Superallowed Transitions Research Articles

Study of [formula omitted]/EC-decay properties of [formula omitted] shell nuclei using nuclear shell model

Our study employs the nuclear shell model to systematically compute the half-lives of β-decay for nuclei in the mass range of A=18−39, encompassing the majority of sd shell nuclei. This analysis utilizes the USDB and SDNN Hamiltonians. The theoretical outcomes contain calculations of various parameters such as Q-values, half-lives, excitation energy, logft values, and branching ratios. We explore these results with axial–vector coupling constant for weak interactions, denoted as gA(=1.27), and κ value (=6289). We perform calculations of Gamow Teller matrix elements for 116 decay processes to calculate the quenching factor; we found a quenching factor of q=0.794±0.05 for the USDB interaction and q=0.815±0.04 for the SDNN interaction. We have also calculated superallowed transitions 0+→0+ for seven nuclei. Further, we have also included the electron capture phase space factor for the required nuclei to calculate the half-lives. This inclusion leads to small contribution in results, particularly for nuclei where electron capture (EC) plays a significant role. The overall results are in agreement with the experimental data.

On-line installation of the Superallowed Transition Beta-Neutrino Decay Ion Coincidence Trap

The Cabibbo-Kobayashi-Maskawa quark mixing matrix currently does not satisfy unitarity at the 2σ-level. This could be the result of an inaccurate value of one or both of its largest matrix elements Vus and Vud. In the case of Vud, the most precise measurement is obtained from the f t-value measurements of superallowed beta-transitions between 0+ states. The accuracy of this determination can, in turn, be tested by extracting Vud in other transitions including superallowed transitions between mirror nuclei. The Superallowed Transition Beta-Neutrino Decay Ion Coincidence Trap (St. Benedict) is currently under construction at the Nuclear Science Laboratory of the University of Notre Dame to perform such a determination, with the goal of shedding more light on this tension with unitarity. St. Benedict will take a radioactive ion beam produced by TwinSol, thermalize it in a large volume gas catcher, then transport it in two separate differentially-pumped volumes using a radio-frequency (RF) carpet and a radio-frequency quadrupole (RFQ) ion guide before injecting it in an RFQ trap to create cool ion bunches for injection in the measurement Paul trap. In this paper, we detail the installation of the beam preparation components of St. Benedict, and present the results of the first RIBs successfully stopped and extracted from its gas catcher.

The St. Benedict Facility: Probing Fundamental Symmetries through Mixed Mirror β-Decays

Precise measurements of nuclear beta decays provide a unique insight into the Standard Model due to their connection to the electroweak interaction. These decays help constrain the unitarity or non-unitarity of the Cabibbo–Kobayashi–Maskawa (CKM) quark mixing matrix, and can uniquely probe the existence of exotic scalar or tensor currents. Of these decays, superallowed mixed mirror transitions have been the least well-studied, in part due to the absence of data on their Fermi to Gamow-Teller mixing ratios (ρ). At the Nuclear Science Laboratory (NSL) at the University of Notre Dame, the Superallowed Transition Beta-Neutrino Decay Ion Coincidence Trap (St. Benedict) is being constructed to determine the ρ for various mirror decays via a measurement of the beta–neutrino angular correlation parameter (aβν) to a relative precision of 0.5%. In this work, we present an overview of the St. Benedict facility and the impact it will have on various Beyond the Standard Model studies, including an expanded sensitivity study of ρ for various mirror nuclei accessible to the facility. A feasibility evaluation is also presented that indicates the measurement goals for many mirror nuclei, which are currently attainable in a week of radioactive beam delivery at the NSL.

Construction of St. Benedict

The Superallowed Transition Beta-Neutrino Decay Ion Coincidence Trap (St. Benedict) is currently under construction at the University of Notre Dame Nuclear Science Laboratory. It is designed to measure the beta-neutrino angular correlation parameter in superallowed mixed beta-decay transitions between mirror nuclei in order to extract the Fermi-to-Gamow Teller mixing ratio and test theoretical corrections entering in the determination of the Vud element of the Cabibbo–Kobayashi–Maskawa quark mixing matrix. St. Benedict includes a large volume gas catcher to thermalize the radioactive ion beam from TwinSol, a differentially-pumped extraction system, a radiofrequency quadrupole ion cooler and buncher, and a Paul trap for the observation and detection of the β decay products. The status of each of these components will be presented.

Commissioning of the St. Benedict RF carpet

The Superallowed Transition BEta NEutrino Decay Ion Coincidence Trap (St. Benedict) aims to measure the beta-neutrino angular correlation parameter for superallowed mixed beta decay transitions between mirror nuclei in order to conduct precision tests of the Standard Model. St. Benedict will include a gas catcher, a differentially pumped extraction system, a radio-frequency quadrupole (RFQ) cooler and buncher, and a Paul trap to utilize radioactive ion beams produced at the Nuclear Science Laboratory of the University of Notre Dame. The two-part differentially pumped extraction system contains a RF carpet at a helium gas pressure of around 3 mbar, and a radio-frequency quadrupole (RFQ) ion guide at a pressure of around 2×10−3 mbar. Transport efficiency of potassium ions in the first part of the extraction system using the RF carpet in the presence of gas flow was determined to be near 100% in the 0.75 to 5 mbar pressure range using the ion surfing transport method.

Transport tests of the St. Benedict first-stage extraction system

The Superallowed Transition BEta NEutrino Decay Ion Coincidence Trap (St. Benedict) aims to conduct precision tests of the standard model by allowing for a determination of the Fermi to Gamow–Teller mixing ratio for many mirror transitions. St. Benedict will include a gas cell, a differentially-pumped extraction system, a radio-frequency (RF) cooler and buncher, and a Paul trap where the measurement will take place. The extraction system will include a first section housing a RF carpet at a pressure of around 3 mbar, followed by a second section housing a radio-frequency quadrupole (RFQ) ion guide at a pressure of 2×10−3 mbar. Transport efficiency of potassium ions using the RF carpet in static gas was determined to be above 90% using the ion surfing method after optimizing the potentials applied to various electrodes.

Precise β branching-ratio measurement for the 0+→0+ superallowed decay of Ar34

We have measured the branching ratio for the superallowed ${0}^{+}\ensuremath{\rightarrow}$ ${0}^{+}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$ transition from $^{34}\mathrm{Ar}$ to be 0.9448(8), and determined its $ft$ value to be 3058.1(28) s, a result with $\ifmmode\pm\else\textpm\fi{}0.09$% precision, which is a factor of 3 improvement over the previous result based on current world data. The $ft$-value ratio for the mirror pair of superallowed transitions $^{34}\mathrm{Ar}$ $\ensuremath{\rightarrow}^{34}\mathrm{Cl}$ and $^{34}\mathrm{Cl}$ $\ensuremath{\rightarrow}^{34}\mathrm{S}$ becomes the most precise yet measured and, in a sensitive test of the method used to calculate the isospin-symmetry-breaking correction, ${\ensuremath{\delta}}_{C}$, it agrees well with the ratio as calculated with Woods-Saxon radial wave functions. This confirms the method used in the most recent survey of superallowed decays to extract ${V}_{\text{ud}}$, the up-down quark-mixing element of the Cabibbo-Kobayashi-Maskawa matrix. In addition, our branching-ratio results for the four observed Gamow-Teller branches to ${1}^{+}$ states in $^{34}\mathrm{Cl}$ are shown to agree well with shell-model calculations based on the same effective interactions that were used in the determination of ${\ensuremath{\delta}}_{C}$.

Development of the St. Benedict Paul Trap at the Nuclear Science Laboratory

There are various tests of the Standard Model aimed at probing new physics. One such test is to check the unitarity of the Cabbibo-Kobayashi-Masakawa matrix, with the most precise result given by the normalization of the top row. The precision of this result is limited by the uncertainty of the Vud and Vus elements. The most precise result for Vud is currently derived from the ensemble of superallowed 0+ → 0+ nuclear β decays, but the determination from other transitions is desirable to check theoretical calculations and the potential presence of unknown systematic contributions. Other candidates include neutron decay, pion decay, and the ensemble of nuclear mirror decays. While the neutron and pion decay present their own unique experimental challenges, the group of mirror decays require the precise determination of the Fermi to Gamow-Teller mixing ratio. This value can be extracted from a measurement of the β-ν angular correlation parameter, αβν, which is currently only known for five mirror decays. A new ion trapping system, St. Benedict (Superallowed Transition Beta-Neutrino Decay-Ion-Coincidence Trap), to be located in the Nuclear Science Laboratory at the University of Notre Dame is currently under construction and will ultimately aim to extract αβν for more mirror decays. The focus of this work is on simulations that will guide the design of the Paul trap which will be used for the measurement.

Stopped, bunched beams for the TwinSol facility

Tests of the unitarity of the Cabbibo-Kobayashi-Masakawa (CKM) matrix offer an important avenue for constraining the Standard Model of the electroweak interaction. Several methods are currently used to determine Vud, the largest element in the top-row normalization test. One such method is through the study of superallowed T = 1/2 mixed mirror transitions, which offers a complementary method to the current most-precise value that is determined from superallowed pure Fermi 0+ → 0+ transitions. The precision currently achievable by this method is currently limited by the very low number of transitions for which the Fermi-to-Gamow-Teller mixing ratio ρ has been measured. St. Benedict, the Superallowed Transition Beta-Neutrino Decay-Ion-Coincidence Trap, is currently under development at the University of Notre Dame’s Nuclear Science Laboratory, and intends to determine ρ for a range of new isotopes through measurements of the β-neutrino asymmetry parameter aβν using a linear Paul trap. In order to trap these ions, the fast, continuous secondary beam separated by the TwinSol twin solenoid separator must be thermalized and bunched. The system through which this will be done will feature a large-volume gas cell in which the ions will be thermalized, a double-RF-funnel-based ion guide system for the extraction of the ions, and a radiofrequency quadrupole (RFQ) to provide cooled ion bunches for capture in the Paul trap.

QEC value of the superallowed β emitter Sc42

Precise measurements of superallowed ${0}^{+}\ensuremath{\rightarrow}{0}^{+}$ $\ensuremath{\beta}$ decay presently provide the most precise value for the weak mixing amplitude ${V}_{u\phantom{\rule{0}{0ex}}d}$. As the largest element of the CKM matrix, ${V}_{u\phantom{\rule{0}{0ex}}d}$ is a critical piece of the Standard Model of the electroweak interaction. The new, precise Penning-trap mass measurement of the decay energy for the superallowed transition in ${}^{42}$Sc opens the door for a much more precise $f\phantom{\rule{0}{0ex}}t$ value determination if its half-life can be measured more precisely as well.

Theoretical corrections and world data for the superallowedftvalues in theβdecays ofTi42,Cr46,Fe50, andNi54

Prompted by recent measurements, we have surveyed world data and calculated radiative and isospin-symmetry breaking corrections for the superallowed 0+ -to- 0+ Fermi transitions from 42Ti, 46Cr, 50Fe and 54Ni. This increases the number of such transition with a complete set of calculated corrections from 20 to 23. The results are compared with their equivalents for the mirror superallowed transitions from 42Sc, 46V, 50Mn and 54Co. The predicted ft-value asymmetries of these mirror pairs are sensitive to the correction terms and provide motivation for improving measurement precision so as to be able to test the corrections. To aid in that endeavor, we present a parameterization for calculating the f values for the new transitions to plus/minus 0.01%.

Isospin Mixing in ^{80}Zr: From Finite to Zero Temperature.

The isospin mixing was deduced in the compound nucleus ^{80}Zr at an excitation energy of E^{*}=54 MeV from the γ decay of the giant dipole resonance. The reaction ^{40}Ca+^{40}Ca at E_{beam}=136 MeV was used to form the compound nucleus in the isospin I=0 channel, while the reaction ^{37}Cl+^{44}Ca at E_{beam}=95 MeV was used as the reference reaction. The γ rays were detected with the AGATA demonstrator array coupled with LaBr_{3}:Ce detectors. The temperature dependence of the isospin mixing was obtained and the zero-temperature value deduced. The isospin-symmetry-breaking correction δ_{C} used for the Fermi superallowed transitions was extracted and found to be consistent with β-decay data.

Precise measurement of branching ratios in theβdecay ofCa38

We present the full description of a measurement of the branching ratios for the $\ensuremath{\beta}$ decay of $^{38}\mathrm{Ca}$. This decay includes five allowed ${0}^{+}\ensuremath{\rightarrow}{1}^{+}$ branches and a superallowed ${0}^{+}\ensuremath{\rightarrow}{0}^{+}$ one. With our new result for the latter, we determine its $ft$ value to be $3062.3\ifmmode\pm\else\textpm\fi{}6.8$ s, a result whose precision (0.2%) is comparable to the precision of the 13 well-known ${0}^{+}\ensuremath{\rightarrow}{0}^{+}$ transitions used up until now for the determination of ${V}_{\text{ud}}$, the up-down quark-mixing element of the Cabibbo-Kobayashi-Maskawa matrix. The $^{38}\mathrm{Ca}$ superallowed transition thus becomes the first addition to this set of transitions in nearly a decade and the first for which a precise mirror comparison is possible, thus enabling an improved test of the isospin-symmetry-breaking corrections required for the extraction of ${V}_{\text{ud}}$.

βDecay ofCa38: Sensitive test of Isospin Symmetry-Breaking Corrections from Mirror Superallowed0+→0+Transitions

We report the first branching-ratio measurement of the superallowed 0+→0+β transition from Ca38. The result, 0.7728(16), leads to an ft value of 3062.3(68)s with a relative precision of ±0.2%. This makes possible a high-precision comparison of the ft values for the mirror superallowed transitions, Ca38→38mK and K38m→Ar38, which sensitively tests the isospin symmetry-breaking corrections required to extract Vud, the up-down quark-mixing element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, from superallowed β decay. The result supports the corrections currently used and points the way to even tighter constraints on CKM unitarity.

Isospin-breaking corrections to superallowed Fermiβdecay in isospin- and angular-momentum-projected nuclear density functional theory

Background: Superallowed $\ensuremath{\beta}$-decay rates provide stringent constraints on physics beyond the standard model of particle physics. To extract crucial information about the electroweak force, small isospin-breaking corrections to the Fermi matrix element of superallowed transitions must be applied.Purpose: We perform systematic calculations of isospin-breaking corrections to superallowed $\ensuremath{\beta}$ decays and estimate theoretical uncertainties related to the basis truncation, to time-odd polarization effects related to the intrinsic symmetry of the underlying Slater determinants, and to the functional parametrization.Methods: We use the self-consistent isospin- and angular-momentum-projected nuclear density functional theory employing two density functionals derived from the density-independent Skyrme interaction. Pairing correlations are ignored. Our framework can simultaneously describe various effects that impact matrix elements of the Fermi decay: symmetry breaking, configuration mixing, and long-range Coulomb polarization.Results: Isospin-breaking corrections to the $I={0}^{+}$, $T=1\ensuremath{\rightarrow}I={0}^{+}$, $T=1$ pure Fermi transitions are computed for nuclei from $A=10$ to $A=98$ and, for the first time, to the Fermi branch of the $I,T=1/2\ensuremath{\rightarrow}I$, $T=1/2$ transitions in mirror nuclei from $A=11$ to $A=49$. We carefully analyze various model assumptions impacting theoretical uncertainties of our calculations and provide theoretical error bars on our predictions.Conclusions: The overall agreement with empirical isospin-breaking corrections is very satisfactory. Using computed isospin-breaking corrections we show that the unitarity of the CKM matrix is satisfied with a precision of better than 0.1$%$.

Microscopic calculations of isospin mixing in N≈Z nuclei and isospin-symmetry-breaking corrections to the superallowed β-decay

Recently, we applied, for the first time, the angular momentum and isospin-projected nuclear density functional theory to calculate isospin-symmetry breaking (ISB) corrections to superallowed β-decay. With the calculated set of ISB corrections, we found that |Vud|=0.97447(23) for the leading element of the Cabibbo–Kobayashi–Maskawa matrix. This is in good agreement with both the recent result of Towner and Hardy (2008 Phys. Rev. C 77 025501) and the central value deduced from the neutron decay. In this paper, we extend our calculations of the ISB corrections covering all superallowed transitions A, Iπ=0+, T=1, Tz→A, Iπ=0+, T=1 and Tz+1 where Tz=− 1, 0, and A ranges from 10 to 74.

Precise half-life measurement of the superallowedβ+emitter46V

The half-life of ${}^{46}$V has been measured to be 422.66(6) ms, which is a factor of two more precise than the best previous measurement. Our result is also consistent with the previous measurements, with no repeat of the disagreement recently encountered with ${Q}_{\mathrm{EC}}$ values measured for the same transition. The $\mathcal{F}t$ value for the ${}^{46}$V superallowed transition, incorporating all world data, is determined to be 3074.1(26) s, a result consistent with the average $\overline{\mathcal{F}t}$ value of 3072.08(79) s established from the 13 best-known superallowed transitions.

βdecay of32Cl: Precisionγ-ray spectroscopy and a measurement of isospin-symmetry breaking

Background: Models to calculate small isospin-symmetry-breaking effects in superallowed Fermi decays have been placed under scrutiny in recent years. A stringent test of these models is to measure transitions for which the correction is predicted to be large. The decay of ${}^{32}$Cl decay provides such a test case.Purpose: The purpose of this paper is to improve the $\ensuremath{\gamma}$ yields following the $\ensuremath{\beta}$ decay of ${}^{32}$Cl and to determine the $ft$ values of the the $\ensuremath{\beta}$ branches, particularly the one to the isobaric-analog state in ${}^{32}$S.Method: Reaction-produced and recoil-spectrometer-separated ${}^{32}$Cl is collected in a tape and transported to a counting location where $\ensuremath{\beta}\ensuremath{-}\ensuremath{\gamma}$ coincidences are measured with a precisely calibrated high-purity germanium detector.Results: The precision on the $\ensuremath{\gamma}$ yields for most of the known $\ensuremath{\beta}$ branches has been improved by about an order of magnitude, and many new transitions have been observed. We have determined ${}^{32}$Cl-decay transition strengths extending up to ${E}_{x}\ensuremath{\sim}11$ MeV. The $ft$ value for the decay to the isobaric-analog state in ${}^{32}$S has been measured. A comparison to a shell-model calculation shows good agreement.Conclusions: We have experimentally determined the isospin-symmetry-breaking correction to the superallowed transition of this decay to be ${({\ensuremath{\delta}}_{C}\ensuremath{-}{\ensuremath{\delta}}_{\mathrm{NS}})}_{\mathrm{exp}}=5.4(9)%$, significantly larger than for any other known superallowed Fermi transition. This correction agrees with a shell-model calculation, which yields ${\ensuremath{\delta}}_{C}\ensuremath{-}{\ensuremath{\delta}}_{\mathrm{NS}}=4.8(5)%$. Our results also provide a way to improve the measured $ft$ values for the $\ensuremath{\beta}$ decay of ${}^{32}$Ar.

THE EXPRESSIONS FOR THE SUPER-ALLOWED BETA-DECAY RATES IN THE CORIOLIS COUPLING MODEL: APPLICATION TO THE 1d5/2 SHELL NUCLEI

The expressions for super-allowed beta-decay transition rates have been derived within the context of the Coriolis coupling model. The derived expressions, valid for the beta-decay between any two mirror nuclei, have been applied to calculate super-allowed beta-decay transition rates of 21 Na , 23 Mg , 25 Al , and 27 Si . Without the use of any hindrance factor, the calculated rates agree well with the data and also with the calculations done using the shell model with configuration admixture.

69Krβ-delayed proton emission: A Trojan horse for studying states in proton-unbound69Br

Particle decay of ${}^{69}$Br and ${}^{65}$As was observed through $\ensuremath{\beta}$-delayed proton emission of ${}^{69}$Kr and ${}^{65}$Se, respectively. Decay spectroscopy was performed through $\ensuremath{\beta}$-$p$ correlations using a position-sensitive silicon-implantation detector surrounded by a $\ensuremath{\gamma}$-ray detector array. A $\ensuremath{\beta}$-decay half-life of 27(3) ms was measured for ${}^{69}$Kr and 33(4) ms for ${}^{65}$Se. The ${}^{69}$Kr ground state decays by a superallowed transition to its unbound isobaric analog state in ${}^{69}$Br which immediately decays by a 2.97(5)-MeV proton group to the first excited state in ${}^{68}$Se at 854.2 keV. This chain of decays constrains both the mass and spin of the ${}^{69}$Kr ground state. We observed no evidence of ground-state proton decay from ${}^{69}$Br.