β衰变与同位旋对称性破缺

β-decay and isospin symmetry breaking

研究同位旋对称性破缺对于深入理解核力与核结构有着重要意义.我们在兰州重离子加速器国家实验室放射性束流线终端采用双面硅条与高纯锗组成强大的探测器阵列,精确测量sd壳质子滴线附近原子核β衰变性质,然后与其镜像核的衰变性质进行对比,系统性研究轻核区同位旋对称性破缺现象.

The elegant concept of isospin symmetry is of fundamental importance in nuclear and elementary particle physics.As aβ-decay process changes an up quark to a down quark or vice versa,studies of nuclei with exchanged numbers of neutrons and protons,known as mirror nuclei,can be a powerful means to probe isospin symmetry breaking.A series ofβ-decay experiments of sd-shell nuclei near the proton drip line produced by projectile fragmentation were performed at the Radioactive Ion Beam Line of the Heavy Ion Research Facility in Lanzhou(HIRFL-RIBLL1)using a silicon array with a high detection efficiency and a low detection threshold.The secondary ions were identified by the combination ofΔE-TOF and magnetic rigidity(Bρ)in which the time of flight(TOF)was measured by two plastic scintillators at the second and fourth focal planes of the RIBLL1 and the energy loss(ΔE)was measured by two silicon detectors in the front of the silicon array.Double-sided silicon strip detectors(DSSDs)in the center of the array served to measure the residual energies of secondary ions on an event-by-event basis and study their decay properties with an implantation-decay correlation.Several quadrant silicon detectors(QSDs)placed behind DSSDs were used for anticoincidences of the penetrating fragments and light particles coming along with the beam,and measurements ofβparticles and high-energy protons.Theβ-delayed proton peaks with known energies and their corresponding absolute intensities were used for the energy and detection efficiency calibrations of the DSSDs.The system allows us to measure protons with energies down to about 200 keV without obviousβbackground in the proton spectrum and was successful to measure the energies,time,and positions of implanted ions and decay protons efficiently,supporting researchers to study the decay properties of nuclei towards the proton drip line.Theβ-decay data,together with the large-scale shell-model calculations,allows detailed investigations on the mirrornuclei systems resulting in the systematic study of isospin symmetry breaking in the sd-shell nuclear region.It was found that the mirror asymmetry parameters become larger due to the smaller separation energies while approaching the proton drip line for the extremely proton-rich sulfur nuclei^(27,28,29)S and the Coulomb forces and other isospin-symmetry nonconserving forces acting between protons would lead to extended proton wave functions and cause mirror asymmetries.The properties of^(22)Siβ-decay for the transitions to the low-lying states of^(22)Al were measured,and the reduced transition probabilities were determined.Comparing with the data on theβ-decay of the mirror nucleus^(22)O,we found a mirror asymmetry ofδ=209(96)%in the transition to the first 1+excited state of the respective daughters.This is by far the largest value ofδobserved in the low-lying states.Shell-model calculations with isospin-nonconserving forces,including the T=1,J=2,3 interactions related to the s1/2 orbit that introduces explicitly the isospin-symmetry breaking force and describes the loosely bound nature of the wave functions of the s1/2 orbit,can reproduce the observed data well and demonstrate that this dramatically large mirror asymmetry is attributed to the significant proton occupation and loosely bound nature of the wave functions of the s1/2 orbit,which suggests that^(22)Al is a proton-halo nucleus.