To measure the radiation in the Van Allen radiation belts, we are using a common radiation detector called a Geiger Tube. Specifically, we are using the LND 71014 from LND, Inc pictured here:

Geiger Tubes are typically cylindrical in shape (see figure below). They are constructed with a conducting metal outer shell which is hooked to ground (cathode) and a thin inner wire along the axis of the cylinder hooked to a high voltage source (anode). Ionizing radiation is allowed into the detector on one end through a thin window (made of Indian Mica in our case). The electrons, being negatively charged, flow towards the anode while the ions flow towards the cathode. When each stream of electrons and ions reach the anode and cathode, respectively, they form a current pulse out of the detector.

Unfortunately, the current pulse from a single electron is usually quite small, >.5mV, which is too small to use with conventional electronics. This problem is solved in the Geiger Tube by holding the anode at about 500V. A 500V potential difference will create a large electric field which is then able to accelerate the ionized electrons. These electrons acquire enough energy to ionize gas atoms that they collide with producing more electrons, which likewise ionize other atoms and the process continues. This production of secondary ionizations is called a Townsend avalanche. The avalanche of charge spreads out along the detector and produces a large output pulse independent of the incident particle energy, perfect for counting radiation.

The remainder of our payload is designed to provide the needed 500V high voltage source and for preparing easily-countable voltage pulses to the satellite processor (see circuit diagram below). For the High Voltage Power Supply (HVPS) we chose a EMCO C06 capable of high voltage, low current output. The EMCO chips will be used for testing the payload circuit, and will fly aboard the BOREALIS test flight mission. For the actual MEROPE launch we will use a more stable, space-rated HVPS from Southwest Research Institute. Once the detector is hit with ionizing radiation, it will send a current pulse of about 10mA amplitude to the charge sensitive preamplifier-discriminator, the A-101 from AmpTek, Inc. The output of the A-101 is a shaped, 5V square pulse which will then be counted by the onboard processor.

Using a Gamma Sensitivity Curve to convert counts per second to dose rates (a measure of radiation), and a graph of Current vs. Dose Rate for our tube, we can calculate the expected current from each pulse the tube sends to the pulse shaping chip. Both graphs are courtesy of LND, Inc. The tube saturates at 4x10⁴ c/sec. Therefore, we want to keep the normal operating count rate somewhere near 10⁴ c/sec. This is nearly equivalent to 30 R/hr for our tube. This is slightly off the left of the Current vs. Dose Rate graph, perhaps at 15 microamps. This is our expected current. The minimum current, of course, will be zero if the tube is not being hit with radiation. Slightly larger current spikes will occur if the tube is hit with a large amount of flux, from a solar flare for example.

A collimator was deemed necessary to prevent the Geiger tube from saturating. A Geiger tube saturates when it receives more particles than it can count. At this point, particles fly into the detector while it is still resetting from counting the last particle, so some counts are missed. For the LND 71014 this occurs at approximately 40,000 counts/sec. Since there is a chance that MEROPE could encounter areas of the radiation belts where the radiation is intense enough to make the Geiger tube saturate, we decided to build a collimator to limit the viewing angle and detection area of the Geiger tube. The collimator was designed to let the Geiger tube reach a maximum of 10,000 counts, thus giving us a nice margin should MEROPE encounter some stronger areas of radiation. The collimator limits the viewing angle of the Geiger tube to 45.24° with an aperture of 1mm. These numbers arise from looking at the data from the NOAA 15 satellite, choosing a 1mm detection area, and calculating for a target value of 10,000 counts/sec. The calculations used can be found here:

Using data obtained from the National Oceanic and Atmospheric Association’s satellite NOAA 15, we were able to get an idea of what kind of data MEROPE will record. The information packet obtained from NOAA 15 was a list of the satellite’s radiation counts from approximately June 2000 to June 2001. The two instruments on the NOAA 15 satellite that we were concerned with were the electron telescope and the proton telescope. The electron telescope gave measurements of the counts in the ranges of 30 – 1100 KeV, 100 – 1100 KeV, and 300 – 1100 KeV. The proton telescope gave measurements in the ranges 30 – 80 KeV, 80 – 250 KeV, 250 – 800 KeV, 800 – 2500 KeV, 2500 – 6900 KeV and >6900 KeV. This data was used to give us an idea of the amount and intensity of the radiation MEROPE could encounter. Once we knew the expected electron flux in our orbit we were able to use this information to design our collimator. After we had designed the collimator we were able to use the geometrical factor from the collimator and the maximum number of counts from NOAA 15 to determine MEROPE’s expected counts/orbit of 3,810,019.

The threshold energy is the energy needed by a particle to move through some material. In our case, we need to know several threshold energies. First, we want to verify the electron threshold energy for the mica Geiger tube window. Ideally, this would be the only energy we would be concerned with, since we only intend to measure counts of electrons entering the detector through the window. In reality, we must also worry about two things: other particles entering through the window and triggering the detector, and electrons and other particles having sufficient energy to shoot through the solid aluminum sides of the CubeSat and enter straight through the stainless steel wall of the detector. Particle ranges are difficult to calculate, especially for particles of energies we are concerned with (< 1 MeV). Fortunately, they can be measured experimentally and we can then estimate the ranges from experimental graphs. The following graphs come from Principles of Modern Physics by Robert Leighton (McGraw-Hill, 1959).

In order to test the operating voltage of each Geiger tube and to analyze which tube has the best operating range, a series of plateau tests were run on the Geiger tubes. The objective of the plateau tests was to generate a graph for each tube that showed the count rate (counts/sec) vs. voltage. Ideally, this should produce a graph showing the count rate rising slightly from the starting voltage (approx. 280 V) to about 400 V where the count rate plateaus and stays roughly the same until about 600 V, where the count rate begins to rise again. The plateau curves were made by increasing the voltage until we found the starting voltage for the given tube. Then the voltage was raised by 10 V increments until we reached 600 V. Three readings of the number of counts in a twenty second time interval were taken at each voltage point. These readings were then averaged and divided by 20 seconds to give us an average counts/sec for each voltage level. What we were looking for was the tube with the longest and flattest plateau region, which would then become our flight Geiger tube. Plateau curves were generated for the Geiger tubes in November of 2001 and then again in May 2002. The only noticeable difference in the plateau curves is that the count rate of all the tubes has shifted down in the latter test. The most likely reason for the count rate going down is that the radiation source being used had decayed slightly in that time, although the possibility of tube properties changing is being studied.

A similar Geiger tube to the one flying on MEROPE was flown on the BOREALIS Balloon project as a means for us to test some of the MEROPE hardware and to get practice in working with Geiger tubes. The Geiger tube flown on BOREALIS helped us test the EMCO C06 high voltage power supply and a pulse shaping circuit used by MEROPE. This test also gave us good experience in hooking up all the hardware for the payload and making it work with the processor.

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