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The energy response of gross gamma dose rate monitors needs to be flat in order to prevent overestimation of dose at low gamma energies. In this paper, a discriminator threshold modulation based electronic energy compensation algorithm has been proposed for SiPM-scintillator based gamma detectors. Theoretical simulation studies were carried out in order to optimize the parameters of the periodic ramp voltage used for modulation of the discriminator threshold of a SiPM-GGAG:Ce,B based gamma dose rate monitor. A customized threshold modulation circuit and signal processing electronics were developed for this gamma detector. For experimentally optimizing the parameters, the energy response studies of the detector, with and without the discriminator threshold modulation, were carried out. With the optimized parameters for a periodic ramp threshold, the count rates for 241Am (60 keV) and 60Co (1173 and 1332 keV) were observed to be within ±30% of the count rate obtained for 137Cs (662 keV). Using the electronic energy compensation techniques presented in this paper, a flat energy response of the SiPM-scintillator gamma detector for the energy range of 60 keV to 1.5 MeV could be achieved. [ABSTRACT FROM AUTHOR]

The Indian Environmental Radiation Monitoring Network continuously monitors the outdoor natural gamma absorbed dose rate in air at different locations throughout India by employing Geiger–Mueller (GM) detector-based field-installed environmental radiation monitors (ERMs). Hyderabad, Telangana, India is known to have high natural background radiation levels due to the presence of high concentrations of primordial radionuclides in its granitic rocks. There are a total of 59 ERMs installed at various locations across Hyderabad. Long-term monitoring data of these locations are presented in this paper. The mean values of outdoor natural gamma absorbed dose rate in air at the monitoring locations were found to vary in the range of 104–258 nGy.h−1 with a mean of 193 ± 40 nGy.h−1. The mean annual effective dose due to outdoor natural gamma radiation was estimated to be 0.24 ± 0.05 mSv.y−1. Analysis of the long-term seasonal variation of outdoor natural gamma absorbed dose rate in air showed that the same was lowest during monsoons. [ABSTRACT FROM AUTHOR]

In this paper, the details of the development and performance characterisation of a compact, low-power gamma spectrometer for environmental gamma radiation monitoring networks are presented. To reduce the power consumption and the size of the spectrometer, a gamma detector comprising a silicon photomultiplier coupled to a Gd3Ga3Al2O12:Ce,B (GGAG:Ce,B) scintillator has been used for gamma spectrometry. Initially, a Monte Carlo simulation study was carried out to verify the suitability of the 5 mm × 5 mm × 5 mm GGAG:Ce,B crystal for spectrometry of gamma sources in the energy range 60–1332 keV. For minimising the power consumption, the signal processing electronics has been custom designed. This electronics was realised using standard off-the-shelf components to reduce the cost. The developed spectrometer is of size 16 cm × 10 cm × 6 cm, weighs 600 g and consumes 600 mW power. The spectrometer is developed such that it could be directly interfaced with GSM/Xbee for wireless communication with the radiation monitoring networks. The lower-level discriminator threshold of the system is 40 keV and the total electronic noise is <20 keV. The experimentally measured sensitivity of the spectrometer for 137Cs (662 keV) is 2.4 cps/μGy/h at 3.5 V overvoltage. The spectrometer offers excellent linearity over the measured energy range of 60–1332 keV and an energy resolution of ~10% for 662 keV gamma-ray at room temperature. [ABSTRACT FROM AUTHOR]

The Indian Environmental Radiation Monitoring Network continuously monitors, throughout India, the absorbed dose rate in air due to outdoor natural gamma radiation, by using Geiger–Mueller detector-based standalone environmental radiation monitors. The network consists of 546 monitors spread across 91 monitoring locations distributed all over the country. In this paper, the countrywide long-term monitoring results are summarised. The measured mean dose rate of the monitoring locations followed a log-normal distribution and ranged from 50 to 535 nGy.h−1 with a median value of 91 nGy.h−1. Due to outdoor natural gamma radiation, the average annual effective dose was estimated to be 0.11 mSv.y−1. [ABSTRACT FROM AUTHOR]

A subtype of retinal amacrine cells displayed a distinctive array of [K.sup.+] currents. Spontaneous miniature outward currents (SMOCs) were observed in the narrow voltage range of -60 to -40 mV. Depolarizations above approximately -40 mV were associated with the disappearance of SMOCs and the appearance of transient ([I.sub.to]) and sustained ([I.sub.so]) outward [K.sup.+] currents. [I.sub.to] appeared at about -40 mV and its apparent magnitude was biphasic with voltage, whereas [I.sub.so] appeared near -30 mV and increased linearly. SMOCs, [I.sub.to], and a component of [I.sub.so] were [Ca.sup.2+] dependent. SMOCs were spike shaped, occurred randomly, and had decay times appreciably longer than the time to peak. In the presence of cadmium or cobalt, SMOCs with pharmacologic properties identical to those seen in normal Ringer's could be generated at voltages of -20 mV and above. Their mean amplitude was Nernstian with respect to [[[K.sup.+]].sub.ext] and they were blocked by tetraethylammonium. SMOCs were inhibited by iberiotoxin, were insensitive to apamin, and eliminated by nominally [Ca.sup.2+]-free solutions, indicative of BK-type [Ca.sup.2+]-activated [K.sup.+] currents. Dihydropyridine [Ca.sup.2+] channel antagonists and agonists decreased and increased SMOC frequencies, respectively. [Ca.sup.2+] permeation through the kainic acid receptor had no effect. Blockade of organelle [Ca.sup.2+] channels by ryanodine, or intracellular [Ca.sup.2+] store depletion with caffeine, eradicated SMOCs. Internal [Ca.sup.2+] chelation with 10 mM BAPTA eliminated SMOCs, whereas 10 mM EGTA had no effect. These results suggest a mechanism whereby [Ca.sup.2+] influx through L-type [Ca.sup.2+] channels and its subsequent amplification by [Ca.sup.2+]-induced [Ca.sup.2+] release via the ryanodine receptor leads to a localized elevation of internal [Ca.sup.2+]. This amplified [Ca.sup.2+] signal in turn activates BK channels in a discontinuous fashion, resulting in randomly occurring SMOCs. KEY WORDS: potassium channels * calcium-induced calcium release * ryanodine receptors * L-type calcium channels * ganglion cells

Spontaneous miniature outward currents (SMOCs) occur in a subset of retinal amacrine cells at membrane potentials between -60 and -40 mV. At more depolarized potentials, a transient outward current ([I.sub.to) appears and SMOCs disappear. Both SMOCs and the [I.sub.to] are [K.sup.+] currents carried by BK channels. They both arise from [Ca.sup.2+] influx through high voltage-activated (HVA) [Ca.sup.2+] channels, which stimulates release of internal [Ca.sup.2+] from caffeine- and ryanodine-sensitive stores. An increase in [Ca.sup.2+] influx resulted in an increase in SMOC frequency, but also led to a decline in SMOC mean amplitude. This reduction showed a temporal dependence: the effect being greater in the latter part of a voltage step. Thus, [Ca.sup.2+] influx, although required to generate SMOCs, also produced a negative modulation of their amplitudes. Increasing [Ca.sup.2+] influx also led to a decline in the first latency to SMOC occurrence. A combination of these effects resulted in the disappearance of SMOCs, along with the concomitant appearance of the [I.sub.to] at high levels of [Ca.sup.2+] influx. Therefore, low levels of [Ca.sup.2+] influx, arising from low levels of activation of the HVA [Ca.sup.2+] channels, produce randomly occurring SMOCs within the range of -60 to -40 mV. Further depolarization leads to greater activation of the HVA [Ca.sup.2+] channels, larger [Ca.sup.2+] influx, and the disappearance of discontinuous SMOCs, along with the appearance of the [I.sub.to]. Based on their characteristics, SMOCs in retinal neurons may function as synaptic noise suppressors at quiescent glutamatergic synapses. KEY WORDS: BK channels * ryanodine receptors * transient potassium current * ganglion cells * dihyropyridines