In this article, a new method to quantify the activity of spatially distributed gamma-emitting isotopes (hotspots) in homogenous content waste drums without the use of a collimator is presented. The method utilizes a spatial digital filter derived using maximum likelihood (ML) to determine multiple sources’ positions and then calculate their activities. To solve the multidimensional maximization problem, we use an alternating projection (AP) technique, which transforms the problem into a considerably simpler 1-D maximization problem. A dynamic grid search was developed to further decrease the computational load. The mathematical simulations demonstrate the improved accuracy when compared to that of industrial segmented gamma scanning (SGS) systems and the same accuracy as that of newer methods. Furthermore, the new method offers the benefit of replacing the heavy mechanical collimator with a “virtual collimator” formed by digital filters and an advanced algorithm to create a “digital virtual scan” of drum volume to locate hotspots.

A novel spectroscopic personal radiation device (SPRD) with sub-milliwatt power consumption is proposed. The SPRD is based on a compact, low-power, high-gain silicon photomultiplier (SiPM) coupled to a high-light-yield CsI(Tl) scintillator. The SiPM is operated in a special mode, in which its output is voltage rather than charge. In this mode, the SiPM output becomes higher and rises more slowly than in the charge amplification mode. Such a mode allows us to use a lower-frequency front-end amplifier with a lower gain and lower power consumption. Moreover, the very beginning of the slower, large-amplitude pulses is easier to detect with a comparator. At each detection event, the rest of the SPRD circuitry is activated only for the time needed for the pulse-processing. Because at background the radiation count rate is very low, some tens per second, and the duration of the signal-processing is very short, about 10μs, the power-demanding SPRD circuitry is not activated most of the time, and its average power consumption is very low. Proper matching of the scintillator and the SiPM helps us to achieve the required gain from the radiation sensor with a relatively low-power, low-gain front-end amplifier. Optimizing the input impedance of the front-end amplifier helps us to obtain the required SiPM output amplitude and shape. Because it takes some time for the signal processing circuit to be activated, an additional passive delay and shaping circuit is used. An experimental model of this device is built, and tested. It is superior to other devices due to its very low power consumption, its portability, and its non-sensitivity to microphonics. The power consumption of the SPRD is about 0.3mW by the radiation sensor and about 0.3mW by the electronics (for a total of 0.6mW as compared to above 20mW consumed by conventional spectroscopic radiation devices). The power consumption has been measured at count rates up to a few hundreds per second, which are much higher than expected in practice.