# A Review of Requirements for Gamma Radiation Detection in Space Using CubeSats
## 3 Gamma Radiation Detection Strategy and Reference Detector
### Readout Electronics  
Both front end (FE) and controller (CTRL) electronics must be able to operate in the typical conditions of the space environment, even though with less stringent operative standards compared to longer life, higher altitude missions. The main characteristics of custom designed electronics for the instrumentation of a payload of CubeSat missions are  
* low cost,
* low power operation,
* temperature compliant components (\(-20\)\({}^{\circ}\)C \(\div\) 60\({}^{\circ}\)C),
* rad-hard components when possible, and
* vibration-tolerant components (i.e., automotive standard). Based on the aforementioned, the electronics concept adopted in this paper is reported in Figure 8.  
The proposed electronics is designed for charge integration and time stamping of the signal generated by a photosensor, with the charge proportional to the energy released in the event. It must also be able to assign an event priority based on the science case and to tag nuisance events (i.e., charged particle interactions). The power supply circuit needed to operate the photosensor array is embodied in the PCB and actively controlled through a slow-control system continuously running. The use of waveform digitizers capable of coping with the typical time response of MPPCs and PMTs would require a high sampling rate (order of \(\sim\)1 GS/s), and so it would have a higher power requirement which may not be available due to the constraints  
Figure 8: Block diagram of the payload electronics needed to operate the reference detector. in CubeSats. Measuring the charge becomes a viable option. The charge integrators (CI; one stage per each photosensor) need to be designed according to the required sensitivity, energy dynamics and decay-time tailored around the time characteristics of the scintillator - photosensor choice. Each CI produces a charge signal output that is in turn sampled (at few tens of MHz) by an Analogue to Digital Converter (ADC). To get the charge value (proportional to the energy released in the interaction) and the event time stamp, the ADC digital output must be processed and evaluated by an algorithm running at the FPGA level. In order to be able to assign an event time stamp at microseconds resolution, an high-precision clock of the order of tens of MHz must be distributed to the payload electronics. This can be easily achieved by implementing an oven-controlled quartz oscillator, or by embodying a compact atomic clock. Both options are viable if power and cost budget allows, however for CubeSat missions it is unlikely that several watts could be invested for this task. A possible power saving solution is to implement and distribute a disciplined clock, by means of a control system, namely, a Phase-Locked Loop (PLL). The typical PLL generates an high-frequency output from a low-frequency reference. A suitable timing circuit concept is based on a PLL making the use of the low-frequency (1 Hz) Pulse-Per-Second (PPS) hardware signal obtained from the spacecraft GPS receiver coupled to a 10 MHz oscillator. A schematic layout of the disciplined clock circuit is reported in Figure 9.  
The time stamp of each event is expressed by the arithmetic sum of values of two different counters, a 26-bit counter (High Counter (HC)) measuring the numbers of the PPS cycles and a 24-bit counter (Low Counter (LC)) for the number of output oscillator clock cycles (each clock cycle lasts 100 ns) in one PPS. The typical PLL brings in a systematic phase noise error due to the precision of the high- frequency oscillator (evaluated in parts per million, PPM), while the PPS signal has an intrinsic precision of 1 ns (i.e., \(10^{-3}\) PPM), the typical precision of off-the-shelf low power oscillator is 15 PPM. In this configuration, the maximum error per each PPS cycle is 15 \(\upmu\)s: note that the error does not accumulate. As shown in Figure 9, signal C realigns the phase of the output oscillator every PPS cycle.