2.1.

CubeSat Design Specifications

NASA’s CubeSat Launch Initiative is a platform that allows researchers to compete for flight opportunities to conduct low-cost space science experiments. Instruments can be from 1 to 6 units (U), with some subincrements of 0.5 units available, with specific weight, size, and power requirements that can be found in Ref. 13. The criteria for a 1.5U include the size of $10\times 10\times 15{\mathrm{cm}}^3$, a weight limit of 2.00 kg (4.4 lbs), and a requirement to run off the power supplied by onboard solar panels or batteries.¹³ The 0.5U IRIS Deep Space Transponder CubeSat, built by JPL, has a power consumption of 26 W during full power transmission, and the 6U JPL MarCO, has a power consumption of 35 W.^14,15

2.2.

CubeSat Design

In the development of our instrument, we have utilized a 1.5-U CubeSat chassis. A block diagram of the instrument is shown in Fig. 1. In this figure, the active area of the detector, the encapsulated ${}^6\mathrm{LiIn}\mathrm{S}{\mathrm{e}}_2$ crystal, is oriented to be at the top. A thermal neutron interacts with the ${}^6\mathrm{Li}$ nucleus, and the resulting nuclear reaction generates an energetic alpha particle that in an ionization cascade generates excited states in the scintillator crystal that ultimately recombine to their ground state by emitting optical photons.⁹ The scintillating photons are then detected by the Si-APD and converted into an electronic signal, or pulse, that is further processed to generate a histogram of all events that constitute the spectrum of incoming radiation. In general, neutron detection takes place in a mixed field of other ionizing particles such as gamma and galactic cosmic rays and the characteristics of the pulse can be used to discriminate among types of particles interacting with the detector.

Fig. 1

Neutron CubeSat prototype schematic.

Figure 1 shows the schematic of our instrument. The instrument has three stages, comprised of a printed circuit board (labeled boards 1, 2, and 3 in all figures) and the associated components. The first board contains the $9\times 9\times 2{\mathrm{mm}}^3$ ${}^6{\mathrm{LiInSe}}_2$-encapsulated crystal and the Si-APD (components 1 and 2 in all figures), which have been packaged together to prevent the crystal from shifting off the active area of the Si-APD that is slightly larger than the crystal at $10\times 10{\mathrm{mm}}^2$. Packaging the crystal and Si-APD together also decreases the amount of ambient light that can reach the Si-APD. Board 2 contains two DC-DC regulated power supplies (components 5 and 6) that convert the 5 V input to the instrument into 12 and 440 V, respectively. The 440-V power supply powers the Si-APD, while the 12-V supply runs the remaining components, the two amplifiers and the Kromek multichannel analyzer (MCA). The third board contains a preamplifier and shaper (component 8), a voltage amplifier (component 9), and the MCA (component 7), which currently, for lab tests, outputs via USB cable to a computer. There is room as well as weight and power available for a microcomputer and wireless transmitter to be included in a later iteration of the instrument, although a RaspberryPi is being used in initial tests. The current instrument, including the RaspberryPi microcomputer, weighs 670 g in its entirety and requires 5 V and 3 W.

The ${}^6{\mathrm{LiInSe}}_2$ crystal being used in the detector was grown using the vertical Bridgman method with details of growth being given by Tupytsin et al. and using a technology that was developed through a partnership between Fisk University and Y-12 National Security Complex.¹⁰ ${}^6\mathrm{Li}$ is a high-density material suitable for producing lightweight and compact neutron detectors due to its high capture cross-section for thermal neutrons. ${}^6\mathrm{Li}$ and other neutron sensitive materials are traditionally used as coatings on neutron-detecting systems to increase the efficiency to a total of a few percent. In comparison, the density of ${}^6\mathrm{Li}$ in ${}^6\mathrm{LiInS}{\mathrm{e}}_2$ is great enough to yield up to 95% detection efficiency of neutrons in a 3.4-mm-thick wafer.¹¹

The ${}^6{\mathrm{LiInSe}}_2$ crystal utilized in the detector is $9\times 9\times 2{\mathrm{mm}}^3$. After characterization and testing, discussed more in Sec. 3, it was encapsulated, covered on five sides with reflective material with a quartz window on the sixth side as shown in the topmost panel of Fig. 2 labeled 1. This side was then placed onto the Si-APD.

Fig. 2

Photo of associated electronics. Labeling of components is the same as the one used in Fig. 1.

The photodetector, in this case the Si-APD (component 2), turns detected events into electrical pulses. Compared to Si-APDs and SiPMs, traditional vacuum PMTs are heavy, large, have high power requirements, and are sensitive to the magnetic environment of space. For these reasons, the use of a silicon-based photodetector is better suited for space applications. In this instrument, a Hamamatsu S8664 1010 Si-APD (component 2) was utilized. APDs utilize the photoelectric effect to turn the scintillated light from the crystal into an electronic signal.⁹

Next the preamplifier and shaper, here an Amptek A225 (component 8), receives the signal from the Si-APD, and preserves the energy of the signal in a pulse that initially has the following characteristics: a sharp rise and a long tail with the total signal on the order of microseconds (shown in Fig. 3). Next, because the Amptek A225 acts as a shaper as well, it then takes the signal and cuts down the pulse length to the order of microseconds, preserving the energy as the height of the pulse and changing the shape of the signal to be closer to a Gaussian shape. Next the voltage amplifier, an Amptek 206 (component 9), amplifies the voltage to the pulse. Then the signals outputted by the voltage amplifier go to the MCA, a Kromek K102 (component 7). The MCA measures the Gaussian peaks from the voltage amplifier by pulse height (with the energy value still represented by height, shown in Fig. 3) and sorts pulses by their energy creating an energy spectrum of the interacting events.

Fig. 3

Block diagram of the detection system including (1) LiInSe2 crystal, (2) Si-APD, (7, 8, 9) associated electronics for signal processing, and (5 and 6) power supplies. The main information extracted from each signal is the height of the pulse, which is linearly proportional to the energy of the ionizing particle detected. Labeling of components is the same used in previous figures.

3.1.

⁶LiInSe₂ Crystal Scintillator

The response to alpha particles is a good indication of the performance of the system before neutron testing. The ${}^6{\mathrm{LiInSe}}_2$ crystal was fabricated into a scintillator and its response to alpha radiation from a ${}^{241}\mathrm{Am}$ source with an activity of $0.9\mu \mathrm{Ci}$ was tested at room temperature before encapsulation and integration into the instrument. The results are shown in Fig. 4. ${}^{241}\mathrm{Am}$ decays via the emission of an alpha particle with energy 5.486 MeV to ${}^{237}\mathrm{Np}$ (Neptunium).¹⁶ The energy resolution of 36% was measured as the full width of the distribution at half the maximum of the peak (FWHM). This demonstrates a relatively high signal-to-noise (or signal-to-background) ratio of our system, which means we will approximately be able to distinguish between two energies separated by 36% or more.⁹ The low energy tail that can be seen in Fig. 4 is due to general background, that includes the low energy, 60 KeV, gammas that are generated by this source.

Fig. 4

${}^6\mathrm{LiInS}{\mathrm{e}}_2$ crystal response to alpha radiation from a ${}^{241}\mathrm{Am}$ source. The spectrum was collected over 100 s.

3.2.

Silicon Avalanche Photodiode

Available scintillators emit in the blue or ultraviolet parts of the spectrum and, therefore, do not pair as well with silicon-based photodetectors as ${}^6\mathrm{LiInS}{\mathrm{e}}_2$ does.^12,13,17 This is shown in Fig. 5, where the quantum efficiency of the Hamamatsu S8664, given by the manufacturer, and the emission spectrum of a ${}^6\mathrm{LiInS}{\mathrm{e}}_2$ crystal, collected using x-ray excited optical luminescence, are shown. It can be seen that the ${}^6{\mathrm{LiInSe}}_2$ crystal has its peak emission at about 510 nm, and that the Si-APD has a high efficiency in this range. The blue or UV part of the spectrum, where most other scintillators emit, corresponds to the 400 nm range and lower. From 500 to 400 nm, there is a drop of efficiency from 80% to 40% of the Si-APD. High efficiency in the photodetector is important because ${}^6\mathrm{LiInS}{\mathrm{e}}_2$ does not have a high light yield, $4400\text{photons}/\mathrm{MeV}$ compared to inorganic and plastic scintillators which have light yields in the 10,000s [e.g., CsI(Tl) has a light yield of $\mathrm{65,000}\text{photons}/\mathrm{MeV}$].^9,12

Fig. 5

Comparison of quantum efficiency of a Hamamatsu S8664 1010 Si-APD and the output spectra of a ${}^6\mathrm{LiInS}{\mathrm{e}}_2$ crystal.

3.3.

Postassembly System Test

Once the instrument was built, it was tested under gamma radiation. After encapsulation, the system was tested by measuring the response to the 1.2-MeV gammas emitted by ${}^{22}\mathrm{Na}$; the spectrum collected is shown in Fig. 6. The height of the ${}^{22}\mathrm{Na}$ gamma signal is significantly lower than the response measured to the alpha particles from an ${}^{241}\mathrm{Am}$ source.

Fig. 6

Response of packaged instrument to gamma radiation from a ${}^{22}\mathrm{Na}$ source. Of the 5095 total counts collected, 70% are from direct interaction with the Si-APD.