SEPICA is the main instrument on ACE responsible for
the determination of the ionic charge states of solar and interplanetary energetic
particles. The charge state of energetic ions is key data, needed to determine
source temperatures, acceleration, fractionation, and transport processes
for these particle populations. Ionic charge results from SEPICA's predecessor
the ISEE ULEZEQ instrument have shown, for example:
- The charge states of heavy elements in long duration solar
flares -- indicating source temperatures of several million degrees -- differs
from element to element. The physical processes behind this finding have
not been completely resolved thus far.
- The charge states of Fe and Si from compact 3He-rich
flares turned out to be significantly higher than those from long duration
flares. No study of individual compact flares had been possible and the
responsible physical processes are not yet clear.
- A substantial enhancement of He+ in interplanetary
particles has been found. It has been suspected that this could be due to
acceleration of interstellar pickup ions. No comprehensive study has been
possible as yet.
To better understand the processes above, SEPICA was
designed to provide superior charge state resolution and a larger geometric
factor than ULEZEQ (see tables below). With these improved capabilities, individual
charge states can now be resolved and counting statistics are more reliable.
Energy Range and Element
Resolution
|
Energy Range
|
Resolution
|
Elements
|
|
0.3 - 6 MeV/N (He)
|
isotopic
|
He
|
|
0.2-18 MeV/N (O)
|
individual elements
|
up to O
|
|
0.1-5.4 MeV/N (Fe)
|
groups,  Z/Z
2-3
|
up to Fe
|
Geometric Factor and
Charge Resolution Q/Q
|
Geometric Factor
|
1 MeV/Q
|
3 MeV/Q
|
|
0.2 cm2sr
|
0.3
|
1
|
|
0.03 cm2sr
|
0.1
|
0.3
|
To accurately measure the charge states of incoming
ions SEPICA uses one high charge resolution sensor section and two low charge
resolution sections. The charge resolution is achieved by focusing of the
incoming ions through a multi-slit mechanical collimator, deflection in an
elctrostatic analyzer with a voltage up to 30 kV, and measurement of the impact
position in the detector system.
To determine the nuclear charge and energy of
the incoming ions, thin-window flow-through proportional counters with isobutane
as the counter gas, and ion-implanted solid state detectors are used. The
counters provide for three independant E
(energy loss) verses E (residual energy) telescopes. The multi-wire proportional
counter simultaneously determines the energy loss (E)
and the impact position of the ions. Supression of background noise from penetrating
cosmic radiation is provided by an anti-coincidence system with a CsI scintillator
and Si photodiodes. The data are compressed and formatted in the S3DPU, which
is shared with SWICS and SWIMS.