current was impressive. The consumption of (48)Ca in VENUS using the low temperature oven was checked roughly weekly, and was found to be on average 0.27 mg/h with an ionization efficiency into the 11+ charge state of 5.0%. No degradation in performance was noted over time. In addition, with the successful operation of VENUS the 88-Inch cyclotron was able to extract a record 2 pμA of (48)Ca(11+), with a VENUS output beam current of 219 eμA. The paper describes the characteristics of the VENUS tune used for maximum transport efficiency into the cyclotron as well as ongoing efforts to improve the transport efficiency from VENUS into the cyclotron. In addition, we briefly present details regarding the recent successful repair of the cryostat vacuum system.

Distinguishing fissions of 232Th, 237Np and 238U with beta-delayed gamma rays

Measurements of beta-delayed gamma-ray spectra following 14-MeV neutron-induced fissions of 232Th, 238U, and 237Np were conducted at Lawrence Berkeley National Laboratory’s 88-Inch Cyclotron. Spectra were collected for times ranging from 1min to 14h after irradiation. Intensity ratios of gamma-ray lines were extracted from the data that allow identification of the fissioning isotope.

Band structure of235U

Over a period of several years we have performed three separate experiments at Lawrence Berkeley National Laboratory's 88-Inch Cyclotron in which ${}^{235}$U (thick target) was Coulomb-excited. The program involved stand-alone experiments with Gammmasphere and with the 8pi Spectrometer using ${}^{136}$Xe beams at 720 MeV, and a CHICO-Gammasphere experiment with a ${}^{40}$Ca beam at 184 MeV. In addition to extending the known negative-parity bands to high spin, we have assigned levels in some seven positive-parity bands which are in some cases (e.g., $[631]1/2$, $[624]7/2$, and $[622]5/2$) strongly populated by $E3$ excitation. The CHICO data have been analyzed to extract $E2$ and $E3$ matrix elements from the observed yields. Additionally, many $M1$ matrix elements could be extracted from the $\ensuremath{\gamma}$-ray branching ratios. A number of new features have emerged, including the unexpected attenuation of magnetic transitions between states of the same Nilsson multiplet, the breakdown of Coriolis staggering at high spin, and the effect of $E3$ collectivity on Coriolis interactions.

To the continuum and beyond: Structure of U nuclei

An experiment was performed at the 88-inch cyclotron at LBNL to investigate the structure of uranium isotopes and concurrently test the so-called surrogate ratio method. A 28 MeV proton beam was used to bombard 236U and 238U targets and the outgoing light ions were detected using the STARS silicon telescope allowing isotopic assignments and the excitation energy of the compound nucleus to be measured. A fission detector was placed at backward angles to give particle-fission coincidences, while the six clover germanium detectors of the LIBERACE array were used for particle-γ coincidences. The (p,d) reaction channels on 236U and 238U targets were used as a surrogate to measure the σ(234U(n,f))/σ(236U(n,f)) cross section ratio. The results give reasonable agreement with literature values over an equivalent neutron energy range between 0 MeV and 6 MeV. Structure results in 235U include a new (3/2−) level at 1035 keV, that is tentatively assigned as the 3/2−[501] Nilsson state. The analogue 3/2−[501] state in 237U may be associated with a previously observed level at 1201 keV, whose spin/parity is restricted to Jπ = 3/2− on the basis of newly observed decays to the ground band.

Upgrade of the 88-Inch Cyclotron Power Amplifier

The radiofrequency (RF) system of the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory is a resonant system based on the quarter-wave cantilever-type resonating structure. Power is fed to the Dee from the anode of the 500 kW RCA 4648 tetrode tube operating in grounded cathode configuration, which is coupled to the side of the Dee stem. The tube is obsolete and makes its continued use impractical. A new final power amplifier was designed and built using the commercially available tube Eimac 4W150,000E. The new amplifier was successfully commissioned and has been reliable and easy to operate.

Utilizing ( p , d ) and ( p , t ) reactions to obtain ( n , f ) cross sections in uranium nuclei via the surrogate-ratio method

The surrogate ratio method has been tested for ($p,d$) and ($p,t$) reactions on uranium nuclei. ${}^{236}$U and ${}^{238}$U targets were bombarded with 28-MeV protons and the light ion recoils and fission fragments were detected using the Silicon Telescope Array for Reaction Studies detector array at the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory. The ($p$,$df$) reaction channels on ${}^{236}$U and ${}^{238}$U targets were used as a surrogate to determine the $\ensuremath{\sigma}$[${}^{236}$U($n$,$f$)]/$\ensuremath{\sigma}$[${}^{234}$U($n$,$f$)] cross-section ratio. The ($p$,$tf$) reaction channels were also measured with the same targets as a surrogate for the $\ensuremath{\sigma}$[${}^{235}$U($n$,$f$)]/$\ensuremath{\sigma}$[(${}^{233}$U($n$,$f$)] ratio. For the ($p$,$df$) and ($p$,$tf$) surrogate measurements, there is good agreement with accepted ($n$,$f$) values over equivalent neutron energy ranges of ${E}_{n}=0$--7 MeV and ${E}_{n}=0$--5.5 MeV, respectively. An internal surrogate ratio method comparing the ($p,d$) and ($p$,$t$) reaction channels on a single target is also discussed. The $\ensuremath{\sigma}$[${}^{234}$U($n$,$f$)]/$\ensuremath{\sigma}$[${}^{233}$U($n$,$f$)] and $\ensuremath{\sigma}$[${}^{236}$U($n$,$f$)]/$\ensuremath{\sigma}$[${}^{235}$U($n$,$f$)] cross-section ratios are extracted using this method for the ${}^{236}$U and ${}^{238}$U targets, respectively. The resulting fission cross-section ratios show relatively good agreement with accepted values up to ${E}_{n}\ensuremath{\sim}5$ MeV.

Recent progress on the superconducting ion source VENUS

The 28 GHz Ion Source VENUS (versatile ECR for nuclear science) is back in operation after the superconducting sextupole leads were repaired and a fourth cryocooler was added. VENUS serves as an R&D device to explore the limits of electron cyclotron resonance source performance at 28 GHz with its 10 kW gryotron and optimum magnetic fields and as an ion source to increase the capabilities of the 88-Inch Cyclotron both for nuclear physics research and applications. The development and testing of ovens and sputtering techniques cover a wide range of applications. Recent experiments on bismuth demonstrated stable operation at 300 eμA of Bi(31+), which is in the intensity range of interest for high performance heavy-ion drivers such as FRIB (Facility for Rare Isotope Beams). In addition, the space radiation effects testing program at the cyclotron relies on the production of a cocktail beam with many species produced simultaneously in the ion source and this can be done with a combination of gases, sputter probes, and an oven. These capabilities are being developed with VENUS by adding a low temperature oven, sputter probes, as well as studying the RF coupling into the source.

Nuclear spectroscopy of the heaviest elements: studies of 254No, 257Rf, and 261Sg

Recently it has become possible to perform detailed spectroscopy on nuclei beyond Z = 100 with the aim of understanding the underlying single-particle structure of superheavy elements. A number of such experiments have been performed at the 88-Inch Cyclotron of the Lawrence Berkeley National Laboratory using the Berkeley Gas-filled Separator (BGS), coupled with delayed γ-ray and electron-decay spectroscopy. Experiments have been performed on 254No (Z = 102), 257Rf (Z = 104), and 261Sg (Z = 106). The results provide new information on the properties of transactinide nuclei, which is important for testing models of the heaviest elements.

Albert Ghiorso

Albert Ghiorso, the celebrated nuclear scientist who co-discovered more elements than anyone in history and a prolific inventor of nuclear techniques and machines, passed away from natural causes on 26 December 2010 in Berkeley, California.Ghiorso was born in Vallejo, California, on 15 July 1915. He became fascinated with airplanes and aeronautics when he moved with his family to Alameda, near the Oakland airport. That exposure triggered his lifelong interest in machines, technology, and electronics. After high school he began experimenting with radio, and soon his projects were exceeding even military capabilities. On a scholarship, he attended the University of California, Berkeley, and in 1937 he received a BS in electrical engineering. His first job, with a firm developing Geiger counters, brought him in contact with the nuclear chemistry group at Berkeley. That led to an invitation from Glenn Seaborg to join him at the University of Chicago’s Metallurgical Laboratory, which was responsible for elaborating the chemistry of plutonium as part of the Manhattan Project. Thus began their lifetime collaboration, which eventually resulted in the addition of 12 new elements to the periodic table and numerous innovations in nuclear chemistry, radiation detection, and accelerator science.Ghiorso’s first work in Chicago involved measuring the energies of alpha particles from isotopes produced by irradiation of uranium, neptunium, and plutonium. Ghiorso and colleagues built an electromechanical 48-channel pulse-height analyzer, which they used to spectroscopically resolve the characteristic nuclear radiation, thus providing the first nonchemical means of isotope identification. In 1944–45 the Seaborg group used the reactor at the Clinton Laboratories (later Oak Ridge National Laboratory) and cyclotrons at Washington University in Saint Louis and at Berkeley to irradiate plutonium-239 with neutrons and deuterons. Ghiorso’s measurements showed the presence of two new elements, subsequently identified as atomic numbers 95 (americium) and 96 (curium).In 1946 Ghiorso and Seaborg returned to Berkeley, where they used the 60-inch cyclotron to bombard minute quantities of americium and curium with alpha particles. In concentrating on the chemicals he expected to find based on Seaborg’s proposed revision of the periodic table of elements, which introduced an actinide series mirroring the rare-earth lanthanides, Ghiorso showed that the radioactivity was due to two more new elements, 97 (berkelium) and 98 (californium).In 1952, following the first thermonuclear explosion, the Seaborg–Ghiorso group received samples of radioactive dust collected by an airplane. The group unambiguously identified the characteristic alpha-particle energies in the samples; that work provided the basis for the discoveries of elements 99 (einsteinium) and 100 (fermium). For elements beyond 100, contamination and background radiation made any new nuclides unobservable. Ghiorso solved that problem with a brilliant invention: the recoil technique. A nucleus that undergoes a nuclear reaction will recoil with enough kinetic energy to exit the target—so long as the target is very thin—and be mechanically collected. The technique made possible the discovery of element 101 (mendelevium) based on just 17 individual atoms.By the mid 1950s it became clear that in order to proceed to heavier nuclei, a new accelerator would be needed, and Berkeley built the Heavy Ion Linear Accelerator (HILAC). With the availability of carbon ions and a curium target, Ghiorso and the HILAC group devised a new technique in which a radioactive element could be identified using its daughter decay products. That enabled the identification of elements 102 (nobelium) and 103 (lawrencium). Subsequently, using targets of californium and berkelium and HILAC ions of nitrogen and oxygen, the group identified elements 104 (rutherfordium), 105 (hahnium, later renamed dubnium), and finally, in 1974, element 106 (seaborgium).Over approximately 30 years, Ghiorso participated in and led groups that discovered dozens of new isotopes in addition to 12 new elements. That work was central to proving the correctness of Seaborg’s revision of the periodic table and to extending the limits of nuclear stability at the top of the chart of nuclides. Ghiorso developed several innovative accelerator concepts for higher-energy heavy-ion beams, including the Omnitron and the Bevalac. The Bevalac opened two new research fields: relativistic heavy-ion collisions and radiation therapy with ion beams. Ghiorso published more than 160 papers in nuclear science, most in Physical Review.Albert Ghiorso became a legend and a metaphor for inspired nuclear research. Physicists and chemists worldwide knew the significance of his work, and those fortunate enough to know him personally were in awe of his brilliance, his hard work and ability to get results when others discounted his chances, and his magical ability to inspire others to devote their careers to the quest. All was done with a quiet, modest personality that belied his power and endeared him to his colleagues. When I remarked to him that nobody gets out of life alive, he replied, with characteristic sagacity, “Well, I don’t regard that as proven!”Albert GhiorsoPeter ArmbrusterPPT|High resolution© 2011 American Institute of Physics.

Integrating a Traveling-Wave Tube into an AECR-U Ion Source

A radio frequency system of 500 W output power from 10.75 to 12.75 GHz frequency range was designed and integrated into the Advanced Electron Cyclotron Resonance-Upgrade (AECR-U) ion source of the 88-inch cyclotron at Lawrence Berkeley National Laboratory. The AECR-U produces ion beams for the cyclotron, giving large flexibility of ion species and charge states. The broadband frequency of a traveling-wave tube (TWT) allows modifying the volume that couples and heats the plasma. The TWT system design and integration with the AECR-U ion source and results from commissioning are presented.

The (3He, tf) as a surrogate reaction to determine (n, f) cross sections in the 10–20 MeV energy range

The surrogate reaction 238U(3He,tf) is used to determine the 237Np(n,f) cross section indirectly over an equivalent neutron energy range from 10 to 20MeV. A self-supporting ∼761μg/cm2 metallic 238U foil was bombarded with a 42MeV 3He2+ beam from the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory (LBNL). Outgoing charged particles and fission fragments were identified using the Silicon Telescope Array for Reaction Studies (STARS) consisted of two 140μm and one 1000μm Micron S2 type silicon detectors. The 237Np(n,f) cross sections, determined indirectly, were compared with the 237Np(n,f) cross section data from direct measurements, the Evaluated Nuclear Data File (ENDF/B-VII.0), and the Japanese Evaluated Nuclear Data Library (JENDL 3.3) and found to closely follow those datasets. Use of the (3He,tf) reaction as a surrogate to extract (n,f) cross sections in the 10–20MeV equivalent neutron energy range is found to be suitable.

Comparison of complementary reactions in the production of Mt

Author(s): Nelson, Sarah; Gregorich, Kenneth; Dragojevic, Irena; Ellison, Paul; Garcia, Mitch Andre; Gates, Jacklyn; Stavsetra, Liv; Ali, Mazhar; Nitsche, Heino | Abstract: The new reaction 208Pb(59Co,n)266Mt was studied using the Berkeley Gas-filled Separator at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron. A cross section of 7.7+5.2-3.3 pb was measured at a compound nucleus excitation energy of 14.9 MeV. The measured decay properties of 266Mt and its daughters correspond well with existing data. We compare this experimental result to transactinide compound nucleus formation model predictions, and the previously studied 209Bi(58Fe,n)266Mt reaction.

Measurement of thePb208(Cr52,n)Sg259excitation function

The excitation function for the {sup 208}Pb({sup 52}Cr,n){sup 259}Sg reaction has been measured using the Berkeley Gas-filled Separator at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron. The maximum cross section of 320{sub -100}{sup +110} pb is observed at a center-of-target laboratory-frame energy of 253.0 MeV. In total, 25 decay chains originating from {sup 259}Sg were observed and the measured decay properties are in good agreement with previous reports. In addition, a partial excitation function for the {sup 208}Pb({sup 52}Cr,2n){sup 258}Sg reaction was obtained, and an improved {sup 258}Sg half-life of 2.6{sub -0.4}{sup +0.6} ms was calculated by combining all available experimental data.

New isotopeHs263

A new isotope of Hs was produced in the reaction $^{208}\mathrm{Pb}(^{56}\mathrm{Fe},n)^{263}\mathrm{Hs}$ at the 88-Inch Cyclotron of the Lawrence Berkeley National Laboratory. Six genetically correlated nuclear decay chains have been observed and assigned to the new isotope $^{263}\mathrm{Hs}$. The measured cross section was ${21}_{\ensuremath{-}8.4}^{+13}$ pb at 276.4 MeV lab frame center-of-target beam energy. $^{263}\mathrm{Hs}$ decays with a half-life of $0.{74}_{\ensuremath{-}0.21}^{+0.48}$ ms by $\ensuremath{\alpha}$-decay and the measured $\ensuremath{\alpha}$-particle energies are 10.57 \ifmmode\pm\else\textpm\fi{} 0.06, 10.72 \ifmmode\pm\else\textpm\fi{} 0.06, and 10.89 \ifmmode\pm\else\textpm\fi{} 0.06 MeV. The experimental cross section is compared to a theoretical prediction based on the Fusion by Diffusion model [W. J. \ifmmode \acute{S}\else \'{S}\fi{}wi\k{a}tecki et al., Phys. Rev. C 71, 014602 (2005)].

Search for oscillation of the electron-capture decay probability of 142Pm

We have searched for time modulation of the electron capture decay probability of 142Pm in an attempt to confirm a recent claim from a group at the Gesellschaft für Schwerionenforschung (GSI). We produced 142Pm via the 124Sn(23Na, 5n)142Pm reaction at the Berkeley 88-Inch Cyclotron with a bombardment time short compared to the reported modulation period. Isotope selection by the Berkeley Gas-filled Separator is followed by implantation and a long period of monitoring the 142Nd Kα x-rays from the daughter. The decay time spectrum of the x-rays is well-described by a simple exponential and the measured half-life of 40.68(53) seconds is consistent with the accepted value. We observed no oscillatory modulation at the proposed frequency at a level 31 times smaller than that reported by Litvinov et al. [Y.A. Litvinov, et al., Phys. Lett. B 664 (2008) 162]. A literature search for previous experiments that might have been sensitive to the reported modulation uncovered another example in 142Eu electron-capture decay. A reanalysis of the published data shows no oscillatory behavior.

Comparison of complementary reactions for the production ofBh261,262

Two heavy-ion induced fusion reactions producing $^{261,262}\mathrm{Bh}$ were studied using the Berkeley Gas-filled Separator at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron. An excitation function for the production of $^{262}\mathrm{Bh}$ via the reaction ${}^{209}\mathrm{Bi}({}^{54}\mathrm{Cr},n){}^{262}\mathrm{Bh}$ was measured with a maximum cross section from a fit to the data of 430 \ifmmode\pm\else\textpm\fi{} 110 pb observed at a compound nucleus excitation energy of 15.7 MeV. New data have been measured for the 1$n$ exit channel of the ${}^{208}\mathrm{Pb}({}^{55}\mathrm{Mn}, n){}^{262}\mathrm{Bh}$ reaction. We present an updated excitation function with an observed maximum cross section of $530\ifmmode\pm\else\textpm\fi{}100$ pb at a compound nucleus excitation energy of 14.1 MeV. Events corresponding to the 2$n$ exit channel for the ${}^{209}\mathrm{Bi}({}^{54}\mathrm{Cr},2n){}^{261}\mathrm{Bh}$ and ${}^{208}\mathrm{Pb}({}^{55}\mathrm{Mn},2n){}^{261}\mathrm{Bh}$ reactions were also observed and are presented as partial excitation functions. The measured decay properties correspond well with existing experimental data. We compare these experimental results to cross section predictions from a model by \ifmmode \acute{S}\else \'{S}\fi{}wia\ifmmode \mbox{\k{}}\else \k{}\fi{}tecki et al. and discuss entrance channel effects on the magnitude of 1$n$ cross sections.

Influence of projectile neutron number in the208Pb(48Ti, n)255Rfand208Pb(50Ti, n)257Rfreactions

Four isotopes of rutherfordium,254-257Rf, were produced by the 208Pb(48Ti, xn)256-xRf and 208Pb(50Ti, xn)258-xRf reactions (x = 1, 2) at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron. Excitation functions were measured for the 1n and 2n exit channels. A maximum likelihood technique, which correctly accounts for the changing cross section at all energies subtended by the targets, was used to fit the 1n data to allow a more direct comparison between excitation functions obtained under different experimental conditions. The maximum 1n crosssections of the 208Pb(48Ti, n)255Rf and 208Pb(50Ti, n)257Rf reactions obtained from fits to the experimental data are 0.38 +/- 0.07 nb and 40 +/-5 nb, respectively. Excitation functions for the 2n exit channel were also measured, with maximum cross sections of nb for the 48Ti induced reaction, and 15.7 +/- 0.2 nb for the 50Ti induced reaction. The impact of the two neutron difference in the projectile on the 1n cross section is discussed. The results are compared to the Fusion by Diffusion model developed by Swiatecki, Wilczynska, and Wilczynski.