Production Rates Of Cosmogenic Radionuclides Research Articles

Holocene solar activity inferred from global and hemispherical cosmic-ray proxy records

Variations in solar activity have been proposed to play an important role in recent and past climate change. To study this link on longer timescales, it is essential to know how the Sun has varied over the past millennia. Direct observations of solar variability based on sunspot numbers are limited to the past 400 years, and beyond this we rely on records of cosmogenic radionuclides, such as 14C and 10Be in tree rings and ice cores. Their atmospheric production rates depend on the flux of incoming galactic cosmic rays, which is modulated by Earth’s and the Sun’s magnetic fields, the latter being linked to solar variability. Here we show that accounting for differences in hemispherical production rates, related to geomagnetic field asymmetries, helps resolve so far unexplained differences in Holocene solar activity reconstructions. We find no compelling evidence for long-term variations in solar activity and show that variations in cosmogenic radionuclide production rates on millennial timescales and longer, including the 2,400-year Hallstatt cycle, are explained by variations in the geomagnetic field. Our results also suggest an on-average stronger dipole moment during the Holocene, associated with higher field intensities in the Southern Hemisphere.

Study of cosmogenic activation in 76Ge enriched germanium detectors during fabrication and transportation above ground

Rare event search experiments using germanium detectors are operated in underground laboratories to minimize the background induced by cosmic rays. However, the cosmogenic activation in germanium crystals on the ground during fabrication and transportation generates long half-life radionuclides and contributes a considerable background. We simulated the production rates of cosmogenic radionuclides in germanium and calculated the specific activities of cosmogenic radionuclides according to the scheduled fabrication and transportation processes of 76Ge enriched germanium detectors. The impact of cosmogenic background in germanium crystals for the next generation CDEX experiment was assessed with the scheduled exposure history above ground.

Effects of Global Geomagnetic Field Variations Over the Past 100,000 Years on Cosmogenic Radionuclide Production Rates in the Earth's Atmosphere

AbstractThe production rates of cosmogenic radionuclides, such as 10Be, 14C, and 36Cl, in the Earth's atmosphere vary with the geomagnetic field and solar activity. For the first time, the production rates of several cosmogenic nuclides are estimated for the past 100 ka based on global, time‐dependent geomagnetic field models and a moderate solar‐activity level. In particular, the production rates were high with no notable latitudinal dependence during the Laschamps geomagnetic excursion (41 ka BP). The mean global production of 10Be over the Laschamps excursion was more than two times greater than the present‐day one, whereas the increase was 1.9 times for the Norwegian‐Greenland Sea excursion (∼65 ka), and only 1.3 times for the Mono Lake/Auckland excursion (∼34 ka). All analyzed geomagnetic field models covering the past 100 ka, including the modern and Holocene epochs, lead to hemispheric asymmetry in the production rates, persistent overall time ranges, and reflected in the time‐averaged nuclide production rates. Production rates predicted by the geomagnetic field models are in good agreement with actual measurements from ice cores and sediment records. These global, long‐term production rates are important for a wide range of studies that employ cosmogenic nuclides as a proxy/tracer of different Earth system processes.

Synchronizing ice-core and U ∕ Th timescales in the Last Glacial Maximum using Hulu Cave 14C and new 10Be measurements from Greenland and Antarctica

Abstract. Between 15 and 27 kyr b2k (thousands of years before 2000 CE) during the last glacial, Greenland experienced a prolonged cold stadial phase, interrupted by two short-lived warm interstadials. Greenland ice-core calcium data show two periods, preceding the interstadials, of anomalously high atmospheric dust loading, the origin of which is not well understood. At approximately the same time as the Greenland dust peaks, the Chinese Hulu Cave speleothems exhibit a climatic signal suggested to be a response to Heinrich Event 2, a period of enhanced ice-rafted debris deposition in the North Atlantic. In the climatic signal of Antarctic ice cores, moreover, a relative warming occurs between 23 and 24.5 kyr b2k that is generally interpreted as a counterpart to a cool climate phase in the Northern Hemisphere. Proposed centennial-scale offsets between the polar ice-core timescales and the speleothem timescale hamper the precise reconstruction of the global sequence of these climatic events. Here, we examine two new 10Be datasets from Greenland and Antarctic ice cores to test the agreement between different timescales, by taking advantage of the globally synchronous cosmogenic radionuclide production rates. Evidence of an event similar to the Maunder Solar Minimum is found in the new 10Be datasets, supported by lower-resolution radionuclide data from Greenland and 14C in the Hulu Cave speleothem, representing a good synchronization candidate at around 22 kyr b2k. By matching the respective 10Be data, we determine the offset between the Greenland ice-core chronology, GICC05, and the Antarctic chronology for the West Antarctic Ice Sheet Divide ice core (WDC), WD2014, to be 125 ± 40 years. Furthermore, via radionuclide wiggle-matching, we determine the offset between the Hulu speleothem and ice-core timescales to be 375 years for GICC05 (75–625 years at 68 % confidence) and 225 years for WD2014 (−25–425 years at 68 % confidence). The rather wide uncertainties are intrinsic to the wiggle-matching algorithm and the limitations set by data resolution. The undercounting of annual layers in GICC05 inferred from the offset is hypothesized to have been caused by a combination of underdetected annual layers, especially during periods with low winter precipitation, and misinterpreted unusual patterns in the annual signal during the extremely cold period often referred to as Heinrich Stadial 1.

Geomagnetic dipole moment variations for the last glacial period inferred from cosmogenic radionuclides in Greenland ice cores via disentangling the climate and production signals

The geomagnetic dipole moment (GDM) modulates the production rates of cosmogenic radionuclides via the shielding of galactic cosmic rays. Therefore, it is possible to use this linkage to reconstruct past changes in the GDM based on cosmogenic radionuclide records from natural archives such as ice cores. Here we present a GDM reconstruction based on 10Be and 36Cl data from two Greenland ice cores from 11.7 ka to 108 ka b2k (before A.D. 2000). We find that the cosmogenic radionuclide records reflect a mixture of climate and production effects that require separation to evaluate the changes in the GDM. To minimize climate-related variations on isotope data, we applied a multi-linear correction method by removing common variability between 10Be and 36Cl and climate parameters (accumulation rates, δ18O and ion data) from radionuclide records. The resulting “climate corrected” radionuclide data are converted to GDM using a theoretical production model. Comparison of “climate corrected” radionuclides based GDM reconstructions with independent paleomagnetic-derived GDM records shows a good agreement. Furthermore, the “climate correction” leads to an improved agreement with GDM reconstructions than simply using radionuclide fluxes, lending support to the validity of our correction method to isolate production rate changes from ice core radionuclide records. With this correction method, we can extend the GDM reconstructions based on the cosmogenic radionuclides in ice cores to a period when there is a strong climate signal in the data.

Study on cosmogenic radioactive production in germanium as a background for future rare event search experiments

Rare event search experiments are one of the most important topics in the field of fundamental physics, and high-purity germanium (HPGe) detectors with an ultra-low radioactive background are frequently used for such experiments. However, cosmogenic activation contaminates germanium crystals during transport and storage. In this study, we investigated the movable shielding containers of HPGe crystals using Geant4 and CRY Monte Carlo simulations. The production rates of 68Ge, 65Zn, 60Co, 55Fe, and 3H were obtained individually for different types of cosmic rays. The validity of the simulation was confirmed through a comparison with the available experimental data. Based on this simulation, we found that the interactions induced by neutrons contribute to approximately 90% of the production rate of cosmogenic activation. In addition, by adding an optimized shielding structure, the production rates of cosmogenic radionuclides are reduced by about one order of magnitude. Our results show that it is feasible to use a shielding container to reduce the cosmogenic radioactivity produced during the transport and storage of high-purity germanium on the ground.

Single-Year German oak and Californian Bristlecone Pine 14C Data at the Beginning of the Hallstatt Plateau from 856 BC to 626 BC

ABSTRACTAs part of the ongoing effort to improve the Northern Hemisphere radiocarbon (14C) calibration curve, this study investigates the period of 856 BC to 626 BC (2805–2575 yr BP) with a total of 403 single-year 14C measurements. In this age range, IntCal13 was constructed largely from German and Irish oak as well as Californian bristlecone pine 14C dates, with most samples measured with a 10-yr resolution. The new data presented here is the first atmospheric 14C single-year record of the older end of the Hallstatt plateau based on an absolutely dated tree-ring chronology. The data helped reveal a major solar proton event (SPE) which caused a spike in the production rate of cosmogenic radionuclides around 2610/2609 BP. This production event is thought to have reached a magnitude similar to the 774/775 AD production event but has remained undetected due to averaging effects in the decadal calibration data. The record leading up to the 2610/2609 BP event reveals a 11-yr solar cycle with varying cyclicity. Features of the new data and the benefits of higher resolution calibration are discussed.

Temporal and Spatial Variations of Cosmogenic Radionuclide Production Rates in Chondrites During Their Passage Through the Inner Heliosphere

To study radiation environment in the interplanetary space, cosmogenic radionuclides in meteorites, the production rates of which are in direct proportionality to the intensity of cosmic rays, are used. The contents of cosmogenic radionuclides of different half-lives T1/2, measured in 42 stony meteorites (chondrites) having sequentially fallen onto the Earth during the period of 1959–2016, are analyzed. They are accumulated by the galactic cosmic rays (GCRs) along the orbits of the chondrites before their falls onto the Earth at some average heliocentric distances, depending on the size of the chondrite orbit and on T1/2 of the radionuclide. The comparison with the calculated production rates of radionuclides in the identical chondrites for isotropic irradiation by the GCRs at ~ 1 AU is demonstrated. The calculations are based on the stratospheric balloon monthly data on the GCR intensity [1] for the periods of accumulation of each radionuclide in each chondrite. The dependence of production rates of the radionuclides of different half-lives upon the GCR variations in the heliosphere is studied. The obtained long set of homogeneous data on cosmogenic radionuclide production rates in consecutively fallen chondrites provides the unique information on the space-time continuum of the cosmogenic radionuclide production rates and their variations over a long-time scale, which could be useful in the correlative analyses of processes in the inner heliosphere and, thus, in the forecast of radiation situation, which is important for the predicted manned flights.

Variations of cosmogenic radionuclide production rates in chondrites of known orbits

The results of multiyear investigations of cosmogenic radionuclide production rates along the orbits of 42 chondrites, fallen successively to the Earth in 1959-2016 are presented. They constitute a long set of homogeneous data, statistical smoothing of which demonstrates some main regularities of distribution and variation of the galactic cosmic rays (GCR) with energy > 100 MeV in the internal (< 5 AU) heliosphere. This set is exclusive, and it has timeless importance for all the future investigations of magneto-hydrodynamic peculiarities of the GCR solar modulation mechanism. Analysis of 26Al contents in 10 chondrites of known orbits allowed us to educe a spatial profile of the GCR intensity (E > 100 MeV), averaged over a 1 My in the internal heliosphere, testifying to permanent development and dissipation of a layer of the solar wind magnetic irregularities, which efficiently modulates cosmic radiation. It suggests a constancy of magneto-hydrodynamic environment in the Solar system at least for the last million years.

Multiradionuclide evidence for an extreme solar proton event around 2,610 B.P. (∼660 BC)

Recently, it has been confirmed that extreme solar proton events can lead to significantly increased atmospheric production rates of cosmogenic radionuclides. Evidence of such events is recorded in annually resolved natural archives, such as tree rings [carbon-14 (14C)] and ice cores [beryllium-10 (10Be), chlorine-36 (36Cl)]. Here, we show evidence for an extreme solar event around 2,610 years B.P. (∼660 BC) based on high-resolution 10Be data from two Greenland ice cores. Our conclusions are supported by modeled 14C production rates for the same period. Using existing 36Cl ice core data in conjunction with 10Be, we further show that this solar event was characterized by a very hard energy spectrum. These results indicate that the 2,610-years B.P. event was an order of magnitude stronger than any solar event recorded during the instrumental period and comparable with the solar proton event of AD 774/775, the largest solar event known to date. The results illustrate the importance of multiple ice core radionuclide measurements for the reliable identification of short-term production rate increases and the assessment of their origins.

Variations of Cosmogenic Radionuclide Production Rates in Chondrites of Known Orbits

The results of multiyear investigations of cosmogenic radionuclide production rates along the orbits of 42 chondrites, fallen successively to the Earth in 1959-2016 are presented. They constitute a long set of homogeneous data, statistical smoothing of which demonstrates some main regularities of distribution and variation of the galactic cosmic rays (GCR) with energy > 100 MeV in the internal ( 100 MeV), averaged over a 1 My in the internal heliosphere, testifying to permanent development and dissipation of a layer of the solar wind magnetic irregularities, which efficiently modulates cosmic radiation. It suggests a constancy of magneto-hydrodynamic environment in the Solar system at least for the last million years.

Patterns of cosmogenic radionuclide production rates in the heliosphere and problems of solar modulation on a long time scale

The results of long-term studies of cosmogenic radionuclide production rates along the orbits of 39 chondrites that fell to the Earth between 1959 and 2013 are presented. They constitute a long series of homogeneous data, a statistical smoothing of which demonstrate some general patterns of the distribution and variations of Galactic cosmic rays (GCRs) with energy >100 MeV in the inner (<5 AU) heliosphere. A correlation analysis of the production rates of radionuclides with different half-lives suggests that the solar GCR modulation mechanism is constant over at least ~1 million years. The role of stochastic factors in the polarity reversal of the general solar magnetic field for the phases of maximum solar activity has been revealed. The subtle sensitivity of the 54Mn production rate in the Chelyabinsk chondrite to the short-term closure of the heliosphere for positively charged particles over 14 months between June 2012 and July 2013 is used as an example to show the high resolution of the method of using cosmogenic radionuclides in meteorites as natural GCR detectors.

Variations of cosmogenic radionuclide production rates along the meteorite orbits

Cosmogenic radionuclides produced by galactic cosmic rays (GCR) in meteorites during their motion in space are natural detectors of the GCR intensity and variations along the meteorite orbits. On the basis of measured and calculated contents of cosmogenic radionuclides in the freshly fallen Chelyabinsk and Košice chondrites some peculiarities of generation of cosmogenic radionuclides of different half-lives in the chondrites of different orbits and dates of fall onto the Earth are demonstrated. Dependence of production rates of the radionuclides on the GCR variations in the heliosphere is analyzed. Using radionuclides with different half-lives it is possible to compare the average GCR intensity over various time periods. The measurement and theoretical analysis of cosmogenic radionuclides in consecutively fallen chondrites provide a unique information on the space–time continuum of the cosmogenic radionuclide production rates and their variations over a long time scale, which could be useful in correlative analyses of processes in the heliosphere. Some applications of cosmogenic radionuclide depth distribution in chondrites for estimation of their pre-atmospheric sizes are illustrated.

Assessing the differences between the IntCal and Greenland ice-core time scales for the last 14,000 years via the common cosmogenic radionuclide variations

Variations in galactic cosmic rays reaching the Earth's atmosphere produce globally synchronous variations in the production rates of cosmogenic radionuclides. In consequence, they leave their imprint in tree-ring 14C and ice-core 10Be records. By identifying this signal and correcting for the known geochemical influences on the radionuclides, it is possible to compare and synchronize the tree-ring chronology and the Greenland ice-core time scale. Here, we compare the IntCal13 and the GICC05 time scales for the last 14,000 years via identification and synchronization of the common short-term variations in the ice-core 10Be and tree-ring 14C records most likely induced by variations in the solar modulation of galactic cosmic rays. We conclude that systematic time-scale differences have to be accounted for if ice-core and 14C-dated records are compared on decadal time scales. These are mostly within the uncertainties of the time scales and the method proposed here. However, for large parts of the mid to late Holocene (i.e. after 7000 yrs BP) the best agreement between ice-core 10Be and tree-ring 14C records is obtained for time shifts outside the stated errors of the respective time scales. A transfer function is proposed that can be applied to synchronize the GICC05 ice-core time scale to the radiocarbon time scale.

氷床コアから復元される地磁気イベント時の宇宙線変動

Cosmogenic Radio Nuclides (CRNs) are produced by nuclear reactions induced by cosmic rays in the Earth's atmosphere. The production rate of CRNs varies because changes in geomagnetic field intensity and solar activity strongly influence galactic cosmic rays, especially the lower part of energy. CRNs are produced in the upper atmosphere and are deposited on ice sheets in both Greenland and Antarctica, hence, ice cores in the polar regions provide excellent records of past histories of cosmic rays because of their continuous and relatively simple depositional processes. In recent studies, high-resolution records of 10Be flux from ice cores have been reported, and these revealed the continuity of 205-year solar cycles over the last glacial period. In particular, the changes induced in the production rate of CRNs by solar modulations increased during periods of low geomagnetic field intensity, such as the Laschamp geomagnetic excursion. The results obtained in this study suggest that it may be possible to obtain a better understanding of long-term solar activity through a high-resolution 36Cl analysis of the polar ice sheet.

In-phase anomalies in Beryllium-10 production and palaeomagnetic field behaviour during the Iceland Basin geomagnetic excursion

Increases in the production rate of cosmogenic radionuclides associated with geomagnetic excursions have been used as global tie-points for correlation between records of past climate from marine and terrestrial archives. We have investigated the relative timing of variations in 10Be production rate and the corresponding palaeomagnetic signal during one of the largest Pleistocene excursions, the Iceland Basin (IB) event (ca. 190 kyr), as recorded in two marine sediment cores (ODP Sites 1063 and 983) with high sedimentation rates. Variations in 10Be production rate during the excursion were estimated by use of 230Th xs normalized 10Be deposition rates and authigenic 10Be/ 9Be. Resulting 10Be production rates are compared with high-resolution records of geomagnetic field behaviour acquired from the same discrete samples. We find no evidence for a significant lock-in depth of the palaeomagnetic signal in these high sedimentation-rate cores. Apparent lock-in depths in other cores may sometimes be the result of lower sample resolution. Our results also indicate that the period of increased 10Be production during the IB excursion lasted longer and, most likely, started earlier than the corresponding palaeomagnetic anomaly, in accordance with previous observations that polarity transitions occur after periods of reduced geomagnetic field intensity prior to the transition. The lack of evidence in this study for a significant palaeomagnetic lock-in depth suggests that there is no systematic offset between the 10Be signal and palaeomagnetic anomalies associated with excursions and reversals, with significance for the global correlation of climate records from different archives.

Palaeomagnetic intensity data: an Achilles heel of solar activity reconstructions

Reconstructions of solar activity prior to AD 1600 rely on estimates of the past production rates of cosmogenic radionuclides in the Earth's atmosphere. These rates are modulated by the strength of the solar magnetic field and the intensity of the Earth's geomagnetic dipole. One of the largest uncertainties associated with these reconstructions originates from palaeointensity data and to show how errors in palaeomagnetic data influence reconstructions of solar activity we present different radiocarbon based reconstructions of the solar magnetic modulation parameter (Φ) during the Holocene. We considered geomagnetic field changes according to (i) a global archaeomagnetic compilation, (ii) a geomagnetic field model covering the last 7000 years and (iii) a regional sediment relative palaeointensity record. Comparison between alternative solar modulation reconstructions shows that the uncertainties in palaeomagnetic data prevent us from making robust conclusions about solar variability at timescales longer than a few hundred years. Our analyses indicate that the geomagnetic dipole field dominates radiocarbon production at timescales longer than 500 years although a combination of solar and geomagnetic modulation could apply to all timescales. Long-term trends depend on which palaeomagnetic data set is used for correction and there are additional unexplained differences between 10Be and 14C records that question results about long-term changes in solar activity. The higher-resolution geomagnetic field records, which are usually very much smoothed in stacked records, also indicate that part of the shorter-term changes (~500 yr) in the radiocarbon production rate could be attributed to the geomagnetic dipole field. To test this hypothesis and improve our knowledge of past solar activity a higher resolution palaeomagnetic reconstruction of geomagnetic dipole moment is required.

A study of the vertical diffusion of the cosmogenic radionuclides, 7Be and 22Na in the atmosphere

The study of the mean vertical concentration profile of cosmogenic radionuclides provides information on the vertical transport in the troposphere and stratosphere. This paper is based on the results of accurate measurements of the mean monthly activity concentrations of 7Be and 22Na in ground-level air, data of the production rate of cosmogenic radionuclides in the atmosphere and a one-dimensional steady-state diffusion equation numerically solving the finite difference approximation.

Applications of accelerator mass spectrometry in extraterrestrial materials

Accelerator mass spectrometry has made it possible to measure the concentrations of 26Al, 10Be, 14C, 36Cl, 41Ca, 59Ni, and 129I in a large number of extraterrestrial samples, mainly because of the small sample mass requirements of the technique. This brief review emphasizes recent (1990–1993) applications of AMS to extraterrestrial problems including the terrestrial ages of meteorites; the production rates of cosmogenic radionuclides; the non-linear effects of chemical composition on production rates; the history of solar cosmic rays; the exposure histories, both simple and complex, of meteorites; and the behavior of a large projectile (the Canyon Diablo iron meteorite) during impact.

On the production of cosmogenic nuclides in meteorites by primary galactic particles: Cross sections and model calculations

Cross sections for the proton‐induced production of radionuclides (40≦A≦60) from natural titanium, iron, and nickel were measured for proton energies from 80 to 200 MeV. To critically survey the theoretical predictions of unknown excitation functions, the experimental data were compared with calculations according to Blann's hybrid model of nuclear reactions and to the semiempirical formulas of Rudstam and of Silberberg and Tsao. Then, based mainly on the new excitation functions, the production rates of cosmogenic radionuclides in meteorites by primary galactic protons were calculated. Since the interaction of primary solar and galactic particles with meteorites can be described with accuracy, the model calculations allow for a quantitative estimate of the action of secondary particles in meteorites. A method of completely describing the production of cosmogenic nuclides in meteorites that takes into account all sources of production is proposed.