Fundamental Technologies Voyager LECP Pages

High Intensity Detector System for the Study of the Magnetospheres of Outer Planets

C. Y. Fan

For the study of the shock transition regions and the radiation belts of outer planets, we propose to use a small 3 detector telescope. Using six rate outputs it should be possible to achieve (1) large dynamic range intensity measurements, (2) identification of high intensity protons and electrons, and (3) obtain limited spectral information. A stack of three AU-Si surface barrier detectors, designated as D₁, D₂, and D₃, 25 mm² in area and 200m in depletion depth, each is placed in a malloy-2000 (tungsten alloy) cylinder of 0.5 mm in wall thickness, to form a particle telescope of angle of acceptance 20 deg. (see Figure 1). A malloy-2000 absorber of 0.5 mm thick is sandwiched between D₁ and D₂ so that D₂ and D₃ are shielded against direct radiation. The outputs of the system will be (a) two counting rates from D₁ at discrimination levels ~60 and 180 keV respectively (N_1l and N_1h), (b) one single rate each from D₂ and from D₃, both at discrimination levels of 60 keV (N₂ and N₃), (c) a D₁D₂ double coincidence rate (N₁₂) and (d) a D₁D₂D₃ triple coincidence rate (N₁₂₃). To illustrate the characteristics of the system we consider the following five cases.

1. Protons and electrons of energies less than 60 keV (i.e., likely populations in shock transition regions). In this case D₁ will register the particles thru pile-up while D₂ and D₃ will not respond. Therefore, we would expect N_1l to be greater than N_1h while N₂, N₃, N₁₂, and N₁₂₃ have their values as in the interplanetary space.
2. Electrons of energies between 0.06 and 1 MeV. In this case, D₁ will be triggered by the electrons directly or by bremsstrahlung produced by these electrons in the spacecraft material. Since the range of 1 MeV electrons is ~0.8 g/cm² in tungsten, D₂ and D₃ will be triggered only by the bremsstrahlung. Therefore, we would expect N_1l to be greater than N_1h, N₂ ~ N₃, N₁₂ approximately equal to or greater than 2t N_1l N₂, and N₁₂₃ approximately equal to or greater than 3 t² N_1l N₂ N₃, where t is the resolving time. The last two equalities are due to the fact that N₁₂ and N₁₂₃ will be mostly accidental coincidences.
3. Protons of energies between 0.06 and 17 MeV. In this case D₁ will count the particles directly but D₂ and D₃ because of the shielding of the tungsten absorber will not respond. The expected rate pattern is then N_1l ~ N_1h while N₂, N₃, N₁₂, and N₁₂₃ still have their interplanetary or background values. This pattern distinguishes this case from case (1).
4. Electrons of energies greater than 1 MeV. In this case, all three detectors will be triggered either by the particles directly or by the bremsstrahlung they produce. Consequently, the rate pattern will be N_1l greater than N_1h greater than N₂ greater than N₃; N₁₂ greater than 2t N₁ N₂, and N₁₂₃ greater than 3t² N₁ N₂ N₃.
5. Protons of energies greater than 17 MeV. All the detectors will be triggered by the particles directly in this case. We would expect N_1l to be greater than N_1h greater than N₂ greater than N₃, N₁₂ ~ F(AW)₁₂ + 2t N₁ N₂, and N₁₂₃ ~ F(AW)₁₂₃ + 3t² N₁ N₂ N₃, where F is the flux and (AW)₁₂ and (AW)₁₂₃ are the geometrical factors for the double and triple coincidences respectively.

From the five sample cases, it is seen that this detector system is a compact but a versatile instrument for the study of the magnetospheres of outer planets. It provides for the detection of shock transition regions, makes a rough estimate of the populations of protons and electrons and their energies. The intensity range is covered through the use of double and triple coincidence counting techniques combined with a coincidence circuit having a resolving time 0.1 msec and a proper scaling factor, a dynamical range of ~10⁶ can be achieved. Including current mode operation for some of these detectors a significant additional increase in intensity dynamical range would be achieved.

Figure 1

Figure 2

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Last modified 8/11/05, Tizby Hunt-Ward
Figure restoration by T. Manweiler.