Search for live 182Hf in deep-sea sediments

doi:10.1016/j.newar.2003.11.023

New Astronomy Reviews

Volume 48, Issues 1–4, February 2004, Pages 161-164

https://doi.org/10.1016/j.newar.2003.11.023 Get rights and content

Abstract

The presence of live ¹⁸²Hf (t_1/2∼9 Myr) in the early solar system is well established but the understanding of its abundance is still challenging. Live ¹⁸²Hf is expected to be present in the interstellar medium (ISM) as a result of recent nucleosynthesis activity. We are attempting a search for live ¹⁸²Hf possibly deposited on Earth. The search focuses on deep-ocean sediments and a method for chemical extraction of the Zr–Hf fraction from sediments has been developed. The detection of ¹⁸²Hf is performed at the Vienna Environmental Research Accelerator (VERA). Measurements of Hf and W isotopic abundances for the Zr–Hf fraction extracted from deep-sea sediment samples were performed. Present limits for the ¹⁸²Hf abundance derived from the measured isotopic abundances are discussed.

Introduction

The presence of live ¹⁸²Hf (t_1/2=(9±2) Myr, Wing et al., 1961) in the early-solar system was first shown (Harper, 1991) through W isotopic anomalies in the iron meteorite Toluca and later confirmed (Lee and Halliday, 1995; Harper and Jacobsen, 1996; Lee and Halliday, 2000). The most recent values for the ¹⁸²Hf/¹⁸⁰Hf initial abundance have been determined as 1.0×10⁻⁴ (Yin et al., 2002; Kleine et al., 2002; Schoenberg et al., 2002); however, this ratio is still considered not well established and seems to be a lower limit (Quitté et al., 2003; Halliday, 2003). ¹⁸²Hf is believed to be produced by r-process nucleosynthesis; its early-solar system abundance seems to be in line with steady-state production but contrasts with the apparent suppression of other r-process nuclides (e.g. ¹²⁹I, Qian and Wasserburg, 2000). A significant amount of ¹⁸²Hf could also be produced through a fast s-process in massive stars (Meyer and Clayton, 2000; Meyer et al., 2003).

In any production scenario, live ¹⁸²Hf is expected to be present in the interstellar medium (ISM) as a result of recent nucleosynthesis. γ-ray detection of ¹⁸²Hf (similarly to ²⁶Al, Plüschke et al., 2001) is unfeasible due to its overall low activity. However, deposition of ISM grains by accretion onto Earth could make direct detection of live ¹⁸²Hf possible in slow-accumulating reservoirs such as deep-sea sediments. Importantly, ¹⁸²Hf is one of the few extinct radionuclides without significant natural or artificial production on Earth. Production from spallation in the atmosphere is prevented by the lack of heavy target nuclei. In the lithosphere, neutron-induced reactions are absent since they start from unstable nuclides and production by μ-induced spallation on stable W isotopes should be extremely small due to the low W abundance. In addition, ¹⁸²Hf is above the fission peaks and has not been produced in nuclear tests. Recently, an indication for a nearby supernova has been found through detection of ⁶⁰Fe (t_1/2=1.5 Myr) in ferro-manganese crusts (Knie et al., 1999). There are two independent measurements of the longer-lived ²⁴⁴Pu (t_1/2=81 Myr) (Wallner et al., 2000; Paul et al., 2001) but more measurements are needed (Winkler et al., 2004). We report here on a first attempt to detect live ¹⁸²Hf in the Hf–Zr separated fraction of deep-sea sediment by accelerator mass spectrometry (AMS).

Section snippets

Chemical preparation and AMS measurements

A sediment core (TRIPOD 7P, 17°30^′N 113°00^′W, water depth 3763 m, sediment depth 3–228 cm) was selected at Scripps Oceanographic Institute (Lal, 2001). The sediment (red clay) 5 cm-diameter core was divided along its length in four quarters and one quarter, divided in sections of 3–8 cm long, was made available to us. Table 1 lists the average concentrations of some relevant elements in the sediment core. A method for extraction of the Zr–Hf and actinides fractions was developed at Hebrew

Present limit for ¹⁸²Hf deposition from ISM and discussion

Following the approach of Paul et al. (2001) for ²⁴⁴Pu, a present limit for ¹⁸²Hf deposition can be derived. Assuming the sediment is homogeneous in ¹⁸²Hf and using the lowest measured ratio $(^{182} Hf +^{182} W)/^{180} Hf =3.2×10$ ⁻⁵, we can correct for the ¹⁸²W contribution. The W isotopic ratios are measured with an accuracy of ∼5%. Thus the correction for the ¹⁸²W using the measured ¹⁸³W/¹⁸⁰Hf ratio and the nominal value for the ¹⁸²W/¹⁸³W ratio results in a limit of ¹⁸²Hf/¹⁸⁰Hf <1×10⁻⁶ or <1×10¹⁰ ¹⁸²Hf

Acknowledgements

We thank D. Lal and W. Smith (Scripps Institute of Oceanography) for their help and for making available to us part of the TRIPOD 7P sediment core. This work was supported in part by Israel Science Foundation grant No. 43/01-1.

References (24)

C.L Harper et al.
Geochim. Cosmochim. Acta
(1996)
D.-C Lee et al.
Chem. Geol.
(2000)
R Schoenberg et al.
Geochim. Cosmochim. Acta
(2002)
Y.-Z Qian et al.
Phys. Rep.
(2000)
C Wallner et al.
NIM B
(2000)
C Vockenhuber et al.
Int. J. Mass Spectrosc.
(2003)
E Anders et al.
Geochim. Cosmochim. Acta
(1989)
J Wing et al.
Phys. Rev.
(1961)
C.L Harper et al.
LPSC
(1991)
D.-C Lee et al.
Nature
(1995)

Q Yin et al.

Nature

(2002)

T Kleine et al.

Nature

(2002)

Cited by (24)

The ILIAMS project – An RFQ ion beam cooler for selective laser photodetachment at VERA
2019, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
Citation Excerpt :
This was attributed to the formation of oxyfluorides, although the exact processes remained elusive. Since such technique may enable measurements of the astrophysically interesting trace isotope 182Hf [15,16], we decided to investigate the behavior of 182WF5− in different buffer gas mixtures by mass analysis of the ejected ion current. The results are shown in Fig. 4b.
Selective laser photodetachment of anions is a novel technique for isobar suppression in Accelerator Mass Spectrometry (AMS). Ion-laser interaction times on the order of ms required for near-complete isobar suppression are achieved by retarding the ions in a gas-filled radio frequency quadrupole cooler. Inside this RFQ, the cooled anion beam is overlapped collinearly with an intense cw-laser beam. Within the Ion Laser InterAction Mass Spectrometry (ILIAMS) project at the University of Vienna, a dedicated injector beamline has been coupled to the VERA-AMS facility to explore and develop this method. In this work, experimental investigations on ion beam transmission, stability and elemental selectivity of the new setup are presented.
A 532 nm laser at 10 W transmitted power provides suppression factors larger than ten orders of magnitude for S⁻ and MgO⁻ under AMS conditions with simultaneous beam transmission for the ions of interest of up to 80%. The excellent ion identification capabilities of the subsequent AMS system also facilitate the study of destruction and formation of molecular anions inside the ion cooler. These kinetic and chemical reactions with the buffer gas provide additional elemental selectivity in certain cases, whereas others constitute a source of background.
The ILIAS project for selective isobar suppression by laser photodetachment
2015, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
Citation Excerpt :
The ultimate goal for the application of our scheme is the separation of 182Hf from the stable isobar 182W. It has been shown in earlier studies [17–20] that 182W can be suppressed in the ion source by extracting the molecules 182HfF5− and 182WF5−. Electron affinities of these molecules are currently unknown.
Laser photodetachment is the process when the extra electron of a negative ion is removed by means of laser radiation. This can happen only if the photon energy is larger than the electron affinity of the ion. The process can be used in mass spectrometry to selectively suppress unwanted isobars, provided that the electron affinity of the unwanted isobar is lower than that of the isobar under investigation.
At the Ion Laser InterAction Setup (ILIAS) at the University of Vienna laser photodetachment of negative atomic and molecular ions is studied and its applicability for selective isobar suppression in accelerator mass spectrometry (AMS) is evaluated. The setup provides mass separated beams of negative ions with energies up to 30 keV. Negative ions are produced in a Middleton type cesium sputter ion source, mass selected and overlapped with a strong continuous wave laser beam. In order to extend the interaction time of ions and laser, the ion beam is decelerated to thermal energies in a gas-filled radio frequency quadrupole cooler. For an appropriate choice of the photon energy, unwanted isobars are neutralized while the isobar of interest is unaffected and remains negatively charged.
A description of the ILIAS setup and results from the commissioning phase of the RFQ cooler are presented. Up to 8% ion beam transmission could be achieved after a recent redesign of the extraction system. Furthermore first results of photodetachment experiments of ⁶³Cu⁻ within the RFQ cooler are presented.
Reassessment of <sup>182</sup>Hf AMS measurements at VERA
2011, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
Citation Excerpt :
The results of the 182Hf AMS measurements comparing the performance of ion source S1 and S2 are shown in Table 1. The measurement using source S1 is in agreement with the already established measurement limits for VERA [1,2]. However, for source S2 the high tungsten background significantly reduces the achievable precision and induces a higher blank level.
The radioisotope ¹⁸²Hf (t_1/2 = 8.9 Ma) is of great interest for astrophysical applications as a chronometer for the early solar system or as possible live supernova remnant on earth. However, AMS measurements of ¹⁸²Hf are seriously influenced by the presence of the stable isobar ¹⁸²W, which cannot be separated at typical AMS energies. Previous studies revealed a possible suppression of ¹⁸²W against ¹⁸²Hf by extracting the negatively charged pentafluoride ${HfF}_{5}^{-}$ from the ion source, leading to a detection limit for ¹⁸²Hf/¹⁸⁰Hf in the order of 10⁻¹¹. However, this suppression behavior is in contrast to theoretical calculations of the electron affinity and recent measurements using SIMS instruments, where the achieved suppression cannot be reproduced. The aim of our study is to determine the effects of ion source background as well as further investigate the suppression of tungsten against hafnium by extracting negatively charged fluoride ions from different sample materials. The previously reported suppression factor of about 6000 could be increased to 36000 by careful tuning of the ion source using HfF₄ as sample material. The trend of the theoretical electron affinities could be reproduced using atomic tungsten and hafnium instead of HfF₄ as sample material. This supports the assumption that the major contribution of the tungsten background is not sputtered from the target matrix but comes from somewhere else in the ion source. Measurements from the second ion source show a higher background of tungsten and a lower suppression factor, i.e. careful design of the ion source is crucial. Moving the sputter beam over the target surface extending over the wheel holding the targets revealed the highest tungsten background was detected outside the sputter target position. Further investigations are necessary to locate the origin of the tungsten background in the ion source. Possible sources are the material used for the ion source construction or contaminations in the cesium used for sputtering.
Improvement on AMS measurement method for <sup>182</sup>Hf
2010, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
In order to improve the detection efficiency and sensitivity for ¹⁸²Hf measurement in HI-13 AMS systems at China Institute of Atomic Energy (CIAE), a new method was developed. Main features of the method include: (1) the installation of a new injection system with high mass resolution, (2) the establishment of a method based on solid-phase reaction to prepare HfF₄ samples from HfO₂. The experimental results show that the, mass resolution (M/ΔM) of the new injector system can reach nearly 700; the beam current of $^{180} {HfF}_{5}^{-}$ ions from HfF₄ samples produced by solid-phase reaction is about two times higher than that of previous method.
Nuclear astrophysics and AMS - Probing nucleosynthesis in the lab
2010, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
Nuclear astrophysics aims at describing nuclear processes relevant to nucleosynthesis. Such reactions can be studied by performing nuclear cross-section measurements at the relevant energy regimes. Accelerator-based experiments allow simulating nucleosynthesis in the laboratory. For specific reactions accelerator mass spectrometry (AMS) offers a powerful tool to measure cross-sections independent on half-lives of reaction products. It represents a complementary, off-line method compared to on-line methods, the latter being sensitive to prompt reaction signatures. An overview over recent experiments using AMS in nuclear astrophysics is given and for selected reactions the potential of AMS is exemplified: limitations and advantages of this method are illustrated for neutron-induced reactions on ⁹Be, ¹³C and ⁵⁴Fe, leading to the long-lived AMS isotopes ¹⁰Be, ¹⁴C, and ⁵⁵Fe, respectively. Measurements on ⁵⁵Fe allow producing highly precise data. The potential of AMS for helping to resolve a recently observed discrepancy between observation and nucleosynthesis models relevant for our understanding of the isotopic abundances is highlighted.
Isobar suppression in AMS using laser photodetachment
2008, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
We are investigating the possibility of using laser photodetachment of negative atomic and molecular ions as an additional isobaric selection filter in accelerator mass spectrometry. The aim of this study is to find a possibility to further improve the detection limit for long-lived heavy radionuclides at AMS facilities. We will focus on the astrophysical relevant radionuclide ¹⁸²Hf, which is one of the isotopes measured with the 3 MV tandem AMS facility VERA (Vienna Environmental Research Accelerator) at the University of Vienna. Laser-induced isobar suppression is also of importance for radioactive-beam facilities.
The present detection limit for measuring the isotope ratio ¹⁸²Hf/Hf at VERA is $1 \times 10^{- 11}$ . The limiting factor is the strong background of the stable isobar ¹⁸²W. Currently this background is suppressed using suitable molecular ions in the injection stage. Selective laser photodetachment of the negative ions at the injector can lead to an additional suppression of the interfering isobar. Test experiments have been carried out at the negative ion laser spectroscopy setup at Göteborg University. In a small ion beam apparatus pulsed tunable laser radiation is used to measure the photodetachment cross-section of different atomic and molecular negative ions. We will present studies of the photodetachment process for various tungsten and hafnium molecules with the aim to find a selective isobaric suppression scheme using laser photodetachment spectroscopy in combination with AMS.

View all citing articles on Scopus

View full text

Search for live 182Hf in deep-sea sediments

Abstract

Introduction

Section snippets

Chemical preparation and AMS measurements

Present limit for 182Hf deposition from ISM and discussion

Acknowledgements

Geochim. Cosmochim. Acta

Chem. Geol.

Geochim. Cosmochim. Acta

Phys. Rep.

NIM B

Int. J. Mass Spectrosc.

Geochim. Cosmochim. Acta

Phys. Rev.

LPSC

Nature

Nature

Nature

Search for live ¹⁸²Hf in deep-sea sediments

Present limit for ¹⁸²Hf deposition from ISM and discussion