Precision mass measurement of 173Hf for nuclear structure of 173Lu and the γ process

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Precision mass measurement of 173Hf for nuclear structure of 173Lu and the γ process © 2023 the Authors Published version Jaries, A.; Stryjczyk, M.; Kankainen, A.; Eronen, T.; Ge, Z.; Hukkanen, M.; Moore, I. D.; Mougeot, M.; Raggio, A.; Rattanasakuldilok, W.; Ruotsalainen, J. Jaries, A., Stryjczyk, M., Kankainen, A., Eronen, T., Ge, Z., Hukkanen, M., Moore, I. D., Mougeot, M., Raggio, A., Rattanasakuldilok, W., & Ruotsalainen, J. (2023). Precision mass measurement of 173Hf for nuclear structure of 173Lu and the γ process. European Physical Journal A, 59, Article 263. https://doi.org/10.1140/epja/s10050-023-01176-4 2023 Eur. Phys. J. A (2023) 59:263 https://doi.org/10.1140/epja/s10050-023-01176-4 Regular Article - Experimental Physics Precision mass measurement of 173Hf for nuclear structure of 173Lu and the γprocess A. Jaries1,a, M. Stryjczyk1,b, A. Kankainen1,c, T. Eronen1,Z.Ge 1,2, M. Hukkanen1,3,I.D.Moore 1, M. Mougeot1, A. Raggio1, W. Rattanasakuldilok1, J. Ruotsalainen1 1Department of Physics, Accelerator laboratory, University of Jyvaskyla, P.O. Box 35(YFL), 40014 University of Jyvaskyla, Finland 2GSI Helmholtzzentrum für Schwerionenforschung, 64291 Darmstadt, Germany 3Université de Bordeaux, CNRS/IN2P3, LP2I Bordeaux, UMR 5797, 33170 Gradignan, France Received: 17 August 2023 / Accepted: 21 October 2023 © The Author(s) 2023 Communicated by Klaus Blaum Abstract We report on the precise mass measurement of the 173Hf isotope performed at the Ion Guide Isotope Separator On-Line facility using the JYFLTRAP double Penning trap mass spectrometer. The new mass-excess value, ME =−55390.8(30)keV, is in agreement with the literature while being nine times more precise. The newly determined 173Hf electron-capture Qvalue, QEC =1490.2(34)keV, allows us to firmly reject the population of an excited state at 1578 keV in 173Lu and 11 transitions tentatively assigned to the decay of 173Hf. Our refined mass value of 173Hf reduces mass-related uncertainties in the reaction rate of 174Hf(γ, n)173Hf. Thus, the rate for the main photodisintegration destruction channel of the pnuclide 174Hf in the relevant temperature region for the γprocess is better constrained. 1 Introduction The radioactive neutron-deficient 173 72 Hf101 isotope (T1/2 =23.6(1)h[1]) was identified for the first time in 1951 [2]. Since then, its decay to 173Lu was studied in several experiments, see Ref. [3] and references therein. The current knowledge of the 173Hf decay is based primarily on the two most recent studies, reported by Funk et al. [4] and by Brenner et al. [5]. In both of these works the radioactive isotope of interest was produced by irradiating enriched Yb targets (172,173Yb in Ref. [4] and 172Yb in Ref. [5]) with an αbeam. The isotope of interest was extracted by means of chemical separation and the γ-ray radiation following the decay was ae-mail: arthur[email protected] (corresponding author) be-mail: [email protected] (corresponding author) ce-mail: [email protected] (corresponding author) measured using Ge(Li) detectors. In addition, in Ref. [4]the conversion electrons were detected using a Si(Li) detector. At the time of 173Hf decay studies publication [4,5], the electron-capture Qvalue (QEC) estimated from the systematics was 1600 keV [6]. Nonetheless, several γ-ray and conversion-electron transitions with an energy above 1.6 MeV were tentatively assigned to its decay [4,5]. To resolve this disagreement Funk et al. assumed that the QEC value is 1900 keV [4]. On the other hand, Brenner et al. indicatedthattheyassignedweakhigh-energyγraystothedecay of 173Hf when these transitions could not be associated with known impurities [5]. A mass measurement of 173Hf performed at the GSI storage ring [7] and the resulting decrease of QEC to 1469(28) keV [8] rendered the decay spectroscopy results incompatible. In addition to the high-energy transitions, a 1578-keV excited state in 173Lu proposed in the work by Funk et al. [4], was also found to be inconsistent with the new QEC value. An independent evaluation of the 173Hf mass and, consequently, of the QEC value would enable a resolution of the aforementioned issues. Such a measurement would allow us to unambiguously establish whether the high-energy transitions were correctly assigned and verify the presence of the 1587-keV state. In addition to the nuclear spectroscopy interest, the mass of 173Hf is also of relevance for the astrophysical pprocess, also known as the γprocess [9,10]. The process proceeds mainly via photodisintegration reactions and takes place in thermonuclear and core-collapse supernovae, when the shock wave passes through the O-Ne layer at typical temperatures of around 1.5–4 GK. The γprocess produces altogether 35 stable isotopes. The production of heavier pprocess isotopes, such as the long-lived radionuclide 174Hf (T1/2=2.0(4)×1015 y[1]), is very sensitive to tempera0123456789().: V,-vol 123 263 Page 2 of 5 Eur. Phys. J. A (2023) 59:263 ture [11]. This is due to the competition between two highly temperature-dependent reactions, namely, (γ, α) and (γ, n). As the level densities are high for such heavy isotopes, the reaction rates are typically calculated using the statistical Hauser-Feshbach (HF) approach [12]. Precise knowledge of the corresponding ground-state properties of the target nucleus and residual nuclei, such as masses, is needed for the HF calculations [10]. In this work, we constrain the mass-related uncertainties related to the 174Hf(γ, n)173Hf reaction via a high-precision mass measurement of 173Hf at the JYFLTRAP double Penning trap. The results are discussed in the context of the nuclear structure and the astrophysical γprocess. 2 Experimental method and results The experiment was performed at the Ion Guide Isotope Separator On-Line (IGISOL) facility [13,14] at the University of Jyväskylä, Finland. Both the 173Hf isotope of interest and the 173Yb reference-mass isotope, were produced in a fusion-evaporation reaction of a 50-MeV αbeam, delivered by the K130 cyclotron with an average current of 1.1 pμA, and a 1.75 mg/cm2-thick natYb target mounted within the light-ion ion guide. The reaction products were stopped in a helium-filled gas cell operating at about 250 mbar. The ions were subsequently extracted with gas flow and guided to the high-vacuum region of the mass separator using a sextupole ion guide [15], accelerated by a 30-kV potential and mass-separated by a 55◦dipole magnet. The continuous beam was injected into the radio-frequency quadrupole cooler-buncher [16] where it was cooled and bunched. From there the radioactive ion beam was finally delivered to the JYFLTRAP double Penning trap [17]. In JYFLTRAP, the singly-charged A=173 ions were first cooled, purified to contain only 173Yb, 173Lu and 173Hf, and centered using a mass-selective buffer gas cooling technique [18] in the first trap. After that, the ions were sent to the second (measurement) trap where their charge-over-massdependent (q/m) cyclotron frequency νc=qB/(2πm)in a magnetic field Bwas measured by using a phase-imaging ion cyclotron resonance (PI-ICR) technique [19–22]. Using the PI-ICR technique, the cyclotron frequency νc of an ion is obtained from the phase differences between its radial in-trap motions during a phase accumulation time tacc (see Fig. 1). In the present case, tacc value was set to 584 ms to avoid an overlap between the projections of 173Hf (ion of interest), 173Lu (isobaric contaminant) and 173Yb (reference mass). The final mass value for 173Hf is obtained by measuring a cyclotron frequency ratio between 173Hf and 173Yb. For the 173Yb reference, the mass excess reported in the Atomic Mass Evaluation 2020 (AME20), MElit.=−57551.234(11)keV [8] and based on the PenFig. 1 Projection of the cyclotron motion of 173Hf+and the isobaric contaminants of 173Lu+and 173Yb+ions onto the position-sensitive detectorobtainedwiththePI-ICRtechniqueusingaphaseaccumulation time tacc =584 ms. The total number of ions is 764 and the number of ions per bunch has not been limited for this figure ning trap measurement [23] was used. The measurements of the ion of interest and the reference ion were alternated every ∼5min to account for the temporal magnetic field fluctuations. The energy difference between 173Yb and 173Hf isotopes, E, was calculated using the cyclotron frequency ratio r=νc,ref /νcof singly-charged ions of both species: E=(r−1)[mref −me]c2,(1) with meand mref being the masses of a free electron and the atomic mass of 173Yb, respectively, and cbeing the speed of light in vacuum. The contribution from electron binding energies are on the order of a few eV and have thus been neglected. To reduce any systematic uncertainty due to ionioninteractions,thecountratewaslimited to one detected ion per bunch. The systematic uncertainties due to the magnetron phase advancement, the angle error and the temporal magnetic field fluctuation δB/B=2.01(25)×10−12 min−1×δt with δtbeing the time between the measurements were taken into account [22]. However, their effect (δr/r∼6×10−9, ∼2×10−9and ∼2×10−11, respectively) is much smaller compared to the statistical uncertainty (δr/r∼2×10−8). The experimental results are summarized in Table 1. The calculated Evalue and the deduced mass excess of 173Hf (ME(173Hf)=ME(173Yb)+E) are in agreement with the literature (ME −MElit.=21(28)keV), however, our result is nine times more precise. To obtain QEC(173Hf)=ME(173Hf)−ME(173Lu),theME lit.(173Lu) =−56881.0(16)keV was taken from AME20 [8] and it 123 Eur. Phys. J. A (2023) 59:263 Page 3 of 5 263 Table 1 A comparison of the energy difference between 173Hf and 173Yb (E), the mass excess of 173Hf (ME(173Hf)) and its electroncapture Qvalue (QEC(173Hf)) between this work and AME20 [8]. The cyclotron frequency ratio r=νc,ref /νcdetermined in this work using the PI-ICR technique is also reported Quantity AME20 This work r=νc,ref /νc1.000013411 (19) E(keV) 2139 (28) 2160.4 (30) ME(173Hf)(keV) −55412 (28) −55390.8 (30) QEC(173Hf)(keV) 1469 (28)1490.2(34) leads to QEC(173Hf)=1490.2(34)keV. The updated value agrees with the literature (1469(28)keV [8]), however, it is eight times more precise. The agreement between the mass measurements reported in Ref. [7] and this work allows us to unambiguously reject the hypothesis of the 1578-keV state being populated in the β decay of 173Hf. We can also remove seven transitions (1505, 1551.0, 1557.7, 1749, 1778.4, 1836 and 1897 keV) assigned to the decay of 173Hf in Ref. [4] and four transitions (1512.5, 2043.0, 2127.7 and 2613.1 keV) from Ref. [5]. We note that there are two transitions at 1485.1 keV [4] and at 1488.9 keV [5] which are within 2σof the updated QEC value, therefore, they cannot be unambiguously removed or kept in the 173Hf decay scheme. There are several possible explanations why the aforementioned transitions were incorrectly assigned to the decay of 173Hf. All of them have a very low absolute intensity (Iγ<10−3) which hindered the γ−γcoincidence analysis. The radioactive samples were prepared using chemical separation methods which are known to have a limited reliability. As a result, transitions originating from different species could also be observed. In addition, the low QEC value prevented the β−γcoincidence analysis as the vast majority of the decays underwent the electron capture channel [3]. This could result in an accidental assignment of the background transitions to the decay scheme. The more precise mass value of 173Hf is also relevant for constraining the calculated photodisintegration reaction rate on the pnuclide 174Hf. Although the natural abundance of the radionuclide 174Hf is rather low, 0.16(1)%, it has been shown that it can be used as a tracer to explore the distribution of supernova material in the early solar system [24]. Constraining the photodisintegration reaction rates of 174Hf has an impact not only on the 174Hf abundance but also on the lighter pnuclide abundances as the process eventually proceedsto lighterelementsvia(γ, p)and(γ, α) reactions.Here we constrain the 174Hf(γ, n)173Hf reaction rate with the new, more precise mass value of 173Hf (see Fig. 2). The astrophysical reaction rates were calculated with the TALYS−1.96 code [25], using the default phenomenological level density Fig. 2 Astrophysical reaction rate ratio using the mass of 173Hf determined in this work (JYFLTRAP) and the AME20 mass values for the 174Hf(γ,n)173Hf reaction as a function of temperature. The (γ, n)is the main photodisintegration destruction channel of 174Hf down to temperatures of 2 GK model based on the Fermi gas model and the local optical model potential parametrization [25]. The masses of 173Hf and 174Hf were adopted from this work (JYFLTRAP) and AME20 [8] and varied up or down by 1σto obtain the maximum and minimum Qvalues for the reaction of interest. The updated Qvalue for the 174Hf(γ, n)173Hf reaction resulted in a reaction rate decrease by up to 13% for the relevant temperature region compared to the rate calculated with theAME20 masses, see Fig.2.The(γ, n)reaction is the main photodisintegration destruction channel of 174Hf for temperatures down to 2 GK below which the (γ, α) starts to dominate. The total photodisintegration reaction rate, however, decreasessignificantlyatthoselowertemperatures.Although there are also many other uncertainties related to the reaction rates and the p-nuclide abundances (see e.g. [26,27]), the mass-related reaction rate uncertainties for the main destructionchannelofthe pnuclide174Hfweresignificantlyreduced in this work, e.g. from ≈14% to ≈2.4% at 2.0 GK. 3 Conclusions The mass of 173Hf was measured with high precision using the PI-ICR method at the JYFLTRAP double Penning trap. The result is in agreement with the literature data, however, it is nine times more precise. The updated QEC value of 173Hf allowed us to exclude one excited state and 11 transitions in the daughter nucleus 173Lu, previously assigned to the decay 173Hf. The high-precision mass measurement also constrained the calculated (γ, n)photodisintegration rate on the pnucleus 174Hf. 123 263 Page 4 of 5 Eur. Phys. J. A (2023) 59:263 Acknowledgements This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreements No. 771036 (ERC CoG MAIDEN) and No. 861198LISA-H2020-MSCA-ITN-2019 and from the Academy of Finland projects No. 295207, 306980, 327629, 354589 and 354968. J.R. acknowledges financial support from the Vilho, Yrjö and Kalle Väisälä Foundation. Funding Open Access funding provided by University of Jyväskylä (JYU). 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