**The University of Chicago ONR--604 Experiment Instrument Description**

The high mass resolution needed for isotopic separation of heavy ions
requires knowledge of the trajectory of each charged particle entering
the acceptance cone of the telescope in order to determine the
pathlengths in the detectors. As shown in FIGURE 1(B), each position
sensing detector (PSD) has a series of gold strips on the top surface of
the detector which collect a fraction of the charge liberated by the
passage of the particle. This signal is read out through a resistive
divider network and compared to the total signal from the detector,
measured by the charge collected on the back surface contact. The ratio
of the two 1024 channel pulse height analyzers provide one coordinate of
the point of incidence of the particle (Lamport, *et al*., 1976). Two
sets of PSDs (three per set for redundancy) were used in the telescope
to obtain the trajectory of the particle, or a total of 12 analyzers.
Calibration of this system with accelerator particles has shown that the
position can be measured to better than 0.1 mm: therefore, the angle of
incidence of the particle could be determined to better than 1°.

The charge and mass of each particle was determined by the well-known
technique of multiple dE/dx versus residual energy using the
measurements provided by the stack of 5 mm thick lithium-drifted silicon
detectors, corrected for the trajectory of the individual particle. For
this analysis we required that the particle came to rest within the
detector stack. This determines the energy range over which each type of
particle can be studied. The signal from the scintillator guard counter
was used in anti-coincidence mode to eliminate out-of-geometry events
and to tag particles that exit from the side of the apparatus.

To separate isotopes for elements as heavy as nickel, it is necessary to
measure the dE/dx and residual energy signals to a precision level of
0.1%. This stringent constraint has been met by developing exceptionally
linear, highly stable electronic circuits to analyze the signals from
each detector. Each analysis chain has been individually calibrated over
a wide temperature range, and the entire instrument performance has been
evaluated using high energy heavy ion beams obtained from the LBL
Bevalac accelerator.

The performance of each analysis chain was monitored during flight by an
on-board calibrator which periodically sent a sequence of 2000 pulses
spanning the entire range of signal sizes. Furthermore, in-flight
calibration and normalization was provided by measurement of galactic
cosmic ray events. These high precision measurements required that the
temperature remained stable and constant to within \~3 °C over each
orbit: consequently the instrument power was on during 100% of the
orbital period.

Because the anti-coincidence scintillator guard counter subtended a
large solid angle and was used in the instrument logic, it was shielded
from the high counting rates induced by low energy particles trapped in
the magnetosphere and by intense solar flare fluxes. The shielding
\[shown in FIGURE 1(A)\] corresponded to complete elimination of \<4 MeV
electrons by the AZ31B magnesium shield and the tantalum collar. The
lower part of the scintillator was located well within the instrument
package and was surrounded on five sides by electronic modules offering
additional shielding. Backward moving particles were additionally
shielded by the spacecraft structure.

Since about 50 percent of the time in each orbit was spent in the
intense radiation portion of the magnetosphere (FIGURE 2) a 3mm external
Al window shield was placed over the telescope in order to protect
further against accumulated radiation damage to the detectors Dl, D2 and
D3 (where the average geometrical factor was \~50 cm^2^-ster). This
shield lowered the integrated proton and electron radiation doses for
detectors Dl, D2, D3 while moving upwards only slightly the energy
intervals for measurement of the heavy particles, which were the main
objective of the experiment.

FIGURE 9 shows a functional block diagram of the instrument electronics.
Besides providing pulse height analysis of each particle, the instrument
had internal accumulators that counted and stored both single detector
counting rates and coincidence counting rates, thus allowing the
absolute sampling rate and the absolute flux levels to be deduced.

In addition to heavy particles the instrument measured protons and alpha
particles over the energy range 25-119 MeV with a priority system to
insure that the rare heavy ion events were preferentially analyzed and
stored. In addition, in regions of expected high background radiation
(*i.e.,* the magnetosphere), a commandable heavy particle mode could be
invoked. This raised the thresholds on detectors D1 through D6 to
suppress the high firing rates induced by trapped protons and electrons
while still permitting the rare heavy ions to be analyzed.

The following are examples of particle range/energy in the telescope:

  -- ---------- --------------------------------------------------- ---------------- ------------------------------------------------------- -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- -------
     Detector   Minimum proton energy to reach detector^\*^ (MeV)   Qmax (degrees)   Maximum proton energy to stop in detector^\*\*^ (MeV)                                                                                                                                                                                                                                                                                                     
     D1         24.8                                                ---              44.5                                                       D2   28.3   87   48.2      D3   31.8   84   49.8      D4   35.2   50   43.9      D5   36.7   48   45.5      D6   38.6   46   46.8      K1   40.0   41   59.4      K2   52.0   39   70.6      K3   62.1   36   79.9      K4   71.3   34   88.5      K5   79.3   32   96.5      K6   87.1   30   103.5
     K7         94.0                                                29               108.4                                                                                                                                                                                                                                                                                                                                                     
     K8         100.8                                               27               116.9                                                                                                                                                                                                                                                                                                                                                     
  -- ---------- --------------------------------------------------- ---------------- ------------------------------------------------------- -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- ------ -- ---- ------ ---- -------

\*normal incidence ^\*\*^Qmax angle of incidence

Examples of the geometrical factors for the telescope in FIGURE 1 are:

**[ ]{.underline}[Logic ]{.underline}**^[\*]{.underline}^**[
]{.underline}[Geometrical Factor
(cm]{.underline}^[2]{.underline}^[-ster)]{.underline}**

D1D2ND3D4N(S) 46.1

D13D46N(K1K2S) 10.9

D13K1K2N(K3S) 7.47

D13K1K8AN(S) 6.08

^\*\ ^D, K, A,S are detectors (see FIGURE 1). "N" indicates that all
following detectors within the following parentheses are in
anti-coincidence.

For further details on the instrument and the CRRES spacecraft, see
Gussenhoven, Mullen, & Sagalyn (1985) and Simpson & Garcia-Munoz (1994).

**APPENDIX 2: The Pathlength Distribution**

The leaky-box equations of interstellar propagation are well-known.
Under certain circumstances which are commonly assumed to be met, these
equations are separable (details in Ginzburg & Syrovatskii 1964;
Garcia-Munoz *et al*. 1987). The resulting "slab" equations can then be
solved as a function of pathlength traversed and energy, while the other
equation has a solution which is the Green's function for the slab
equations, namely, the pathlength distribution (PLD). This technique of
solution is known as the "weighted-slab" method (Fichtel & Reames 1968).
In the slab equations, spallation effects, energy loss due to ionization
and radioactive decay are taken into account. The PLD is used to correct
for the distribution of pathlengths traversed by the cosmic rays. The
solutions of the coupled slab equations are number densities as a
function of pathlength and energy Ni(x,E). These are integrated using a
particular PLD(x,E) over the pathlength x to derive a local interstellar
flux of nuclear species i.

The choice of the shape of the PLD used in our calculations is generally
constrained by the requirement that the calculations agree with measured
secondary/primary ratios. Garcia-Munoz *et al. *(1987) found below \~1
GeV/n that a pure exponential PLD with an energy-dependent escape length
"tuned" to agree with the observed B/C data did not agree with the
available experimental data on the (Sc-Mn)/Fe ratio. They showed that a
simultaneous fit of both ratios could be obtained if an energy-dependent
truncation of the PLD was used below \~1 GeV/n, and that the truncation
increased with decreasing energy.

As described in SECTION 5, the PLD is required to fit measured
secondary/primary elemental ratios. It has been found that while a
single exponential PLD may agree with the Ulysses B/C and (Sc-Mn)/Fe
ratios (DuVernois, Simpson, & Thayer, 1995), some slight truncation at
low energies still appears necessary for agreement with all of the
available data, including other recently published data (FIGURE 10). The
truncation was modeled by the use of a double exponential (DE) PLD, as
suggested by Simon (1977). The form of the DE PLD is:

This is properly normalized, and the energy dependence of the individual
means x1(E) and x2(E) is chosen to agree with elemental
secondary/primary ratios. FIGURE 11 shows the PLD means used to generate
the curves in Fig. 10. It is important to note again that the particular
choice of PLD (single exponential *vs*. double exponential or energy
dependence of mean) has very little effect on the isotopic ratios,
however. This can be seen by looking at the curves in FIGURES 6, 7 and
8. The solid lines in all these figures are the results of using a DE
PLD which was used for previous CRRES source calculations (DuVernois *et
al*., 1993; Garcia-Munoz *et al*., 1993), the dot-dashed lines are the
result of using a DE PLD (FIGURE 11) with a much lower amount of
truncation (controlled by mean x2(E)), and the short dashed lines (seen
in FIGURES 6 and 8) are derived from using a single exponential PLD with
the mean as determined in GM. The differences between the curves are in
all cases \<10%, and are certainly much smaller than the uncertainties
in the fragmentation cross sections. It is clear that truncation of the
PLD has a much smaller role in the question of determining isotopic
ratios than may have been previously thought. The minor truncation
required is in qualitative agreement with Webber (1993), who also found
a slight truncation of the PLD at low energies necessary when performing
a similar analysis.

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