Voyager-1 LET B
Voyager-2 LET C
Pulse Height Multiplication in the LET 35 micron detectors
Three Parameter Analysis for Iron Nuclei
Two Parameter Analysis for Iron Nuclei
Pulse Height 'Multiplication' Effect
LET ID Tag Bit Errors at High Counting Rates
LET L1 Detector Damage and Annealing
Voyager-2 LET C Temporary Gain Shift
CRS Instrument Configuration Changes
Voyager-1 Block I PHA Problem
From Appendix B of Dr. Rick Cook's thesis:
Appendix B Problems
LET B of Voyager-1
The response of this telescope was unusual in that:
The problem may be electronic instability, but was not
investigated in detail; instead the data from this telescope were
excluded from use in the measurements presented in Chapter 4,
Tables 4.1 through 4.12.
LET C of Voyager-2
On April 1, 1978 the L1 detector of this telescope experienced
a simultaneous shift in energy calibration and an increase in count
rate, perhaps as the result of a light leak in the thin AL entrance
window. Subsequent data from this telescope were not used in the
measurements of Chapter 4, Tables 4.1 through 4.12.
Pulse Height Multiplication in the LET 35 um Detectors
A background effect specific to the iron group nuclei was
identified in Sections 3.3 and 3.5 and necessitated the further
study and special handling which is discussed here. The affected
PHA events were clearly seen in Figure 3.7, a plot of the charge
measurements Z1 versus Z2, as a clump of points near (Z1=26,
Z2=28). Inspection of the L1, L2 and L3 pulse heights of the
events in this clump revealed a very specific signature:
The effect also occurs for a fraction of the iron nuclei
which barely penetrate the L1 detectors. In this case, the
affected events may be seen as the unusual clump of points near
Z1=28 in Figure 3.6.
The above signature suggests a pulse height multiplication
effect in the LET 35 micron surface barrier detectors that is very
similar to one observed earlier in the response of such detectors
to fission fragments. The effect (discussed by Walter 1969)
occurs for nuclei which deposit a large ionization charge density
in the depleted silicon near the gold electrode. The large
ionization charge density is thought to induce tunneling of
additional carriers (electrons) from the electrode through the
thin oxide layer which separates the electrode frm the depleted
bulk silicon. The orientation of the LET 35 micron detectors is
consistent with this multiplication hypothesis: The gold electrodes
face toward the L3 detector, such that the ionization charge
density near a gold electrode is largest for nuclei which just barely
penetrate the detector, as in (2) above.
To further study the problem, three detector PHA events with
charge measurements Z1> 24 were re-analyzed. For each event the
L1 and L3 energy measurements (E{L1} and E{L3}) were used to
calculate a new charge measurement Z' and obtain an estimate
E'{L2} of the energy deposited in detector L2 by solving:
R is the generalized range-energy function discussed in Section
3.4. T{L1} and T{L2} are the mean pathlengths for the L1 and L2
detectors respectively. The results are illustrated in Figure
B.1, a plot of E{L2} (the measured L2 energy) versus E'{L2} (the
L2 energy inferred from the L1 and L3 energy measurements).
Normal events lie along the diagonal, while events with abnormally
large L2 energy measurements lie above the diagonal. (Events
below the diagonal may be due to L2 edge effects and other
processes discussed in Section 3.5.2) The abnormal events occur
predominantly at large values of E'{L2} which can only be obtained
by nuclei which barely penetrate L2, consistent with observation
(2) above. Figure B.2 shows two histograms of E{L2}/E'{L2}; one
histogram for events which barely penetrate L2 (E{L3} < 170 MeV)
and one for the remaining events (E{L3} > 170 MeV). These
histograms are summed over all the LETs used on both Voyagers and
indicate that (1) about 15 percent of the events with E{L3} less
than 170 MeV have L2 energy measurements which are abnormally large
by an average of about 12 percent and (2) the L2 energy measurements
are essentially normal for events with E{L3} greater than
170 MeV.
Three Parameter Analysis for Iron Nuclei
The results for iron nuclei presented in Table 4.2 (8.7-15.0
MeV/nucleon) are based on PHA events selected as follows:
PHA events with abnormal L2 pulse heights constitute about 15
percent of the iron nuclei counts presented in Table 4.2. Any
additional systematic error (beyond that discussed in Section 4.4.1)
in the iron nuclei counts due to the abnormal L2 pulse heights
should be small. For example, if no corrections at all are applied
to the abnormal L2 energy measurements (as was the case for the iron
abundances reported earlier in Cook et. al. 1979 and Cook, Stone
and Vogt 1980), the resulting iron abundance measurements differ
from those of Table 4.2 by less than 5 percent.
Two Parameter Analysis for Iron Nuclei
In the two parameter analysis of iron (for the fluence measurements
of Tables 4.4 through 4.11) the multiplication effect in the
L1 detectors was taken into account as follows:
In the two parameter iron analysis, no special treatment was
applied to account for multiplication effects in the L2 detectors.
They do not significantly affect the Z1 charge measurement and
at most shift a small fraction (about 5 percent) of the PHA events
which should fall in th 7.8-8.7 MeV/nucleon interval into the 8.7
- 10.6 MeV/nucleon interval.
The inclusion of nickel and other nuclei (27 <= Z <= 32) increases
the fluence measurements over measurements of pure iron by roughly
the combined abundance of these elements relative to iron, or about
10 percent. However, the inclusion of these nuclei probably has
only a negligible effect on the spectral index( gamma, see Section 4.5),
since the energy spectra of the various elements of the iron group are
likely to be similar. (Spectral index variations among the elements
appear to be roughly ordered by nuclear charge Z, such that, in a
given flare event, neighboring elements have similar spectral
indices; see Section 5.2. Further, the nickel to iron ratio {8.7
-15 MeV/nucleon, Table 4.2} is nearly constant from flare to flare.)
The multiplication effect in the L1 detectors is potentially an
additional source of systematic error (beyond those discussed in
Section 4.5) in the iron fluence measurements of Tables 4.4 through
4.11 and the iron spectral index measurements of Tabl 4.12. Upper
limits to these possible additional errors were obtained by re-
computing the fluences and spectral indices without the L1 energy
corrections discussed in (2) above. The changes in the iron fluence
measurements were typically less than 10 percent for energy bins in
the range 5.0-8.7 MeV/nucleon, and wer zero for the other higher
energy bins. The iron spectral indices changed by less than 5
percent. Since the actual systematic error induced in the iron
spectral indices is probably much less than 5 percent (and therefore
is small compared to statistical error and the possible systematic
error from other sources discussed in Section 4.5) this error is
negligible.
Appendix G Instrumental Anomalies and Other Problems
The analysis of data form the Voyager CRS LET and HET telescopes
was complicated by several instrumental problems. These are
described in detail elsewhere (Breneman 1984); they are summarized
here. The Flare analysis was treated differently from interplanetary data.
Pulse Height 'Multiplicaton' Effect
This effect has been observed many times in numerous surfacebarrier
detectors, both in flight and in the laboratory (Cook
1981, Breneman 1982). Particles passing completely through
a detector sometimes yield pulse heights that are anomalously
high By about 10 - 30%). It occurs most often for particles
with high dI/dx in the detector in question, and therefore
at a given initial energy, the errect occurs more often for
elements higher on the charge scale; in the data it is most
prominent for Fe. On a delta E versus E' plot, the effect
appears as a more or less diffuse 'track' above and roughly
parallel to the nominal track for the element, since delta
E is anomalously high for the affected particles (Fig. G.1).
A charge determination of such an event will of course be high,
generally by ~2-3 charge units at Fe. Since the effect is
strongly dependent on dE/dx, it is usually evident only in
the delta E detector immediately before the E' detector.
When z is calculated for 3-parameter events involving such
anomalous pulse heights, Z2 is more strongly affected than
Z1, since the anomalous pulse height has the role of delta
E for Z2, while for Z1 the same detector PHA usually makes
only a modest contribution to E' with delta E normal. On a
Z1 versus Z2 plot (e.e., Fig. 3.1), the effect takes the form
of a cluster fo events to the right of, and slightly above, the
main cluster along the diagonal. All of the Voyager LETs show
the effect for Fe; although its rate of occurrence varies
somewhat between the different telescopes, it is generally in
the range of ~5 - 10% of all the 3-parameter Fe SEP events in
a given telescope. The fraction of 2-parameter events affected
is larger, since the E' detector is thinner and therefore a
larger fraction of the data has high dE/dx in the delta E
detector. At least one LET (Voyager 1 LET A) shows evidence
for the effect at charges as low as 20.
In the HETs the problem is worse in several respects. Its
rate of occurrence at Fe, as a percentage of the total Fe
event ample, is generally much larger than in the LETs
(~40 % for Voayger 1 HET 2); it is clearly seen for elements
as low on the charge scale as Mg in some telescopes (Voyager
1 HET 2 and Voyager 2 HET 2); and for 3-parameter events it
can sometimes be seen occurring in either (or both) delta E
detectors, rather than just the last one, resulting in
several displaced clusters of events in those telescopes
(Voyager 1 HET 1 and Voayger 2 HET 2).
Two actions were necessary to deal with this problem. For
abundant elements affected by the problem, mainly Fe and Ni,
the 3-parameter charge consistency requirement was made
lenient enough to include the particles affected by pulse
height multiplication. The rate of occurrence in LET for
elements lower than Fe was negligible compared to other sources
of uncertainty in the abundance determination, and no correction
was made for these elements. Based on the observed rates
in some HET telescopes for the abundant elements (e.e., Fig.
G.2 for Voayger 1 HET 2), the rate of occurrence in these
thelscopes for elements in the Z = 17-25 charge range was
significant even though limited statistics make it less
apparent and less quantifiable. However, HET data were not
used for these elements for the reasons given in Section 3.3.
In addition, the energy loss in the delta E detector had to
be corrected in an approximate way for affected events, so that
the total incident energy, which is required for constructing
energy spectra, would be accurate.
For 2-parameter events, there is no second independent
determination fo Z to permit unambiguous separation of normal
and abnormal events. This is not a serious problem for
abundant elements, since the only abundant element
significantly affected is Fe, which has no other elements of
comparable abundance near it on the charge scale with which
it could be confused. For rare elements the situation would
be more serious, but as noted in Section 3.3, 2-parameter
data were not used for rare element abundances due to
excessive background contamination from other sources.
LET Telescope ID Tag Bit Errors at High Counting Rates
For each Block I or Block II LET event, there is a single
tag bit which specifies from whih LET telescope in that Block
the event originated. A near coincidence in the triggering
of LETs A and B, as is possible during periods of very high
count rates, can result in the bit being set for a LET A
event, causing that event to be read out as a LET B event
(similarly for C and D). Since the bit is ordinarily set
only for LET B events and is otherwise not set, LET A
events can be misidentified as LET B events but never the
reverse.
The telescope identification bit is used in all subsequent
data analysis to determine the appropriate detector thicknessess
and gains to use in calculating energy losses in the
detectors and, ultimately, the charge of the particle. If
the telescope identification is erroneous, incorrect thicknesses
and gains are used in the calculations, resulting in
incorrect determinations of Z. The magnitude and sign of
the discrepancy in Z depends only on the (coincidental)
relationship between the thicknesses of the detectors in the
paired telescopes, and, to a lesser degree, differences in
the energy calibrations fo the respective detectors.
On a Z1 versus Z2 plot of 3-parameter Voyager data, this
effect has the appearance of small clusters of events displaced
slightly from the main clusters along the diagonal
for all the more abundant elements. It appears only in
plots of the B and D telescopes, sine it is events with
these identifications which contain some misidentified
particles. In the Voyager flight data the effect is noticeable
only during flare period 7, for which the peak LET B
singles rate is ~ 5 * 103/sec, the highest seen during
the Voayger mission through August 1984. Its rate of occurrence
is about 3% at this peak rate, an average of about
1% for flare period 7 as a whole, and is the same for all
elements for which statistics permit a measurement. On delta
E versus E' plots of both 2- and 3-parameter data, these
effects have the appearance of 'ghost' tracks falling
between or partially overlapping the real tracks of nearby
elements (Fig. G.3).
Later laboratory work using the backup GRS and pulse
generators (Martin 1983) was able to reproduce the effect
with greatly improved statistics, and verified the magnitude
of the time constant (~6 micro-seconds) implied by the flight data
while extending coverage to event rates more than an order
of magnitude above the highest seen in the flight data.
The impact of this problm on the data analysis is relatively
minor. For 3-parameter data, the previously chosen charge
consistency requirement is restrictive enough to easily
exclude the misidentified events; the amount of data lost
is an insignificant 0.2% of the total, and the remaining data
set is as 'clean' as that from the other telescopes. The
problem is more serious for the 2-parameter data, since there
is no second determination of Z to permit separation of the
normal and abnormal events; it is an unremovable source of
background in the data. For abundant elements this is unimportant,
since the error introduced by this background is
on the order of 1 % or less. But the problem would be serious
for rare elements in cases where the 'ghost' track of an
abundant element overlaps the true location fo a rare element,
since even 1 % of an abundant element could seriously contaminate
a much rarer element. However, as noted previously,
the 2-parameter LET data are not useful in obtaining rare
element abundances on account of other background contributions.
LET L1 Detector Jupiter Encounter Radiation Damage
and Post-Encounter Annealing
As a result of their exposure to intense charged particle
fluxes in the inner Jovian magnetosphere during the 1979
Jupiter encounters, the L1 detectors of the LETs experienced
radiation damage which can be modeled as a reduction in the
'effective thickness' of the detectors. It is thought to be
due to the implantation of energetic oxygen and sulfur ions
known to be present in the inner Jovain magnetosphere (Gehrels
1982). Although all LETs on both spacecraft were affected to
some degree, the Voayger 1 LETs were affected much worse than
those on Voyager 2, since the former spacecraft passed closer
to Jupiter and experienced a more intense radiation environment.
On each spacecraft, LET C was by far the most seriously
affected; this telescope was sparially oriented so as to
receive the most intense radiation exposure during the Jupiter
encounters. (LET B on Voyager 2 experienced unrelated types
of radiation damage during the Jovian encounter and has returned
no data since the encounter.) The front detectors of
the HETs, being much thicker than those of the LETs and pro-
tected by a thicker window, showed no detectable effective
thichness reduction.
The impact of the radiation damage on the post-Jupiter data
is apparent as a shift in the location fo the element tracks on
a delta E versus E' plot of data from flares 16 and 17, the
first large post-Jupiter flares, relative to their location in
plots of pre-Jupiter flares. Similarly, Z1 versus Z2 plots
of flares 16 and 17 show Z values shifted from their proper
values when Z is calculated using the detector thicknesses
measured before launch, which served adequately for all pre-
Jupiter flares. The change in effective thickness appears to
be somewhat dependent on Z, with the magnitude of the reduction
increasing with Z for any given detector. Furthermore, with
the passage of time the radiation damage seems to gradually
undergo a partial reversal. This 'annealing' effect is
evident in data from the later large flares, 20 and 24, which show
less shift in Z than do the flares immediately following
Jupiter encounter.
In the analysis of post-Jupiter data, this problem was dealt
with by adjusting the L1 detector thicknesses used in the calculation
of Z so as to make the calculated charges fall in the
proper places on the charge scale. Flares 16 and 17 were used
to define the required shift for the more abundant elements;
a linear or weakly quadratic function of Z was fit to these to
define the Z dependence for all Z. Data from flare periods
20 and 24 were used in conjunction with the flare 16/17 data
to mathematically characterize the time-dependence of the
annealing effect, by fitting to a decaying-exponential
function of time. This procedure, repeated for each LET,
defined the L1 thickness to be used in analyzing any given
post-Jupiter event. The adjustment of the L1 thickness was
then incorporated into the iterative cycle for calculating
Z. The radiation damage did not have a noiceable effect
on the inherent charge resolution of the telescopes, so with
the above modifications the post-Jupiter flare data could be
treated the same way as the pre-Jupiter data.
Table G.1 lists the adjustment made to the thickness for
each L1 detector cor carbon and iron at two different times
during the post-Jupiter phase. The actual expression for the
thickness L(Z,t) of each L1 detector was given by
L(Z,t) = L0 - deltaL0(Z) + K exp(delta t / 562.56)
where K is a constant, L0 is the pre-Jupiter thickness,
delta t is the time since the Jupiter encounter in days, and
deltaL0(z) is the linear or quadratic function of Z that
closely fits the required thickness changes for the first
post-Jupiter flares.
Voyager 2 LET C Temporary Gain Shift
During the time period 1978 APR 3 - June 9, the L1 detector
of LET C on Voayger 2 experienced, for unknown reasons, a
temporary gain shift (~ 47 % decrease) and an associated
excessively high L1 count rate (~ 9* 103/sec). The gain
shift and excessive count rate set in abruptly, remained
nearly constant until about May 29, and then gradually
reverted to their former levels; a very slight decline during
the central phase was consistent with the decrease in the
intensity of sunlight during the smae time period and suggests
the possibility of a light leak in the telescope's aluminum
window.
The effect on the data was to shift the locations of the
element tracks on a delta E versus E' plot, and yield shifted
charge estimates when nominal gain factors were used in the
analysis. The only flares occurring duting this time period
were the three large events of flare period 7. Since the
gain was constant at the shifted value during these flares,
the data could be analyzed by generating the appropriate
gain factors by fitting the oxygen flight data from flare
period 7 in the manner described in Appendix A. The resulting
values are included in Table A.1.
This anomaly was previously noted (Cook 1981), and the
telescope was rejected for analysis because of the problem.
However, since the energy calibration used in the analysis
is easily adjusted to compensate for the problem, since the
charge resolution and background of the telescope do not
seem to be affected by the problem and since the three 7
flares include a major fraction of all SEP data, it was
decided to inlude Voayger 2 LET C in the analysis with
the special treatment described above.
CRS Instrument Configuration Changes
At certain times during the Voayger mission, the configuration
of the CRS instruments was changed in ways that
influence data analysis. At the beginning of each flight
the LETs were configured to require triggering of the L3
detector for pulse height analysis; that is, only 3-
parameter events were analyzed. About 12 days after launch
the L3 coincidence requirement was removed, permitting
both 2- and 3-parameter events to be analyzed. For Voyager
2 this occurred before the first flares were seen, but on
Voyager 1 flares 1a and 1b occurred before the configuration
was changed, so no 2-parameter events were obtained from
these flares. A similar situation occurred on 17 Jun 1978
when Voyager 1 LET C was switched back to requiring L3
coincidence. Thus for all flares from 8 onward there are no
2-parameter events from this telescope. These situations
required changes in the weightings of 2-parameter relative
to 3-parameter events for the affected spacecraft and
flares. There could still be a residual abundance bias in
flares 1a dna 1b if the particle composition was energy-dependent
and if the two spacecraft saw particles with
different spectra, but the possibility of a bias comparable
to the statistical uncertainty in the abundances is very
unlikely.
Another important configuration change is the HET gain state.
Normally the instrument cycles between high and low gain
modes, but after Jupiter encounter HET 1 on Voyager 2 was
switched to a high-gain-only mode, so no post-Jupiter SEP
heavy ion data were obtained from this telescope. This
required changes in the partile wwighting factors of HET
relative to LET for Voyager 2 for all post-Jupiter flares.
The event weighting factors tabulated in Table 3.8 include
the effects of all instrument configuration changes.
Voyager 1 Block I PHA Problem
On 1982 Feb 8, the Voayger 1 CRS experienced a failure
affecting the readout of PHA information from the Block I
telescopes (LETs A and B and HET 1). The result of the
failure is that in place of PHA2, the instrument reads out
whichever of the three PHAs has the largest numerical value.
If PHA2 happens to be numerically the largest pulse height,
as is true over some energy ranges, the event is read out
normally; otherwise some information si lost. The effect
of this problem is that some of the 2-parameter events are
lost completely, and that some of the 3-parameter events
are degraded to 2-parameter events.
T{L1} = R(E{L1} + E'{L2} + E{L3}, Z', M) - R(E'{L2} + E{L3}, Z', M)
T{L2} = R(E'{L2} + E{L3}, Z', M) = R(E{L3}, Z', M)
taking M = 2.132 * Z'
From Appendix G of Dr. H.H. Breneman's thesis:
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