Decays of TZ = − 3/2 nuclei 23Al, 31Cl, and 41Ti

This article gives an overview on the decay spectroscopy of TZ = − 3/2 nuclei 23Al, 31Cl, and 41Ti performed at the Ion Guide Isotope Separator On-Line (IGISOL) facility. The results of the IGISOL experiments are compared to the experimental results that have been published since. The isobaric multiplet mass equation (IMME) has been studied for the T = 3/2 quartets at A = 23 and A = 31. For 41Ti, a detailed comparison to the Gamow–Teller strengths obtained for the analog transitions via charge-exchange reactions has been done. Further improvements in the experimental instrumentation and methods and possible implementations for studying TZ = − 3/2 nuclei at the new IGISOL facility are discussed.

analogous transitions studied via charge-exchange reactions, such as in the case of the T = 3/2 quartet at A = 41 [1].
The relative β-delayed proton emission window (1 − S p /Q EC ) of the T Z = −3/2 nuclei depends strongly on whether they belong to the A = 4n + 3 (odd Z ) or to the A = 4n + 1 series (even Z ) [2]. Generally for the T Z = −3/2 nuclei with odd Z , such as 23 Al and 31 Cl, the β-delayed proton emission window is between 0.3 and 0.45, whereas for even-Z nuclei, like 41 Ti, it is between 0.8 and 1. This gives a rough indication of the expected β-delayed proton versus γ -ray emission probability. As the β-decay phase space favors the decay to the low-energy states, the γ decay by slow electromagnetic interaction is especially pronounced for the A = 4n + 3 series β-decay precursors. Also due to Coulomb and angular momentum barriers, γ decay starts to compete with isospin-forbidden proton decay for low-lying proton unbound states. Therefore, in addition to protons, detecting γ rays with good sensitivity is essential.
According to the Isobaric Multiplet Mass Equation (IMME) [3][4][5], masses of the members of an isobaric multiplet (T) should lie along a parabola M(T Z ) = a + b T Z + cT 2 Z . In order to study the IMME, masses and excitation energies of the isobaric analog states (IAS) belonging to the T multiplet should be precisely measured. Previously, masses were determined from beta-decay energies, but nowadays Penning trap mass spectrometry offers a more accurate method for measuring the ground state mass excesses directly. Beta-decay studies of T Z = −3/2 nuclei offer information on the excitation energies of the IAS in the T Z = −1/2 nuclei. In addition, isospin mixing in the wavefunctions of the T = 3/2 IAS, manifested in isospin-forbidden proton decays of these IAS, can be systematically studied in these nuclei.
The studied T Z = −3/2 nuclei 23 Al, 31 Cl, and 41 Ti lie at the path of the rapid proton capture (rp) process [6,7]. In particular, beta-decay studies of 23 Al and 31 Cl yield information on the states in the daughter nuclei, 23 Mg and 31 S, whose properties are relevant for the modeling the nucleosynthesis in ONe novae. Namely, the reactions 22 Na(p, γ ) 23 Mg and 22 Mg(p, γ ) 23 Al have to be well-known in order to model the production of 22 Na in ONe novae [8]. The non-observation of 1275-keV γrays from 22 Na with COMPTEL telescope [9] has drawn attention to the production mechanisms of 22 Na and constrained nova models during the last years. However, there are recent papers on possible observation of these γ -rays based on long-term COMPTEL observations [10,11]. Iyudin et al. [10] reports possible detection from a diffused source in the galactic bulge, explaining it to originate most likely from photoactivation of 22 Ne by cosmic rays whereas [11] claims a more localized source from a very slow Nova Cassiopeia 1995. The reaction 30 P(p, γ ) 31 S is important since 30 P is a mandatory passing point in ONe novae and it will stop further nucleosynthesis unless proton captures on 30 P (T 1/2 = 2.5 min) are fast enough [12].

Experimental details
2.1 Production of T Z = −3/2 nuclei at IGISOL At IGISOL [13], the ions of interest with T Z = −3/2 have been produced in light ion fusion evaporation reactions with a proton or 3 He beam from the K-130 cyclotron on a thin (few mg/cm 2 ) target. The recoiling products from the target are thermalized in the gas cell where they undergo charge-exchange reactions and a good fraction of the ions end up at a charge state 1 + . The ions are extracted from the gas cell with the help of differential pumping and a skimmer electrode. During recent years, the skimmer electrode has been replaced by a sextupole ion beam guide SPIG [14]. After the gas cell, the ions are accelerated typically to 40q kV energy and mass-separated by a 55 • dipole magnet providing a mass resolving power of M/ M ≈ 500. The massseparated beam is implanted on a thin carbon foil for the studies of beta-delayed protons and/or on a movable tape for beta-delayed γ -rays at the experimental station of the IGISOL facility. This on-line mass separation provides much cleaner spectra of the studied nuclides compared to experiments performed with the He-jet technique [15]. Experimental setups and production reactions used for 23 Al, 31 Cl, and 41 Ti are summarized below. 23 Al was produced at IGISOL via 24 Mg(p,2n) 23 Al reactions with a 7-10 μA, 40-MeV proton beam on nat Mg target [16]. The observed yields for mass-separated 23 Al and 23 Mg were ≤ 20 atoms/s and about 4000 atoms/s, respectively. 31 Cl was produced with the same type of reaction, 32 S(p,2n) 31 Cl, using a 10-20 μA, 40 or 45-MeV proton beam on a thin ZnS target [17]. The yields for 31 Cl and 31 S at 40 MeV were around 14 atoms/s and 20000 atoms/s, respectively [17]. For the production of 41 Ti, a 1-7 eμA, 40-MeV 3 He beam on nat Ca target was used to produce the ions of interest via 40 Ca( 3 He,2n) 41 Ti [18]. The production rate of 41 Ti was about 1 atom/s with the highest beam intensity [18].

Detector setups for observing β-delayed γ -rays and protons
Identification of beta-delayed protons from the beta-particle background is a key issue in beta-delayed proton and gamma spectroscopy. Therefore, different E − E detectors have been used, since the energy deposit in a E detector depends on the type of the particle. With the same initial energy, protons leave more energy than beta particles but less than alpha particles. A E gas − E Si detector telescope [19] was used to detect beta-delayed protons of 23 Al [16] and 41 Ti [18]. The telescope consisted of an E detector which was an Ortec Ultra series silicon detector with an active area of 300 mm 2 and a thickness of 300 μm [19]. A proportional counter mode was applied to the CF 4 gas E detector in order to reach a large enough signal-to-noise ratio. A lower detection limit of 155 keV and an energy resolution of 20 keV was achieved for the telescope [19].
In addition to the E gas − E Si detector telescope for detecting protons, a 1-mmthick plastic E β detector and a HPGe (37.5% relative efficiency) were used in the initial IGISOL 23 Al experiment for detecting beta particles and γ -rays, respectively. The 40 keV 23 Al + beam was implanted on a 40 μg/cm 2 -thick carbon foil surrounded by the three detectors. This setup is illustrated in Fig. 1. In the experiment on the beta decay of 41 Ti, two different measurement setups were used. For detecting betadelayed protons, the E gas − E Si detector telescope was used behind a 40 μg/cm 2thick carbon foil into which the mass-separated beam was implanted. Beta-and proton-delayed γ -rays were observed with a setup consisting of a 0.9-mm-thick plastic scintillator for detecting beta particles, a 50% HPGe detector for γ -rays, and an ion-implanted silicon detector for both beta detection and proton energy measurements [18]. There, the 40 keV 41 Ti + beam was implanted into aluminized mylar tape.  23 Al β-decay experiment reported in [16,23]. The 40-keV beam was implanted into 40 μg/cm 2 carbon foil surrounded by 37.5% HPGe detector, 1-mm-thick plastic β-detector and the E gas − E Si detector telescope The mass-separated 25-keV 31 Cl + was implanted into a 30 μg/cm 2 -thick carbon foil surrounded by a novel state-of-the-art silicon detector assembly consisting of the ISOLDE Silicon Ball [20], three double-sided silicon strip detectors (DSSSDs) [21,22] backed with three thick silicon detectors, and a 70% HPGe detector [17]. The DSSSDs were about 60 μm thick, and therefore, beta particles left very little energy in them. This made DSSSDs ideal for detecting beta-delayed protons from 31 Cl. The ISOLDE Silicon Ball and three thick Si detectors were used for detecting betas. The total beta efficiency was measured as 24.9(19)%. Protons below 700 keV could not be observed due to noise and β-tail caused mainly by 31 S in the spectrum.
3 Beta-delayed gamma and proton spectroscopy of T Z = −3/2 nuclei at IGISOL 3.1 23 Al β-delayed proton emitter 23 Al can be used as a tool to probe astrophysically interesting states just above the proton separation threshold in 23 Mg. These states are relevant for understanding resonant proton capture 22 Na(p,γ ) 23 Mg and the amount of 22 Na ejected in ONe novae. Measuring this reaction directly is challenging because the need for radioactive 22 Na targets complicates the measurement [24,25]. 23 Al was first produced in the early 1970's in Berkeley with the He-jet technique where a single proton group with an energy of 870(30) keV and a half-life of 470(30) ms was discovered [26]. In the mid-90's Tighe et al. [27] extended the study to lower energies and found a low-energy proton group with high intensity, which was assigned to originate from the T = 3/2 isobaric analog state (IAS) of the ground state of 23 Al. This was interpreted to occur due to extremely strong isospin-mixing as proton decay from the IAS does not conserve isospin.
Due to the astrophysical importance, and in order to confirm the results of Tighe et al. [27], a project to investigate the decay of 23 Al was initiated in Jyväskylä in Fig. 2 The high energy part of the γ -spectrum from β-decay of 23 Al as reported in [16] the late 1990's. An experimental setup capable of detecting γ -rays, electrons and heavier charged particles was installed to the end of the central beam line at IGISOL, see Fig. 1. Experimental details related to the 23 Al production are summarized in Section 2.1. This was the first ever study of β-decay of 23 Al to have a capability for simultaneous observation of both protons and γ -rays.
As the spectroscopy setup was also capable of detecting γ -rays, a γ -decay of the IAS to the ground and first excited state of 23 Mg was reported in [16] for the first time. These observations were confirmed later with much higher statistics by Iacob et al. [28]. Figure 2 presents Fig. 2a and b of [16], where peak labelled with 5γ is related to the transition from the IAS to the ground state and 4γ from the IAS to the first excited state.
From the decay spectroscopy point of view, it turned out to be that this initial IGISOL experiment [16] did not confirm the low energy part (below 400 keV) of the proton spectrum reported in [27]. At these energies significantly less intensity was observed at IGISOL [16] compared to [27]. Iacob et al. [28] suggested that this difference was caused by the higher cut-off energy in the E gas − E Si telescope at IGISOL [16]. However, in a recent experiment at Texas A&M, β-delayed proton spectrum with high statistics [29,30] was found out to be closer to the IGISOL spectrum [16] than the Berkeley spectrum [27]. A comparison of these spectra is illustrated in Fig. 3. 23 Mg was known to have a level with J π ≥ 3/2 + at 17 keV below the IAS already in [16]. Using shell-model calculations and the data obtained in [16] it was concluded that the spin and the parity of the state is not 3/2 + , 5/2 + or 7/2 + , i.e., allowed betadecay is not populating it. This conclusion also meant that all observed 200-keV protons were associated to the decay of the IAS. This was, however, a wrong Fig. 3 Comparison of the proton spectra from IGISOL and Texas A&M. The IGISOL spectrum is multiplied ×25 to scale with statistics of the Texas A&M experiment. Note that the IGISOL spectrum has its energy recorded as proton energy in lab, while the Texas A&M spectrum is the recorded total decay energy conclusion made due to the low statistics of the experiment. Namely, recent studies [28,31] have assigned this state to be (7/2) + and to receive a β-feeding of 4.89(25)%. As also correctly predicted by shell-model calculations [16], this (7/2) + state primarily decays to the first excited state of 23 Mg at 450.71 (15) keV. γ -rays with an energy of 7335.1 keV are associated to this transition [28]. The location of this line is marked into Fig. 2a. As shown, a small peak, almost at the level of the surrounding background, is visible, but it basically vanishes into the background when the full statistics is employed (Fig. 2b). Notice also, that due to the higher statistics and better energy resolution the more recent proton experiment (Fig. 3) assigned the lowest proton group to the 7787-keV state instead of the IAS.
Using a setup of four DSSSD-thick Si-pad telescopes, data from β-decay of 23 Al was measured at IGISOL during the calibration phases of another experiment in 2008. Improvements in the front-end after the first 23 Al experiment at IGISOL [16] increased the yield of 23 Al by a factor of around 35. This fact and the high efficiency of the setup enabled collection of sufficient statistics to observe 13 proton groups above the two main groups at 554 and 839 keV in only about 28 hours [32,33]. One of these groups was observed in the earlier experiment [16]. β-tail in the spectrum caused by the isobars at A = 23 extended all the way up to 700 keV, preventing the studies of low-energy proton groups.
Soon after this experiment a high-precision mass measurement of 23 Al and 23 Mg ground states was done with JYFLTRAP. The resulting masses, reported in [34], were used to test the IMME at A = 23. The quadratic form of the IMME was found to hold well for this T = 3/2 quartet [34]. The new mass values also confirmed the Q EC value deduced in [16].

31 Cl
Beta-decay of 31 Cl has been previously studied with the He-jet technique at the MC-35 cyclotron of the University of Oslo [35] and at the Lawrence Berkeley Laboratory 88-inch Cyclotron [36,37] by using proton beam with an energy of 34 or 45 MeV on ZnS targets, respectively. In these studies, eight proton peaks in the energy range of 845 to 2204 keV and a half-life of 150(25) ms [35] have been observed. However, these He-jet experiments suffer from contaminants with different mass numbers, such as 32 Cl [35][36][37] and 25 Si [37]. Therefore, it was important to study the beta decay of 31 Cl at IGISOL where on-line mass-separation is used.
Whereas the previous studies of the beta decay of 31 Cl have aimed for determination of experimental Gamow-Teller strength distribution and its comparison to the shell-model predictions [36,37], the recent study has also been motivated by nuclear astrophysics. Namely, the reaction rate of 30 P(p, γ ) 31 S plays a key role in ONe novae. 30 P is a mandatory passing point to 32 S via 30 P(p, γ ) 31 S(p, γ ) 32 Cl(β + ) 32 S or via 30 P(p, γ ) 31 S(β + ) 31 P(p, γ ) 32 S [12]. With a half-life of about 2.5 min, 30 P stops further nucleosynthesis unless the proton captures are fast enough. In addition, the knowledge of the final abundance pattern is important for example in the identification of the possible nova origin of presolar grains [40,41]. Large excesses in 30 Si/ 28 Si and close-to-solar 29 Si/ 28 Si abundance ratios have been measured in these grains. In fact, the calculations based on several hydrodynamic nova simulations show that if the 30 P(p, γ ) rate is reduced by a factor of 100, an enhancement in 30 Si is obtained [12,42].
Since the direct study of this reaction rate is currently limited by the difficulty of producing an intense 30 P beam for studies in inverse kinematics, most of the experimental effort has been concentrated on the studies of the excitation energies of the states in 31 S either via β + decay [17] or different reactions, such as 12 C( 20 Ne, n) 31 S [43], 32 S(p, d) 31 S [44], or 31 P( 3 He, t) 31 S [45][46][47]. Previously, the calculated reaction rates were based on statistical Hauser-Feshbach [48] calculations, which can have uncertainties as high as a factor of 100 up or down. In [43], a first evaluation of the 30 P(p, γ ) 31 S astrophysical reaction rate based on calculated resonant reaction rates of 13 experimentally determined states in 31 S was performed.
The IAS (T = 3/2) state at 6280(2) keV [17] was confirmed in [45] where an excitation energy of 6283(2) keV was measured from the triton spectrum of the 31 P( 3 He, t) 31 S reaction. In a later publication on 31 P( 3 He, t) 31 S reactions measured at Yale University's Wright Nuclear Structure Laboratory [46], 17 new levels and 5 new tentative levels were found in 31 S. The obtained reaction rate for 30 P(p, γ ) 31 S [46] was about a factor of two larger than that from [43,44] for 0.4 ≤ T ≤ 1.9 GK. Above 1 GK the reaction rate calculated in [46] is a factor of seven larger than in [44].
All states observed in the beta decay of 31 Cl at IGISOL [17], were confirmed in [46], except the very weakly populated state at 8669(40) keV [17]. This peak was already marked as a smaller than a 3σ peak [17], and thus, it can be due to random statistical fluctuations. The beta-delayed proton peak observed at 1174 (14) keV [17,36] was assumed to correspond to the proton decay to the 30 P ground state, and an excitation energy of 7347(14) keV [17] was deduced for the state in 31 S. This was assumed to correspond to a known state at 7311(11) keV in 31 S. However, in Jenkins et al. [43,49], an excitation energy of 7302.8(7) keV and a spin 11/2 + was determined for the 7311(11) keV state. Thus, this state cannot be strongly fed in the beta decay of 31 Cl having a ground-state spin of 3/2 + . Wrede et al. [46] have found that several states in 31 S proton decay also to states at 677.01(3) keV (0 + ) and 708.70(3) keV (1 + ) [50] in 30 P. They have also suggested that the 1174(14)-keV beta-delayed protons [17,36] would result from a proton decay to the 0 + state at 677.01(3) keV in 30 P. Then, the corresponding excitation energy would be 8024 (14) keV, close to the excitation energy of 8021(16) keV deduced from the 1826(15)-keV proton peak [17]. In future, coincidences of the beta-delayed protons with the γ -rays in 30 P should be searched for with better statistics.
The most recent study on 31 P( 3 He, t) 31 S reactions [47] performed at Maier-Leibnitz-Laboratorium (MLL) in Garching, Germany confirmed the results from previous ( 3 He, t) experiments. However, no evidence for the suggested doublets [46] at 6835 keV or 7030 keV was found. In addition, the angular distributions were analyzed and spin constraints were obtained for almost all critical states [47]. The hydrodynamic nova simulations performed in [47] show that the remaining uncertainties in the spin values and in the relevant proton spectroscopic factors may still lead to a factor of up to 20 variation in the proton capture rate on 30 P and a factor of up to four in the nova yields in the Si-Ar region.
The recent 31 P( 3 He, t) 31 S studies [45][46][47] have improved the precision of the IAS in 31 S. A weighted mean of 6283.2(8) keV [17,[45][46][47] for the IAS can be utilized in a more precise IMME fit to the A = 31 T = 3/2 quartet. At JYFLTRAP, the ground state mass of 31 S has been measured very precisely, −19042.55 (24) keV [51], and found to deviate from the Atomic Mass Evaluation 2003 (AME03) [52] by more than 1σ . In this paper, we have adopted the mass of 31 S from [51], the mass of 31 P from [53], the latest value for the IAS in 31 S, and used otherwise the same, most recent results for the T = 3/2 quartet at A = 31, as in [54], and performed a new IMME fit. The resulting mass excess value for 31 Cl is −7048(5) keV, which is 10 keV higher than the value obtained in [54] and about 20 keV higher than in the AME03 [52]. In future, JYFLTRAP aims to measure the mass of 31 Cl produced via 32 S(p, 2n) reactions.
β-decay of 31 Cl has been studied also at Texas A&M in two experiments with a similar setup as in the case of 23 Al [29,30]. These experiments confirm the results of the IGISOL experiment [17] for the proton spectrum up to about 2 MeV, even though suffering from 29 S impurities and the fact that the higher-energy protons escaped the implantation detector. Final analysis of the proton data from these experiments is an ongoing process. The first experiment yielded a good statistics for βγ -data, see [55][56][57] for the preliminary results. In the second experiment, the γ -data were extended further by acquiring βγ γ -coincidences to allow building up a more complete level scheme [57].
A more precise proton separation energy of 31 S, S p = 6130.95(39) keV, could be determined from the mass excess of 31 S measured at JYFLTRAP [51]. This new value deviates from the adopted AME03 value [52] by more than 1σ [51]. Taking into account the new proton separation energy and the new excitation energy for the T = 3/2 IAS in 31 S [17,[45][46][47], β-delayed protons from the IAS should have laboratory energies of about 147.3(9) keV.

41 Ti
Beta-decay of 41 Ti offers a possibility to study beta-decay strength over a wide energy range and compare the strength and its sum to the shell-model calculations in the crossing region of sd and f p shells. In addition, isospin mixing in the wave function of the T = 3/2 isobaric analog state (IAS), which makes the isospin-forbidden proton decay from the IAS possible, can be investigated.
Delayed proton emission following the beta-decay of 41 Ti was observed for the first time at Brookhaven National Laboratory [58,59]. Seventeen beta-delayed proton peaks of 41 Ti were observed and a half-life of 88(1) ms was determined [59]. Later, a high-resolution study of the 41 Ti decay was performed at Berkeley with the He-jet technique and E − E counter telescopes [60,61]. A half-life of 80(2) ms and 27 proton peaks belonging to 41 Ti beta decay were observed [61]. Most of the peaks agreed with the peaks observed in [59]. The location of the lowest T = 3/2 in 41 Sc was determined as 5935 (8)keV [60] disagreeing with the old 40 Ca(p, p) resonance measurements. Studies at Berkeley continued in the 1980s and six new beta-delayed proton peaks were found in [62]. 41 Ti was also used for calibration purposes in experiments at GSI and GANIL where half-lives of 81(4) ms [63] and 80.1(9) ms [64], respectively, were measured for 41 Ti.
In the experiment conducted at IGISOL, no beta-or proton-delayed γ -rays were observed [18]. Therefore, the intensities of the observed 25 proton peaks were directly proportional to the beta-decay transition intensities. For three peaks, betaproton summing was taken into account in the analysis of peak intensities. The main differences from [61] were that the intensity of the 986-keV proton peak was about half of the one given in [61], the proton group just below 1.6 MeV was found to be a double peak, and the 2063-keV proton group was not detected at IGISOL. On the other hand, proton peaks with energies of 754(12), 1586(11), 4298, and 4684(11) keV not observed in [61,62] were observed at IGISOL [18]. Of these peaks, the 4684(11)-keV protons originated from a new level at an energy of 5886 keV [18]. Extensive shell-model calculations of the 41 Ti beta decay were performed with the code OXBASH [18]. The observed experimental decay strength was found to be 0.64 times the theoretical strength corresponding to a quenching factor of 2 ), the isospins T = 3 2 and T = 1 2 of these near-lying states can mix with each other with an amplitude b according to a simple perturbation theory. Then, the perturbed IAS has a structure a|T = 3 2 + b |T = 1 2 and the other state is In order to explain the observed Fermi strength, a fraction b 2 = 10(8)% of the Fermi strength of the IAS (B(F) theor = 3) is shifted to this other state.
After the experiment performed at IGISOL, beta decay of 41 Ti has been studied at GSI [65]. The isotopically separated 41 Ti beam from FRS was implanted into a silicon detector stack consisting of eight 300 μm thick, 30-mm diameter Si detectors [65]. An array of 14 large-volume Crystal Ball NaI detectors were used to measure emitted γ rays [65]. The energy resolution of around 40 − 100 keV [65] was worse than at IGISOL, and some of the peaks observed at IGISOL [18] could not be resolved. In [65], the total number of implanted 41 Ti ions could be deduced from the E versus A/q data after correction for the intensity loss due to secondary reactions in the energy degrader at F 4 [65]. In addition, proton-γ coincidence data were collected and five transitions were assigned to populate the 3904-keV state in 40 Ca [65]. With the absolute counting of 41 Ti ions and proton-γ coincidence data, Liu et al. [65] obtain significantly larger B(GT) values at high excitation energies in 41 Sc than at IGISOL [18] where the branching ratios were based on the total number of observed protons. The total experimental B(GT) of 3.6(5) determined at IGISOL [18] is also smaller than the value obtained at GSI, 4.83 (29)  In order to normalize this proportionality, a B(GT) value known well from beta-decay studies is typically used. For the A = 41 system, such well-known beta transition does not exist. Therefore, the total sum of the B(GT) CE was normalized to the total sum of B(GT) β , which was obtained as an average from [18,65]. The transition to the IAS in 41 Ca contains both Fermi and GT components, and the B(GT) CE was estimated to be 0.24 (4). If a similar isospin impurity of 10(8)% [18] is assumed as for 41 Sc, the B(GT) CE value would be increased by 0.055 (45).
The B(GT) CE distribution [1] is quite similar to the B(GT) β distribution obtained at IGISOL [18] (see Fig. 4). Almost one-to-one correspondence of the observed states and GT transition strengths to them is observed up to around 6 MeV. The energy resolution of around 40 − 100 keV in [65] did not allow a good comparison to the charge-exchange reactions. In addition, the observed rather strong GT strength to states around 7 MeV in 41 Sc [65] was not observed via charge-exchange reactions c) Liu et al. 41 Ti β + decay to 41 Ca [1] nor at IGISOL [18]. Most probable J π values were deduced for each analog pair, which lead to a confirmation of J π = 1/2 + for the states at 3951 (14) and 6038(25) keV, J π = 3/2 + for the states at 3562.6(3), 5576(4) and 5939(4) keV, and J π = 5/2 + for the states at 4928(5), 5774(4), 5840(5) and 5886(12) keV suggested as 1/2 + , 3/2 + , 5/2 + states in the beta-decay study at IGISOL [18]. The main part of the GT strength was found to the 5/2 + states in charge-exchange reactions [1]. Mass of 41 Ti has been directly measured at the FRS-ESR facility at GSI [66]. However, the obtained precision for the mass excess, −15090(360) keV, is rather poor compared to Penning trap measurements. In future, the mass of 41 Ti could be measured with the purification trap of JYFLTRAP (as was done e.g. for 97 Kr at ISOLTRAP [67]) if the half-life is too short for precision trap measurements.

Discussion
Various experiments on beta decays of other T Z = −3/2, A = 4n + 1 nuclei than 41 Ti have been performed in the past: 9 [63] have been studied. At IGISOL, some of these beta decays could be investigated in future. JYFLTRAP mass spectrometer could be applied for example in the direct mass measurement of 31 Cl. Future measurements on these nuclei are also motivated by possible one-proton halos in 17 Ne, 23 Al, 31 Cl, and 35 K, and two-proton halos in 9 C and 13 O [80].
Straightforward experimental interpretation of the low-energy part of the 23 Al β-delayed proton spectrum, around 200 keV, is a challenging task. To distinguish unambiguously protons from the IAS and the neighboring 7787 keV state a detection setup that has a particle identification capability, excellent proton-energy resolution and a thin dead-layer is required. The challenge is similar in the case of β-decay of 31 Cl, where the β-delayed protons from the IAS have an energy of about 147 keV as discussed in Section 3.2.
This is a non-trivial experimental challenge. For example, with the telescope used in the studies of 23 Al and 41 Ti decays at IGISOL, the lower energy limit was about 160 keV. Even the high-end DSSSDs with ultra-thin dead layers have the detection thresholds around 150-200 keV, mostly due to the electronics noise in the measurement area. In addition, reducing the β-background requires extremely pure source, preferably with capability of removing the activity from the following daughter decays. In future, these requirements could possibly be fulfilled at IGISOL by combing a Penning trap and a micro-calorimeter detector [81] that employs digital electronics.
Novel detector assemblies could provide cleaner spectra and better energy resolution in future. An energy resolution of 1.06 keV, excellent compared to modern silicon detectors with resolutions of around 8.8 keV, has been achieved for 5.3-MeV alpha particles with a cryogenic microcalorimeter detector [82]. A microcalorimeter is based on the conversion of the particle's kinetic energy into thermal excitations in a tin absorber. The temperature change is measured with a superconducting transition edge sensor (TES) at its superconducting transition temperature of 140 mK where a small change in temperature results in a large change in resistance. An advantage of this technique is that it does not suffer from the surface dead layer effect but the disadvantage is the required low temperature. Typically, an adiabatic demagnetization refrigerator at 80 mK is used for a copper mount, and the TES is heated into its resistive transition by electrical bias [82]. This method requires that the sample is cooled and maintained at 80 mK in the same vacuum as the microcalorimeter. Therefore, it is not suitable for on-line experiments where the sample should be implanted and moved in short time periods unless a clever way to maintain the low temperature for TES is invented. Microcalorimeters can also be used for detecting γ -rays: a 47-eV resolution was achieved for a 103-keV γ -peak [83]. Major drawback of this method is that these microcalorimeters are small (≈ 1 mm 2 ) and slow (50 − 100 counts/s), and a large array of these microcalorimeters would be required for decent efficiency.
After the IGISOL upgrade in 2002-2003 [84] (including e.g. higher pumping efficiency, better radiation shielding to fully gain the high-intensity light ion beams, and improved gas purification control) and after the replacement of the skimmer by SPIG [14], the yields have increased a lot. For example, with a 8-10 μA, 40-MeV proton beam on nat Mg, yields of 700 atoms/s and 150000 atoms/s were observed for 23 Al and 23 Mg, respectively [34]. Thus, the production rates of 23 Al and 23 Mg were about 35 times higher than with the skimmer at the old IGISOL facility. Similar improvement for the production of 41 Ti can also be expected. 31 Cl was measured already after the IGISOL upgrade using the skimmer. The introduction of SPIG should increase the chlorine yields with a factor of about 4 − 10, as reported in [14] .
The production of 31 Cl has been challenging at IGISOL. This is seen for example from the production rates of 23 Al and 31 Cl produced via similar ( p, 2n) reactions at IGISOL: the yield of 23 Al has been higher than for 31 Cl (see Section 2.1). Chlorine is a halogen and it has a high electron affinity of 3.612724 (27) eV [85] (compare to 0.43283(5) eV [86] for Al). 31 Cl is, unfortunately, in the mass region where major molecular contaminant beams are present in form of different nitrogen and oxide compounds. These molecular beams, e.g. 15 N 16 O, arise mostly from either dirty He-gas or leaks in the gas feeding system. In the past these background beams have been so intense that they have overloaded the Penning trap and thus prevented even purification of the samples for mass measurements. Improvements for the gasfeeding system and He-gas purification for the new IGISOL facility are in the high priority, thus making measurements around A = 30 region with the JYFLTRAP a more viable venture. In future, it would be interesting to study the fraction of negative chlorine ions produced at IGISOL and to search for possibilities to produce a beam of these negative ions.
The experiments performed at IGISOL providing mass-separated, thin sources of short-lived nuclei have demonstrated the potential of high-resolution detection technique for β-delayed protons and γ -rays. Taking into account the expected improvement in the yields, these experiments also pave the way for the studies of more exotic nuclei, such as 22 Al and 40 Ti. 22 Al has a very exotic and interesting β-delayed decay spectrum from γ -rays to two-proton and alpha emissions. The decay of 40 Ti has significance in determining the detection efficiency of the ICARUS detector for the study of solar neutrinos by utilizing the inverse β-decay of 40 Ar. These two nuclei have earlier been studied using the He-Jet technique [87,88], fragment separator at GANIL [75,89,90] and FRS at GSI [65]. The new IGISOL facility may offer an alternative way for studying these nuclei.