# A Review of Requirements for Gamma Radiation Detection in Space Using CubeSats
## 3 Gamma Radiation Detection Strategy and Reference Detector
### The Detection Target  
In this section, we present the performance offered by different choices of scintillating crystals considered for building the detection target. Table 2 reports a selection of possible target candidates, along with their main characteristics.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Scintillating Crystal** & \begin{tabular}{c} **Density** \\ **[gcm\({}^{-3}\)]** \\ \end{tabular} & \begin{tabular}{c} **Weight** \\ **[g]** \\ \end{tabular} & \begin{tabular}{c} **Energy Resolution** \\ **at 662 keV [\%]** \\ \end{tabular} & \
\begin{tabular}{c} **Decay** \\ **Time [ns]** \\ \end{tabular} &
\begin{tabular}{c} **Emission** \\ **Peak [nm]** \\ \end{tabular} & **Hygroscopic** \\ \hline Low background & & & & & & \\ Cerium Bromide (CeBr\({}_{3}\)) & 5.2 & 495 & \(\sim\)4–5 & 18–20 & 370 & yes \\ \hline Thallium Activated Sodium & & & & & & \\ Iodide(NaI(Tl)) & 3.67 & 350 & \(\sim\)7–8 & 250 & 415 & yes \\ \hline Thallium Activated Cesium & & & & & & \\ Iodide (CsI(Tl)) & 4.51 & 430 & \(\sim\)5–6 & 1000 & 550 & slightly \\ \hline Lanthanum Bromide (LaBr\({}_{3}\)) & 5.08 & 485 & \(\sim\)3 & 16 & 385 & yes \\ \hline Lanthanum Bromochloride (LBC) & 4.90 & 465 & \(\sim\)3 & 35 & 380 & yes \\ \hline Bismuth Germanate (BGO) & 7.13 & 680 & \(\sim\)11–12 & 300 & 480 & no \\ \hline Lutetium-yttrium oxyorthosilicate & & & & & & \\ (LYSO) & 7.1 & 675 & \(\sim\)8–9 & 45 & 420 & no \\ \hline Gadolinium Aluminum Garnet (GAGG) & 6.6 & 630 & \(\sim\)6 & 90 & 520 & no \\ \hline \end{tabular}
\end{table}
Table 2: Physical characteristics of scintillating crystals considered in the paper as building block of the payload target. In red, the crystals affected by unavoidable intrinsic radioactivity (parameter values can change at the level of few percent depending on the manufacturing conditions).  
Figure 4: The core of the reference detector consists of a 4-fold scintillating crystal array read out by as many photosensors and encapsulated in an aluminum enclosure (transparent body visible in the figure).  
For a given volume, the higher the density, the more efficient is the detector. However, higher densities translate into heavier payloads. Given the weight constraints typical of a CubeSat missions, the use of heavy targets could prevent the usage of other satellite subsystems. It is therefore necessary to determine the best trade-off between the required detection capabilities of the science case and the needs of the supporting infrastructure functional to the mission operations.  
The intrinsic radioactivity can also be a nuisance parameter when operating a gamma-ray detector on-board a CubeSat mission. An internal conversion would mimic a gamma-ray interaction (the typical event in which there is no VETO fired and energy deposited in one or more target channels). A standard technique in order to kill this contribution consists in building up a series of coincidence patterns between two or more detection channels.  
However, due to power constraints and the limited number of detection channels that can be operated in a CubeSat, this approach can possibly affect the overall detection capabilities of the instrument, especially when the science case demands for single gamma-ray observations.  
As expected, the efficiency detection depends on the physical properties of the target (density, atomic number, etc.). Below 30 keV, the largest fraction of gamma-rays gets absorbed by the surrounding passive materials.  
For a NaI (BGO) target, the efficiency curve shows a maximum value of 85% (90%) at around 200 keV (400 keV) and becomes 45% (70%) at 3 MeV. At highest energy, the apparent efficiency drop is mainly due to the high penetration power of gamma-rays and to the interactions of secondary particles with the VETO, mimicking a charged particle induced event.