## Fundamental Technologies## Voyager LECP Pages |

*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 D_{1}, D_{2}, and
D_{3}, 25 mm^{2} 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 D_{1} and D_{2}
so that D_{2} and D_{3} are shielded against *direct* radiation.
The outputs of the system will be (a) two counting rates from D_{1} at
discrimination levels ~60 and 180 keV respectively (N_{1l} and N_{1h}),
(b) one single rate each from D_{2} and from D_{3}, both at discrimination
levels of 60 keV (N_{2} and N_{3}), (c) a D_{1}D_{2}
double coincidence rate (N_{12}) and (d) a D_{1}D_{2}D_{3}
triple coincidence rate (N_{123}). To illustrate the characteristics of the system
we consider the following five cases.

**Protons and electrons of energies less than 60 keV**(i.e., likely populations in shock transition regions). In this case D_{1}will register the particles thru pile-up while D_{2}and D_{3}will not respond. Therefore, we would expect N_{1l}to be greater than N_{1h}while N_{2}, N_{3}, N_{12}, and N_{123}have their values as in the interplanetary space.**Electrons of energies between 0.06 and 1 MeV.**In this case, D_{1}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/cm^{2}in tungsten, D_{2}and D_{3}will be triggered only by the bremsstrahlung. Therefore, we would expect N_{1l}to be greater than N_{1h}, N_{2}~ N_{3}, N_{12}approximately equal to or greater than 2t N_{1l}N_{2}, and N_{123}approximately equal to or greater than 3 t^{2}N_{1l}N_{2}N_{3}, where t is the resolving time. The last two equalities are due to the fact that N_{12}and N_{123}will be mostly accidental coincidences.**Protons of energies between 0.06 and 17 MeV.**In this case D_{1}will count the particles directly but D_{2}and D_{3}because of the shielding of the tungsten absorber will not respond. The expected rate pattern is then N_{1l}~ N_{1h}while N_{2}, N_{3}, N_{12}, and N_{123}still have their interplanetary or background values. This pattern distinguishes this case from case (1).**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 N_{2}greater than N_{3}; N_{12}greater than 2t N_{1}N_{2}, and N_{123}greater than 3t^{2}N_{1}N_{2}N_{3}.**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 N_{2}greater than N_{3}, N_{12}~ F(AW)_{12}+ 2t N_{1}N_{2}, and N_{123}~ F(AW)_{123}+ 3t^{2}N_{1}N_{2}N_{3}, 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 ~10^{6} can be achieved. Including current mode operation for
some of these detectors a significant additional increase in intensity dynamical range
would be achieved.

**Figure 2**

Next: MJS-77 Science Capability Review on July 29-30, 1975

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Last modified 8/11/05, Tizby Hunt-Ward

tizby@ftecs.com

*Figure restoration by T. Manweiler.*