## The Galileo Energetic Particles Detector

## Galileo EPD Handbook

### Chapter 1. Instrument Summary

#### CMS Subsystem (Pre-Challenger Information)

*Source: JOMPI Proposal, November
1976*

The CMS is a three parameter
(time-of-flight, dE/dx and E) solid state detector telescope
designed to measure with high resolution the elemental
composition, energy spectra and pitch angle distribution of
the Jovian magnetospheric Z>=2 energetic particle
environment throughout the Orbiter mission. The
telescope consists of a mosaic of three very thin front
elements (2 and 5 micron thick totally depleted surface
barrier detectors) that are separated by a 7.5 cm time of
flight path from the rear detectors. Incident particle
energy loss (dE/dx) is measured in the front element,
particle time-of-flight (TOF) is measured between the front
and rear detectors, and residual energy is measured in the
rear detector. Three parameter analysis is thus performed on
particles penetrating the front detector, providing accurate
elemental resolution with high immunity to accidental events
over an energy range (for Na ions, for example) from H100
keV/nuc to >10 MeV/nuc. Single parameter rate channels
extend coverage for medium-Z nuclei to 20 to 50 keV/nuc, and
unique two and three parameter particle identifier circuits,
based on logarithmic amplifiers, provide rate data and
priority information with excellent temporal and angular
resolution. An adaptive electronic priority system
(utilizing the particle group identifier channels)
establishes priority for full three-parameter analysis on
the basis of atomic number, thus assuring adequate sampling
for all species, over the full range of nuclear charge and
energy. These proven design factors, plus a low energy
threshold, a good geometry factor (total G = 0.01 cm^{2} ster.
for three-parameter analysis and = 0.31 cm^{2} ster.
for single parameter analysis), and wide dynamic range allow
the CMS to accurately analyze the composition of the Jovian
radiation environment from the low fluxes expected at
apojove and off the equatorial plane to the regions of
intense flux at the orbits of Io and Ganymede.

#### Geometrical Factors

*Source: Ted Fritz, SEL Memo,
April 24, 1978*

During the March 15-21 meetings held at MPAE I was given the action item of calculating the geometric factor of the CMS telescope design as schematically outlined by Herr Bolme. Upon returning from Germany I discussed the problem with Harold Leinbach, and he agreed to use his computer program to make these calculations. The resultant geometric factor for the front elements is:

Detector |
Area (mm^{2}) |
g (cm^{2} ster.) |
Max θ/Z* |

Ja | 25 | 0.1044 | 28 degrees |

Jb | 25 | 0.1044 | 28 degrees |

Jc | 5 | 0.0211 | 24.5 degrees |

Total J (single parameter) | 0.2299 |

Geometric factor of detector K (100 mm2).

Through detector |
g (cm^{2} ster.) |
Max θ/Z** |

Ja | 0.00497 | 6.8 degrees |

Jb | 0.00497 | 6.8 degrees |

Jc | 0.00099 | 5.5 degrees |

Total K (coincidence) | 0.01093 |

*This angle represents the largest possible half-angle at which a particle still strikes the J detector, relative to the axis of the collimator.

**This angle represents the largest possible angle at which a particle still strikes the K detector after passing through the respective J detector, relative to an axis through detectors Ja, Jb, or Jc.

These numbers are in reasonable
agreement with the proposal values of 0.31 cm^{2} ster.
(single parameter) and 0.01 cm^{2} ster.
(coincidence).

**Note on Geometric Factor for
0 Degree End in CMS Unit**

*Source: B. Wilkin, MPAE, July
24, 1979*

The 5μ/50 mm^{2} telescope
in the ΔE/E-telescopes is designed with a 45º opening
angle centered on the surface of the back detector. The
total collimator length of 21 mm (from surface of front
detector) defines then a collimator angle of β =
18.5º. (Any growth of the collimator length l will be done
with β = 18.5 = const.) The collimator ratio is
presently l/r = 5.4. (According to T. Mueller it can be
increased to 6.0.)

The differential geometric factor dG/dα is shown in Figure 1-13 as a function of the angle α (curve (2)). (α denotes the angle of incidence formed by a parallel beam with the collimator center line; compare Figure 1-14). In Figure 1-13 the effect of the collimator is described by curve (3), curve (1) is the dependence of dG/dα with no collimator at all. The function dG/dα reaches the maximum value (modal value) at an angle somewhat larger than the collimator angle β = 18.5º. α(mode) = 200 dG/dα drops to zero for the maximum angle accepted by the collimator αgr=dG/dα drops to zero for the maximum angle accepted by the collimator αgr = 35º. (Note: Figures 1-13 and 1-14 are not available.)

It turns out that the median value am
for the distribution dG/dα is pretty close to β: αm
= 18.5º. The median value of the detector thickness
corresponding to αm is then: dm = 1.054 d. If
we define a FWHM value for the distribution dG/dα we
find FWHM - 20.5º corresponding to an effective
range for dm: dm = (1.054 +/- 0.05) d, i.e., the effective
thickness of the detector is increased by 5%. The maximum
thickness occurs for a beam with α=αgr=35º:
d_{max} =
1.225 d.

Figure 1-15 shows the effect of the
thickness variation in the ΔE/E curves of a 5μ detector.
The FWHM variation seems to be comparable with the other
statistical fluctuations in the signal. This effect seems to
be acceptable. The upper extreme limit, on the other hand,
is quite large and may in fact generate some minor overlap.
However, d_{max} occurs only with a
very small probability. (Note: Figure 1-15 is not
available.)

Conclusion: The large opening in the CMS ΔE/E telescopes seems to be an adequate compromise if a large geometric factor is the more important feature. Some additional spread is to be expected from the large range permitted for the angle α. The FWHM fluctuation introduced by thickness variations seems to be comparable with contributions from other noise sources in the ΔE/E system.

According to T. Mueller, the total length of the CMS unit can be increased to 150 mm. As a result we modified the backlooking (dual telescope) collimator as follows:

l = 300 mm (surface front det.-edge
of collimator)

β = 16.74º

αmax = 22.5º

αgr = 30º

The angles are somewhat smaller than before and this will reduce the signal spread introduced by thickness variations.

Next: CMS Analog Electronics (Post-Challenger Information)

Return to the CMS Subsystem Index

Return to Galileo EPD Handbook Table of Contents Page.

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Galileo Table of Contents Page.

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Technologies Home Page.

Updated 8/23/19, Cameron Crane

## QUICK FACTS

**Manufacturer:**The Galileo Spacecraft was manufactured by the Jet Propulsion Laboratory, Messerschmitt-Bölkow-Blohm, General Electric, and the Hughes Aircraft Company.

**Mission Duration:**Galileo was planned to have a mission duration of around 8 years, but was kept in operation for 13 years, 11 months, and 3 days, until it was destroyed in a controlled impact with Jupiter on September 21, 2003.

**Destination:**Galileo's destination was Jupiter and its moons, which it orbitted for 7 years, 9 months, and 13 days.