Momentum Achromat Research Articles

Multinucleon Transfer in 25 MeV/nucleon 86Kr + 64Ni, 124Sn Peripheral Collisions

In this work we investigate multinucleon transfer reaction channels in peripheral collisions between a 86Kr projectile at 25 MeV/nucleon with 64Ni and 124Sn targets. The experimental data of these reactions were obtained with the Momentum Achromat Recoil Separator (MARS) at Texas A&M University in previous works of our group. We extracted and compared experimental mass and momentum per nucleon distributions of ejectiles with a two step model calculation. For the interaction of the projectile with the target, the phenomenological Deep Inelastic Transfer model (DIT) and the microscopic Constrained Molecular Dynamics model (CoMD) were used, both followed by the GEMINI model for the de-excitation of the primary excited projectile-like fragments. Generally, both DIT and CoMD appear to describe the trend of the data for both systems. However, possible further improvements in the models may be necessary. In conjunction with our recent work with a 86Kr beam at 15 MeV/nucleon with a 64Ni target, this study provides further insights into the reaction mechanisms of peripheral collisions in the Fermi energy regime and the production of neutron rich nuclides.

Measurement of fusion-evaporation cross sections for reactions of Ca44 with Gd154,156,157,160 targets

Background: Previously reported fusion-evaporation cross sections of residues in $^{45}\mathrm{Sc}$-induced reactions with lanthanide targets are much smaller than $^{48}\mathrm{Ca}$-induced reactions on the same targets. $^{44}\mathrm{Ca}$ is one proton removed from $^{45}\mathrm{Sc}$ and could be used to produce nuclei with a relative neutron content between those produced in the $^{45}\text{Sc-}$ and $^{48}\mathrm{Ca}$-induced reactions.Purpose: Several experiments worldwide have attempted to discover elements beyond the currently heaviest known element, oganesson ($Z=118$). Due to a lack of appropriate targets, these efforts focused on projectiles other than $^{48}\mathrm{Ca}$, which has been widely used for a number of successful element discovery experiments. The present study continues our previous work to understand the influence of various projectiles on the compound nucleus in fusion-evaporation reactions, and addresses the influence of target neutron number on fusion-evaporation cross sections.Methods: In experiments performed at the Cyclotron Institute at Texas A University, a beam of $^{44}\mathrm{Ca}^{6+}$ with an energy of $\ensuremath{\approx}5\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}/u$ was delivered by the K500 superconducting cyclotron to the Momentum Achromat Recoil Spectrometer (MARS). The $^{44}\mathrm{Ca}$ projectiles bombarded various isotopically enriched Gd targets in the MARS target chamber to create evaporation residues, which were spatially separated from unreacted projectiles by MARS and identified via their characteristic $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{decay}$ energies. Excitation functions for the reactions of $^{44}\mathrm{Ca}$ with $^{154,156,157,160}\mathrm{Gd}$ were measured at several projectile energies each.Results: The maximum $4n$ cross sections in the $^{44}\mathrm{Ca}{+}^{154,156,157,160}\mathrm{Gd}$ reactions were $0.038\ifmmode\pm\else\textpm\fi{}0.008, 0.83\ifmmode\pm\else\textpm\fi{}0.08, 3.8\ifmmode\pm\else\textpm\fi{}0.2$, and $3.0\ifmmode\pm\else\textpm\fi{}0.5$ mb, respectively. Production cross sections for the more neutron-rich targets were surprisingly constant even given the substantial changes in the difference in neutron binding energy and fission barrier of the compound nuclei.Conclusions: Collective enhancements to level density caused a reduction in compound nucleus survivability for all targets. While this effect was required to obtain good agreement between theoretical calculations and experimental data, it was not sufficient to explain the cross sections for the reaction with the most neutron-deficient target studied $(^{154}\mathrm{Gd})$. Instead, it appears that the difference in the fission barrier and neutron binding energy is the dominant factor affecting the survival of the compound nucleus in this case.

The Super Separator Spectrometer (S3) for the SPIRAL2 facility

The Super Separator Spectrometer S3 is, with the NFS (Neutrons For Science) facility, a major experimental system developed for SPIRAL2. It is designed for very low cross section experiments at low (<15MeV/u) energy. It will receive the very high intensity (more than 1pμA) stable ion beams accelerated by the superconducting LINAG accelerator of SPIRAL2. S3 will be notably used for the study of rare nuclei produced by fusion evaporation reactions, such as superheavy elements and neutron-deficient isotopes. Such experiments require a high transmission of the products of interest but also a separation of these nuclei from unwanted species. Hence S3 must have a large acceptance but also a high selection power including physical mass resolution. These properties are reached with the use of seven large aperture superconducting quadrupole triplets which include sextupolar and octupolar corrections in a two - stage separator (momentum achromat followed by a mass spectrometer) that can be coupled to the SIRIUS implantation-decay spectroscopy station or to the REGLIS3 gas cell with laser ionization to provide very pure beams for low energy experiments. S3 is now in the construction phase. We will present the scientific objectives of S3 as well as the current status of the facility and its different elements.

SU-F-T-368: Improved HPGe Detector Precise Efficiency Calibration with Monte Carlo Simulations and Radioactive Sources

Purpose: To obtain an improved precise gamma efficiency calibration curve of HPGe (High Purity Germanium) detector with a new comprehensive approach. Methods: Both of radioactive sources and Monte Carlo simulation (CYLTRAN) are used to determine HPGe gamma efficiency for energy range of 0–8 MeV. The HPGe is a GMX coaxial 280 cm3 N-type 70% gamma detector. Using Momentum Achromat Recoil Spectrometer (MARS) at the K500 superconducting cyclotron of Texas A&M University, the radioactive nucleus 24 Al was produced and separated. This nucleus has positron decays followed by gamma transitions up to 8 MeV from 24 Mg excited states which is used to do HPGe efficiency calibration. Results: With 24 Al gamma energy spectrum up to 8MeV, the efficiency for γ ray 7.07 MeV at 4.9 cm distance away from the radioactive source 24 Al was obtained at a value of 0.194(4)%, by carefully considering various factors such as positron annihilation, peak summing effect, beta detector efficiency and internal conversion effect. The Monte Carlo simulation (CYLTRAN) gave a value of 0.189%, which was in agreement with the experimental measurements. Applying to different energy points, then a precise efficiency calibration curve of HPGe detector up to 7.07 MeV at 4.9 cm distance away from the source 24 Al was obtained. Using the same data analysis procedure, the efficiency for the 7.07 MeV gamma ray at 15.1 cm from the source 24 Al was obtained at a value of 0.0387(6)%. MC simulation got a similar value of 0.0395%. This discrepancy led us to assign an uncertainty of 3% to the efficiency at 15.1 cm up to 7.07 MeV. The MC calculations also reproduced the intensity of observed single-and double-escape peaks, providing that the effects of positron annihilation-in-flight were incorporated. Conclusion: The precision improved gamma efficiency calibration curve provides more accurate radiation detection and dose calculation for cancer radiotherapy treatment.

AstroBox: A novel detection system for very low-energy protons from β-delayed proton decay

An instrument, AstroBox, has been developed to perform low energy proton spectroscopy from β-delayed proton emitters of interest to astrophysics studies. Energetic precursor nuclei are identified and stopped in the gas volume of the detector. The subsequent β or β-proton decay traces ionized paths in the gas. The ionization electrons are drifted in an electric field and are amplified with a Micro Pattern Gas Amplifier Detector (MPGAD). The system was tested in-beam using the β-delayed proton-emitter 23Al, which was produced with the p(24Mg,23Al)2n reaction and separated with the Momentum Achromat Recoil Spectrometer (MARS) at the Cyclotron Institute at Texas A&M University. Off-beam proton spectra have essentially no β background down to ∼100keV and have a resolution of ∼15keV (fwhm) for proton-decay lines at Ep=197 and 255keV. Lines with βp-branching as low as 0.02% are observed. In addition, the device also gives good mass and charge resolution for energetic heavy ions measured in-beam.

Prospects for the discovery of the next new element: Influence of projectiles with Z > 20

The possibility of forming new superheavy elements with projectiles having Z > 20 is discussed. Current research has focused on the fusion of 48Ca with actinide targets, but these reactions cannot be used for new element discoveries in the future due to a lack of available target material. The influence on reaction cross sections of projectiles with Z > 20 have been studied in so-called analog reactions, which utilize lanthanide targets carefully chosen to create compound nuclei with energetics similar to those found in superheavy element production. The reactions 48Ca, 45Sc, 50Ti, 54Cr + 159Tb, 162Dy have been studied at the Cyclotron Institute at Texas A&M University using the Momentum Achromat Recoil Spectrometer. The results of these experimental studies are discussed in terms of the influence of collective enhancements to level density for compound nuclei near closed shells, and the implications for the production of superheavy elements. We have observed no evidence to contradict theoretical predictions that the maximum cross section for the 249Cf(50Ti, 4n)295120 and 248Cm(54Cr, 4n)298120 reactions should be in the range of 10-100 fb.

Implantation-decay station for low-energy [formula omitted] proton measurements

We have built an implantation-decay station for β-delayed proton and α decay studies at the focal plane of the Momentum Achromat Recoil Spectrometer (MARS) at the Cyclotron Institute of Texas A&M University. Energetic secondary beams with a small momentum spread are stopped in a controlled manner into a very thin silicon strip detector. In addition, high-purity germanium detectors are installed for γ ray detection. Here we give a description of the setup and the observed performance down to Ep≈200keV using implanted 23Al and 31Cl sources.

Development of the MARS separator for heavy element studies

The Momentum Achromat Recoil Spectrometer (MARS) is a high-resolution separator at the Cyclotron Institute at Texas A&M University. MARS consists of an achromatic section followed by a velocity filter, and its suitability as a separator for heavy element research has been studied. Alpha particle sources have been used to measure the acceptance of the velocity filter and extend its calibration to slow ions (v/c≈2%). An online method for measuring the beam dose using “Rutherford” scattering has been implemented. The performance of the entire system has been checked in experiments to produce At and Po isotopes with the 40Ar+165Ho reaction, and a beam rejection factor of ≳1015 has been obtained. The setup is currently suitable for measuring cross sections on the order of millibarns and microbarns, and further improvements in sensitivity are planned.

Experimental study ofβ-delayed proton decay ofAl23for nucleosynthesis in novae

The $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ and proton decay of $^{23}\mathrm{Al}$ has been studied with an alternative detector setup at the focal plane of the momentum achromat recoil separator MARS at Texas A University. We could detect protons down to an energy of 200 keV and determine the corresponding branching ratios. Contrary to results of previous $\ensuremath{\beta}$-decay studies, no strong proton intensity from the decay of the isobaric analog state (IAS) of the $^{23}\mathrm{Al}$ ground state at ${E}_{x}=7803$ keV in $^{23}\mathrm{Mg}$ was observed. Instead we assign the observed low-energy group ${E}_{p,\mathrm{c}.\mathrm{m}.}=206$ keV to the decay from a state that is 16 keV below the IAS. We measured both proton and gamma branches from the decay of this state at ${E}_{x}=7787$ keV in $^{23}\mathrm{Mg}$, which is a very rare case in the literature. Combining our data with its measured lifetime, we determine its resonance strength to be $\ensuremath{\omega}\ensuremath{\gamma}={1.4}_{\ensuremath{-}0.4}^{+0.5}$ meV. The value is in agreement with older direct measurements, but disagrees with a recent direct measurement. This state is the most important resonance for the radiative proton capture $^{22}\mathrm{Na}$($p,\ensuremath{\gamma}$)$^{23}\mathrm{Mg}$ in some astrophysical environments, such as novae.

Fusion-evaporation studies with the Super Separator spectrometer (S3) at Spiral2

The Super Separator Spectrometer S3 is a device designed for experiments with the very high intensity stable ion beam of the superconducting linear accelerator of the SPIRAL2 facility. Its Physics goals cover the study of radioactive ions produced by fusion-evaporation reactions, like superheavy elements or neutron deficient nuclei close to the proton drip line, but also neutron rich nuclei produced by multi-nucleon transfer reactions as well as ion-ion atomic interactions. It is composed of a two-step separator, with a momentum achromat followed by a mass spectrometer. Superconducting multipole triplets, combining quadruple, sextuple and octupole fields, allow a combination of high transmission and mass resolution. A specific open multipole has been designed to stop the high beam power at the first momentum dispersive plane. A decay spectroscopy detection set-up or a low energy branch can be coupled to S3 for a wide range of studies.

SUPER SEPARATOR SPECTROMETER FOR THE LINAG HEAVY ION BEAMS

The Super Separator Spectrometer (S3) will receive the very high intensity heavy ion beams from the LINAG accelerator of SPIRAL2. Its privileged fields of physics are the delayed study of rare nuclei and secondary reactions with exotic nuclei. The project is presently in a phase of conceptual design. It includes a rotating target to sustain the high energy deposit, a two stages separator (momentum achromat) and spectrometer (mass spectrometer). Various detection set-ups are foreseen, especially a delayed α, γ, and electron spectroscopy array and a gas catcher coupled to a low energy branch. We present here the current status of the project and its main features.

SU-GG-T-240: New Method of An HPGe Detector Precise Efficiency Calibration with Experimental Measurements and Monte Carlo Simulations

Purpose: Developing a method of an HPGe detector precise γ efficiency calibration which is very important for accurate radiation detection during cancer radiotherapy practices. Method and Materials: radioactive nucleus produced and separated with Momentum Achromat Recoil Spectrometer (MARS) at the K500 superconducting cyclotron of Texas A&M University has positron decays followed by γ transitions up to 8 MeV from excited states, which is used for a β-γ coincidence measurement with a 1-mm-thick BC404 plastic scintillator, an HPGe detector and a fast tape-transport system to calibrate the HPGe detector. Results: By carefully considering the effects of summing, positron annihilation, internal conversion, and β detector efficiency when analyzing spectrum, we got the efficiency for γ-ray 7070 keV at 49 mm distance away from the source sample , which was 0.192(6)%. The Monte Carlo (MC) simulation with CYLTRAN code gave a value of 0.189%, which was in agreement with our measurements. The precise efficiency calibration curve of the HPGe detector up to 7070 KeV at 49 mm distance away from the source sample was obtained. By using the same procedure, we got the efficiency for the 7070 keV γ-ray at 151 mm distance away from the source sample , which was 0.0385(8)%. MC simulation value was 0.0399%, which differed from measurement by 4(2)%. This discrepancy led us to assign an uncertainty of 4% to our efficiencies at 151 mm up to 7070 KeV. The Monte Carlo calculations also reproduced the intensity of observed single-and double-escape peaks, providing that the effects of positron annihilation-in-flight were incorporated. Conclusion: A new method was established. The precise calibration curves obtained from this work are useful for accurate radiation detection and improving quality control to quality assurance (QA) for intensity-modulated radiation therapy (IMRT). Research sponsored by Department of Energy and Robert Welch Foundation.

β decay of proton-rich nucleusAl23and astrophysical consequences

We present the first study of the \ensuremath{\beta} decay of $^{23}\mathrm{Al}$ undertaken with pure samples. The study was motivated by nuclear astrophysics questions. Pure samples of $^{23}\mathrm{Al}$ were obtained from the momentum achromat recoil separator (MARS) of Texas A University, collected on a fast tape-transport system, and moved to a shielded location where \ensuremath{\beta} and \ensuremath{\beta}-\ensuremath{\gamma} coincidence measurements were made. We deduced \ensuremath{\beta} branching ratios and log $\mathit{ft}$ values for transitions to states in $^{23}\mathrm{Mg}$, and from them determined unambiguously the spin and parity of the $^{23}\mathrm{Al}$ ground state to be ${J}^{\ensuremath{\pi}}=5/{2}^{+}$. We discuss how this excludes the large increases in the radiative proton capture cross section for the reaction $^{22}\mathrm{Mg}(p,\ensuremath{\gamma})^{23}\mathrm{Al}$ at astrophysical energies, which were implied by claims that the spin and parity is ${J}^{\ensuremath{\pi}}=1/{2}^{+}$. The log $\mathit{ft}$ for the Fermi transition to its isobaric analog state (IAS) in $^{23}\mathrm{Mg}$ is also determined for the first time. This IAS and a state 16 keV below it are observed, well separated in the same experiment for the first time. We can now solve a number of inconsistencies in the literature, exclude strong isospin mixing claimed before, and obtain a new determination of the resonance strength. Both states are resonances in the $^{22}\mathrm{Na}$($p,\ensuremath{\gamma}$)$^{23}\mathrm{Mg}$ reaction at energies important in novae. The reactions $^{22}\mathrm{Mg}(p,\ensuremath{\gamma})^{23}\mathrm{Al}$ and $^{22}\mathrm{Na}(p,\ensuremath{\gamma})^{23}\mathrm{Mg}$ have both been suggested as possible candidates for diverting some of the flux in oxygen-neon novae explosions from the $A=22$ into the $A=23$ mass chain.

A recoil mass spectrometer for HHIRF

A recoil mass spectrometer (RMS) is to be built that will carry out a broad research program in heavy-ion science at the Holifield Heavy Ion Research Facility at Oak Ridge, Tennessee. The RMS will make possible the study of otherwise inaccessible exotic nuclei. Careful attention has been given to match the RMS to all the beams available from the HHIRF accelerators, including those beams with the highest energy as well as massive particles for use in inverse reactions. The RMS is to be a momentum achromat followed by a split electric dipole mass spectrometer of the type operating at NSRL at the University of Rochester. The RMS is essential for many of the proposed experiments on short lived and/or low cross section products. The spectrometer design will be discussed, with attention paid to the aberrations present and their corrections.

MARS: a status report

We are building a momentum achromat recoil spectrometer (MARS) for use with the new K500 superconducting cyclotron at Texas A&M University. MARS uses a unique optical design utilizing two dispersive planes to combine a momentum achromat with a recoil mass spectrometer. This configuration makes MARS applicable to a broad range of nuclear reaction studies utilizing inverse kinematics. It also leads to a system that is well matched to the range of secondary particle energies that will be produced in reactions with K500 beam. MARS will have a typical mass resolution of δM M ≈ 1 300 , with an energy acceptance of ± 9% ΔE E and a geometric solid angle of up to 9 msr. A beam swinger system will allow reaction products in the angular range 0° to 30° to be studied. MARS will be used to study both the excited states and decay properties of very proton- and neutron-rich nuclei. MARS will also be used to provide a reaction mechanism filter to assist investigations of the dynamics of heavy ion collisions and to produce secondary radioactive beams for reaction and spectroscopic studies of particular interest for nuclear astrophysics. We briefly describe the design of MARS, give a status report on its construction and an overview of the scientific program planned for it.

MARS: A momentum achromat recoil spectrometer

We are building a Momentum Achromat Recoil Spectrometer (MARS) for use with the new K500 superconducting cyclotron at Texas A&M University. MARS uses a unique optical design utilizing two dispersive planes to combine a momentum achromat with a recoil mass spectrometer. This configuration makes MARS applicable to a broad range of nuclear reaction studies utilizing inverse kinematics. It also leads to a system that is well matched to the range of secondary particle energies that will be produced in reactions with K500 beams. MARS will have a typical mass resolution of δM M ≅ 1 300 , with an energy acceptance of ± 9% ΔE E and a geometric solid angle of up to 9 msr. A beam swinger system will allow reaction products in the angular range 0° to 30° to be studied. MARS will be used to study both the excited states and decay properties of very proton- and neutron-rich nuclei. MARS will also be used to provide a reaction mechanism filter to assist investigations of the dynamics of heavy ion collisions and to produce secondary radioactive beams for reaction and spectroscopic studies of particular interest for nuclear astrophysics. We describe the design of MARS, together with a brief discussion of the scientific program planned for it.