Barry J. Brennan

Beta doses to spherical grains

Abstract The classic paper of Mejdahl (1979) on beta doses to quartz grains has been a standard reference for many years. In this paper we present an update of the data for beta doses to spherical quartz grains from internal and external natural sources, using dose point kernels based on modern spectra and Monte Carlo calculations. The differences from the Mejdahl results are not large, reflecting the modest adjustments in nuclear data and in the physics of electron–photon cascades in recent decades. It is recommended that, for a grain of original diameter 100 μm which has had a 9 μm layer etched away, the typical attenuation factor should be approximately 0.92 rather than the traditional 0.90. In addition, theory and data for the effects of etching on the average beta dose are presented, and we outline procedures for the estimation of beta doses for any situation that can be modelled using a series of concentric uniform shells.

Beta doses in tooth enamel by “one-group” theory and the ROSY ESR dating software

Abstract A new method of calculating beta doses for one-dimensional geometries, based on “one-group” theory, is presented. In typical situations it is found that this approach leads to average beta doses which are significantly lower (by up to 50%) than those from Gruns DATA software. For various natural sources, our beta dose rate estimates are slightly greater than expected from calculations based on Monte Carlo data, while the DATA results agree well with the experimental results of Aitken et al. (1985) for the uranium and thorium decay chains. Further experimental evidence is desirable to resolve this issue. We compare age estimates calculated using new ESR dating software called ROSY, which incorporates this approach, with those from DATA. Because of the difference in beta dose estimates, ROSY ages are generally older than the corresponding DATA ages, there being little difference in gamma and alpha dose estimates. We do however observe significant discrepancies in cosmic dose estimates. Examination of results for El Castillo Cave, Spain, by Rink et al. (1996) suggests that the ROSY age estimates are the more accurate.

Monte Carlo determinations of the beta dose rate to tooth enamel

The results of Monte Carlo simulations of the beta dose rate to enamel are presented. The dose rates are the most comprehensive to date, incorporating the beta spectrum from 21 different radionuclides, all internal conversion and Auger electrons, the majority of the bremsstrahlung radiation, effects due to radon loss in the uranium series, and variations in the moisture content of the sediment. Applications to a new dating technique in archaeology using electron spin resonance and beta-gradient isochrons are discussed.

Simulation of the gamma radiation field in lumpy environments

Abstract Many of the environments used for ESR and luminescence dating are inhomogeneous over the distances up to 0.3 m which contribute to gamma doses, and the consequent variability in gamma dose usually cannot be ascertained on a sample by sample basis. In this paper we examine the characteristics of such “lumpy” environments by simulation. Random environments were generated in which spherical source-free calcite inclusions were embedded in a source-rich sediment matrix. Using dose point kernels based on build-up factors, the gamma dose rate at a large number of random points in the sediment was then calculated. Various values for the sphere radius and volume fraction of the inclusions were tested. The gamma dose rate frequency distributions exhibit negative skewness, with the low outliers arising from points close to one or more inclusions. The variance of the distribution and the significance of the negative tail increase with the inclusion radius. Lognormal and gamma distributions were found to fit these histograms quite well. By sampling from the dose rate distribution, it was possible to repeatedly generate two hypothetical samples representing the positions of contemporaneous datable objects (and thus their accumulated doses) and of dose meters. Various statistical estimators were trialled to establish the optimum approach to extracting a site age from given sets of accumulated dose and gamma dose rate values. For ease of analysis, it was assumed that the gamma dose is the total dose, but the techniques can be generalised to realistic dose environments also. Most estimators produced values accurate to a few percent for typical environment parameters, and the one with the least variance was based on a two-stage maximum likelihood approach to age estimation. In the absence of such a sophisticated approach, culling of low valued outliers for both accumulated dose and dose rate leads to improved site age estimation.

Experimental determinations of beta attenuation in planar dose geometry and application to ESR dating of tooth enamel

Abstract Two experiments were performed to determine the distribution of beta dose in a target medium as a function of distance from a planar source (2 π geometry). In Experiment ♯1 planar absorbers were used to simulate depth in the target medium, and the attenuated dose that penetrated the intervening absorbers was detected using thermoluminescence from CaSO 4 :Tm. In experiment ♯2 pellets of powdered tooth enamel were situated in a cavity in solid tooth enamel. Dose response of the electron spin resonance (ESR) signals in the pellets allowed direct detection of dose distribution within the target medium. The dose distributions were found to agree more closely with Monte Carlo calculations and with one-group transport theory ( Prestwich et al., 1997 and Brennan et al., 1997 ) than with approximations used widely in ESR dating applications ( Grun, 1986 ). Although the latter were supported by experiments by Aitken et al. (1985) , the results of experiment ♯1 provided data to suggest that departure from 2 π geometry was unacceptably large in the Aitken et al. (1985) configuration. The beta doses obtained by one-group theory in typical target volumes of enamel were 17 to 40% lower than those obtained using the Grun approximation. It was concluded that one-group transport theory provides the best available routine tool for calculations of beta dose attenuation in near-planar target materials (e.g. tooth enamel and shell) irradiated by natural sources, although Monte Carlo methods are better and could eventually be introduced into the ROSY ESR dating program ( Brennan et al., 1997 ). It was also concluded that a revision of previously published ESR ages using one-group transport for beta dosimetry will be needed. For cases where the U concentrations in enamel are generally low (less than a few ppm), previously published ESR ages on tooth enamel are systematically too young by 5–30%.

Results from the new seismic monitoring network at Egmont Volcano, New Zealand: Tectonic and hazard implications

Abstract Data from the new Taranaki Volcano‐Seismic Network provide the first detailed information on microseismicity in the vicinity of Egmont Volcano (Mt Taranaki) and central Taranaki. During the period January 1994 to March 1995, 133 earthquakes were located within a 40 x 40 km area centred on Egmont Volcano. Earthquake magnitudes varied from 1.5 to 3.4, with five events being of magnitude 3 or greater. Most earthquakes were located in the northwest of the study area, with a distinct cluster of earthquakes occurring c. 12 km WNW of the summit of Egmont Volcano. Levels of seismicity were low in the remainder of the area and occurred throughout the crust, with depths ranging to just over 30 km. The location and character of the observed seismicity, and especially that of the cluster, suggest sources related to tectonic processes rather than volcanic activity. A likely cause of this earthquake cluster is movement on the Oaonui Fault, which is known to be recently active. A composite fault‐plane solution ...

Auditory click stimuli evoke event-related potentials in the visual cortex.

The objective of this study was to determine whether unimodal auditory stimuli evoke event-related potentials (ERPs) in brain areas normally designated as the visual cortex (VC). The topographical distribution of ERPs evoked by auditory click stimuli was measured from (a) electroencephalographic electrodes on the scalp of six neurologically normal adult human participants and (b) intracranial electrodes implanted on the cortex of one epileptic adult human participant. In all participants, unimodal click stimuli evoked ERPs over both the auditory cortex (AC) and the VC. Relative amplitudes of ERPs at different scalp electrodes did not support the idea that the ERPs over VC were volume-conducted versions of those over AC, and intracranial records confirmed the origin of some click-evoked ERPs in both V1 and other regions of VC. We conclude that unimodal auditory stimuli can evoke ERPs in VC. This finding adds to the earlier evidence for the effect of visual stimuli on AC by providing new evidence for bidirectional functional connectivity in the audio-visual network of the human brain. The implication is that not only do visual stimuli affect hearing; auditory stimuli also affect visual perception.

Alpha and beta dose gradients in tooth enamel

Abstract New results describing gradients in effective alpha and beta doses within a layer of tooth enamel in planar geometry are presented. The alpha (track) dose to an enamel layer is calculated using an algorithm similar to that of Aitken (Aitken, M.J., 1987. Alpha dose to a thin layer. Ancient TL 5, 1–3.). The code for ROSY version 1.4 ( Brennan, B.J., Rink, W.J., Rule, E.M., Schwarcz, H.P., Prestwich, W.V., 1999. The ROSY ESR dating software. Ancient TL, in press. ) incorporates this algorithm. The approach allows for variation of the alpha track dose near the edges of the enamel, and we describe the gradient of the alpha track dose within 40 μm of each edge of the enamel for natural sources. In ESR or luminescence dating of naturally thin layers, for which stripping of the surface layer containing the alpha dose gradients may not be feasible, age estimates may change by as much as 5–10% when the detailed alpha dose calculation is included. Modern Monte Carlo-based results for the variation of beta dose for depths up to 2 mm are compared with ROSY results. For external irradiation by different sources, the attenuation of the ROSY dose estimate with depth is usually less rapid than that of the Monte Carlo-based estimate. The ROSY estimate of average beta dose to a layer in this case is between 5 and 18% higher than the Monte Carlo estimate.

A possible physiological basis for the discontinuity of consciousness

A comparison is made between the frequency of local minima in the analytic power (AP) of intracranial EEG (ECoG) from waking and unconscious human subjects and the frequency of putative frames of consciousness reported in earlier psychological literature. In ECoG from unconscious subjects, the frequency of deep minima in AP is found to be a linear function of bandwidth. In contrast, in ECoG from conscious subjects, the bandwidth/minima-frequency curve saturates or plateaus at minima frequencies similar to the frequencies of previously reported frames of consciousness. This result is consistent with the hypothesis that local minima in AP may act as the shutter in a cinematographic model of consciousness. The fact that artificially generated samples of black noise with power spectra similar to ECoG data give similar results in the analyses above suggests that the discontinuous nature of consciousness is not due to some specifically biological factor, but is simply a consequence of the physical properties of the 1/f (aka power law) oscillations that are widely found in nature.

Systematic underestimation of the age of samples with saturating exponential behaviour and inhomogeneous dose distribution

In luminescence and ESR studies, a systematic underestimate of the (average) equivalent dose, and thus also the age, of a sample can occur when there is significant variation of the natural dose within the sample and some regions approach saturation. This is demonstrated explicitly for a material that exhibits a single-saturating-exponential growth of signal with dose. The result is valid for any geometry (e.g. a plain layer, spherical grain, etc.) and some illustrative cases are modelled, with the age bias exceeding 10% in extreme cases. If the dose distribution within the sample can be modelled accurately, it is possible to correct for the bias in the estimates of equivalent dose estimate and age. While quantifying the effect would be more difficult, similar systematic biases in dose and age estimates are likely in other situations more complex than the one modelled.