arXiv subjects

X. W. Ma

Publications and source records attributed to X. W. Ma.

15 recordsLinked to original sources

arXiv2022

X-ray morphology due to charge-exchange emissions used to study the global structure around Mars

Soft x-ray emissions induced by solar wind ions that collide with neutral material in the solar system have been detected around planets, and were proposed as a remote probe for the solar wind interaction with the Martian exosphere. A multi-fluid three-dimensional magneto-hydrodynamic model is adopted to derive the global distributions of solar wind particles. Spherically symmetric exospheric H, H$_2$, He, O, and CO$_2$ density profiles and a sophisticated hybrid model that includes charge-exchange and proton/neutral excitation processes are used to study the low triplet line ratio $G=\frac{i+f}{r}$ (0.77$\pm$0.58) of O VII and total x-ray luminosity around Mars. We further calculate the emission factor $α$-value with different neutrals over a wide ion abundance and velocity ranges. Our results are in good agreement with those of previous reports. The evolution of the charge stage of solar wind ions shows that sequential recombination due to charge-exchange can be negligible at the interaction region. This only appears below the altitude of 400~km. The anonymous low disk $G$ ratio can be easily explained by the collisional quenching effect at neutral densities higher than 10$^{11}$cm$^{-3}$. However, the quenching contribution is small in Mars' exosphere and only appears below 400~km. Charge-exchange with H$_2$ and N$_2$ is still the most likely reason for this low $G$-ratio. X-ray emissivity maps in collisions with different neutrals differ from each other. A clear bow shock in the collision with all the neutrals is in accordance with previous reports. The resulting total x-ray luminosity of 6.55~MW shows a better agreement with the XMM-Newton observation of 12.8$\pm$1.4~MW than that of previous predictions.

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arXiv2019

New Test of Modulated Electron Capture Decay of Hydrogen-Like $^{142}$Pm Ions: Precision Measurement of Purely Exponential Decay

An experiment addressing electron capture (EC) decay of hydrogen-like $^{142}$Pm$^{60+}$ ions has been conducted at the experimental storage ring (ESR) at GSI. The decay appears to be purely exponential and no modulations were observed. Decay times for about 9000 individual EC decays have been measured by applying the single-ion decay spectroscopy method. Both visually and automatically analysed data can be described by a single exponential decay with decay constants of 0.0126(7) s$^{-1}$ for automatic analysis and 0.0141(7) s$^{-1}$ for manual analysis. If a modulation superimposed on the exponential decay curve is assumed, the best fit gives a modulation amplitude of merely 0.019(15), which is compatible with zero and by 4.9 standard deviations smaller than in the original observation which had an amplitude of 0.23(4).

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arXiv2019

Masses of ground and isomeric states of $^{101}$In and configuration-dependent shell evolution in odd-$A$ indium isotopes

We report first precision mass measurements of the $1/2^-$ isomeric and $9/2^+$ ground states of $^{101}$In. The determined isomeric excitation energy continues a smooth trend of odd-$A$ indium isotopes up to the immediate vicinity of $N=50$ magic number. This trend can be confirmed by dedicated shell model calculations only if the neutron configuration mixing is considered. We find that the single particle energies are different for different states of the same isotope. The presented configuration-dependent shell evolution, type II shell evolution, in odd-$A$ nuclei is discussed for the first time. Our results will facilitate future studies of single-particle neutron states.

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arXiv2019

Masses of neutron-rich $^{\operatorname{52-54}}$Sc and $^{54,56}$Ti nuclides: The $N=32$ subshell closure in scandium

Isochronous mass spectrometry has been applied in the storage ring CSRe to measure the masses of the neutron-rich $^{\operatorname{52-54}}$Sc and $^{54,56}$Ti nuclei. The new mass excess values $ME$($^{52}$Sc) $=$ $-40525(65)$ keV, $ME$($^{53}$Sc) $=$ $-38910(80)$ keV, and $ME$($^{54}$Sc) $=$ $-34485(360)$ keV, deviate from the Atomic Mass Evaluation 2012 by 2.3$σ$, 2.8$σ$, and 1.7$σ$, respectively. These large deviations significantly change the systematics of the two-neutron separation energies of scandium isotopes. The empirical shell gap extracted from our new experimental results shows a significant subshell closure at $N = 32$ in scandium, with a similar magnitude as in calcium. Moreover, we present $ab$ $initio$ calculations using the valence-space in-medium similarity renormalization group based on two- and three-nucleon interactions from chiral effective field theory. The theoretical results confirm the existence of a substantial $N = 32$ shell gap in Sc and Ca with a decreasing trend towards lighter isotones, thus providing a consistent picture of the evolution of the $N = 32$ magic number from the $pf$ into the $sd$ shell.

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arXiv2019

Improving the resolving power of Isochronous Mass Spectrometry by employing an in-ring mechanical slit

Isochronous Mass Spectrometry (IMS) in heavy-ion storage rings is an excellent experimental method for precision mass measurements of exotic nuclei. In the IMS, the storage ring is tuned in a special isochronous ion-optical mode. Thus, the mass-over-charge ratios of the stored ions are directly reflected by their respective revolution times in first order. However, the inevitable momentum spread of secondary ions increases the peak widths in the measured spectra and consequently limits the achieved mass precision. In order to achieve a higher mass resolving power, the ring aperture was reduced to 60 mm by applying a mechanical slit system at the dispersive straight section. The momentum acceptance was reduced as well as better isochronous conditions were achieved. The results showed a significant improvement of the mass resolving power reaching $5.2 \times 10^{5}$, though at the cost of about 40\% ion loss.

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arXiv2018

Application of isochronous mass spectrometry for the study of angular momentum population in projectile fragmentation reactions

Isochronous mass spectrometry was applied to measure isomeric yield ratios of fragmentation reaction products. This approach is complementary to conventional gamma-ray spectroscopy in particular for measuring yield ratios for long-lived isomeric states. Isomeric yield ratios for the high-spin I = 19/2 states in the mirror nuclei 53Fe and 53Co are measured to study angular momentum population following the projectile fragmentation of 78Kr at energies of 480A MeV on a beryllium target. The 19/2 state isomeric ratios of 53Fe produced from different projectiles in the literature have also been extracted as a function of mass number difference between projectile and fragment (mass loss). The results are compared to ABRABLA07 model calculations. The isomeric ratios of 53Fe produced using different projectiles suggest that the theory underestimates not only the previously reported dependence on the spin but also the dependence on the mass loss.

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arXiv2018

Isochronous mass measurements of $T_z=-1$ $fp$-shell nuclei from projectile fragmentation of $^{58}$Ni

Atomic masses of seven $T_z=-1$, $fp$-shell nuclei from $^{44}$V to $^{56}$Cu and two low-lying isomers, $^{44m}$V ($J^π=6^+$) and $^{52m}$Co ($J^π=2^+$), have been measured with relative precisions of $1-4\times 10^{-7}$ with Isochronous Mass Spectrometry (IMS) at CSRe. The masses of $^{56}$Cu, $^{52g,52m}$Co, and $^{44m}$V were measured for the first time in this experiment. The Mass Excesses ($ME^{\prime}$s) of $^{44}$V, $^{48}$Mn, $^{50}$Fe, and $^{54}$Ni are determined with an order of magnitude improved precision compared to the literature values. $^{52g,52m}$Co and $^{56}$Cu are found to be $370$~keV and $400$~keV more bound, respectively, while $^{44g,44m}$V are $\sim 300$~keV less bound than the extrapolations in the Atomic-Mass Evaluation 2012 (AME$^{\prime}$12). The masses of the four $T_z=-1/2$ nuclei $^{45}$V, $^{47}$Cr, $^{49}$Mn, and $^{51}$Fe are re-determined to be in agreement, within the experimental errors, with the recent JYFLTRAP measurements or with the previous IMS measurements in CSRe. Details of the measurements and data analysis are described, and the impact of the new $ME$ values on different aspects in nuclear structure are investigated and discussed.

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arXiv2018

The Masses of the $T_z=-3/2$ Nuclei $^{27}$P and $^{29}$S

Isochronous mass spectrometry has been applied in the storage ring CSRe to measure the masses of the $T_z=-3/2$ nuclei $^{27}$P and $^{29}$S. The new mass excess value $ME$($^{29}$S) $=-3094(13)$~keV is 66(52)~keV larger than the result of the previous $^{32}$S($^3$He,$^{6}$He)$^{29}$S reaction measurement in 1973 and a factor of 3.8 more precise. The new result for $^{29}$S, together with those of the $T=3/2$ isobaric analog states (IAS) in $^{29}$P, $^{29}$Si, and $^{29}$Al, fit well into the quadratic form of the Isobaric Multiplet Mass Equation IMME. The mass excess of $^{27}$P has been remeasured to be $ME(^{27}$P$)=-685(42)$ keV. By analyzing the linear and quadratic coefficients of the IMME in the $T_z=-3/2$ $sd$-shell nuclei, it was found that the ratio of the Coulomb radius parameters is $R\approx0.96$ and is nearly the same for all $T=3/2$ isospin multiplets. Such a nearly constant $R$-value, apparently valid for the entire light mass region with $A>9$, can be used to set stringent constraints on the isovector and isotensor components of the isospin non-conserving forces in theoretical calculations.

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arXiv2018

First application of combined isochronous and Schottky mass spectrometry: Half-lives of fully ionized 49Cr24+ and 53Fe26+ atoms

Lifetime measurements of b -decaying highly charged ions have been performed in the storage ring CSRe by applying the isochronous Schottky mass spectrometry. The fully ionized 49Cr and 53Fe ions were produced in projectile fragmentation of 58Ni primary beam and were stored in the CSRe tuned into the isochronous ion-optical mode. The new resonant Schottky detector was applied to monitor the intensities of stored uncooled 49Cr24+ and 53Fe26+ ions. The extracted half-lives T1/2(49Cr24+) = 44.0(27) min and T1/2(53Fe26+) = 8.47(19) min are in excellent agreement with the literature half-life values corrected for the disabled electron capture branchings. This is an important proof-of-principle step towards realizing the simultaneous mass and lifetime measurements on exotic nuclei at the future storage ring facilities.

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arXiv2018

Mass Measurements of Neutron-Deficient Y, Zr, and Nb Isotopes and Their Impact on $rp$ and $νp$ Nucleosynthesis Processes

Using isochronous mass spectrometry at the experimental storage ring CSRe in Lanzhou, the masses of $^{82}$Zr and $^{84}$Nb were measured for the first time with an uncertainty of $\sim 10$ keV, and the masses of $^{79}$Y, $^{81}$Zr, and $^{83}$Nb were re-determined with a higher precision. %The latter differ significantly from their literature values. The latter are significantly less bound than their literature values. Our new and accurate masses remove the irregularities of the mass surface in this region of the nuclear chart. Our results do not support the predicted island of pronounced low $α$ separation energies for neutron-deficient Mo and Tc isotopes, making the formation of Zr-Nb cycle in the $rp$-process unlikely. The new proton separation energy of $^{83}$Nb was determined to be 490(400)~keV smaller than that in the Atomic Mass Evaluation 2012. This partly removes the overproduction of the $p$-nucleus $^{84}$Sr relative to the neutron-deficient molybdenum isotopes in the previous $νp$-process simulations.

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arXiv2016

Identification of the Lowest $T=2$, $J^{π=}0^+$ Isobaric Analog State in $^{52}$Co and Its Impact on the Understanding of $β$-Decay Properties of $^{52}$Ni

Masses of $^{52g,52m}$Co were measured for the first time with an accuracy of $\sim 10$ keV, an unprecedented precision reached for short-lived nuclei in the isochronous mass spectrometry. Combining our results with the previous $β$-$γ$ measurements of $^{52}$Ni, the $T=2$, $J^π=0^+$ isobaric analog state (IAS) in $^{52}$Co was newly assigned, questioning the conventional identification of IASs from the $β$-delayed proton emissions. Using our energy of the IAS in $^{52}$Co, the masses of the $T=2$ multiplet fit well into the Isobaric Multiplet Mass Equation. We find that the IAS in $^{52}$Co decays predominantly via $γ$ transitions while the proton emission is negligibly small. According to our large-scale shell model calculations, this phenomenon has been interpreted to be due to very low isospin mixing in the IAS.

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arXiv2016

The decay characteristic of $^{22}$Si and its ground-state mass significantly affected by three-nucleon forces

The decay of the proton-rich nucleus $^{22}$Si was studied by a silicon array coupled with germanium clover detectors. Nine charged-particle groups are observed and most of them are recognized as $β$-delayed proton emission. A charged-particle group at 5600 keV is identified experimentally as $β$-delayed two-proton emission from the isobaric analog state of $^{22}$Al. Another charged-particle emission without any $β$ particle at the low energy less than 300 keV is observed. The half-life of $^{22}$Si is determined as 27.5 (18) ms. The experimental results of $β$-decay of $^{22}$Si are compared and in nice agreement with shell-model calculations. The mass excess of the ground state of $^{22}$Si deduced from the experimental data shows that three-nucleon (3N) forces with repulsive contributions have significant effects on nuclei near the proton drip line.

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arXiv2014

Charge and frequency resolved isochronous mass spectrometry in storage rings: First direct mass measurement of the short-lived neutron-deficient $^{51}$Co nuclide

Revolution frequency measurements of individual ions in storage rings require sophisticated timing detectors. One of common approaches for such detectors is the detection of secondary electrons released from a thin foil due to penetration of the stored ions. A new method based on the analysis of intensities of secondary electrons was developed which enables determination of the charge of each ion simultaneously with the measurement of its revolution frequency. Although the mass-over-charge ratios of $^{51}$Co$^{27+}$ and $^{34}$Ar$^{18+}$ ions are almost identical, and therefore, the ions can not be resolved in a storage ring, by applying the new method the mass excess of the short-lived $^{51}$Co is determined for the first time to be ME($^{51}$Co)=-27342(48) keV. Shell-model calculations in the $fp$-shell nuclei compared to the new data indicate the need to include isospin-nonconserving forces.

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arXiv2013

Nuclear Physics Experiments with Ion Storage Rings

In the last two decades a number of nuclear structure and astrophysics experiments were performed at heavy-ion storage rings employing unique experimental conditions offered by such machines. Furthermore, building on the experience gained at the two facilities presently in operation, several new storage ring projects were launched worldwide. This contribution is intended to provide a brief review of the fast growing field of nuclear structure and astrophysics research at storage rings.

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arXiv2013

High-resolution measurement of the time-modulated orbital electron capture and of the $β^+$ decay of hydrogen-like $^{142}$Pm$^{60+}$ ions

The periodic time modulations, found recently in the two-body orbital electron-capture (EC) decay of both, hydrogen-like $^{140}$Pr$^{58+}$ and $^{142}$Pm$^{60+}$ ions, with periods near to 7s and amplitudes of about 20%, were re-investigated for the case of $^{142}$Pm$^{60+}$ by using a 245 MHz resonator cavity with a much improved sensitivity and time resolution. We observed that the exponential EC decay is modulated with a period $T = 7.11(11)$s, in accordance with a modulation period $T = 7.12(11)$ s as obtained from simultaneous observations with a capacitive pick-up, employed also in the previous experiments. The modulation amplitudes amount to $a_R = 0.107(24)$ and $a_P = 0.134(27)$ for the 245 MHz resonator and the capacitive pick-up, respectively. These new results corroborate for both detectors {\it exactly} our previous findings of modulation periods near to 7s, though with {\it distinctly smaller} amplitudes. Also the three-body $β^+$ decays have been analyzed. For a supposed modulation period near to 7s we found an amplitude $a = 0.027(27)$, compatible with $a = 0$ and in agreement with the preliminary result $a = 0.030(30)$ of our previous experiment. These observations could point at weak interaction as origin of the observed 7s-modulation of the EC decay. Furthermore, the data suggest that interference terms occur in the two-body EC decay, although the neutrinos are not directly observed.

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