HRC Anticoincidence Shield Rates vs EPHIN Rates

Introduction

The detectors (HRC-I and HRC-S) are surrounded by a five-sided plastic scintillator anticoincidence shield designed to reject events within the MCPs that are induced by high energy charged particles. The figure below illustrates the principle of the shield.

The shield provides 4pi coverage. The shield consists of two types of Bicron plastic scintillators, BC 404V (blue) and BC 428V (green)optically coupled to two EMR photomultipliers (only one is used at a time). The walls of the shield are BC 404V scintillator and the baseplate is BC 428 V scintillator. The light pipe is coupled to the BC 428V scintillator with BC-600 optical expoxy and the photomultiplers are coupled to the light pipes via elastomer pads. The pads are made of Dow Corning 93-500 space grade encapsulant. The two figures below are schematic of the shield and its electronics.

The quiescent (no solar flares) shield rate in-orbit varies between 3000 and 4500 ct s^-1. The maximum telemetered rate is limited by a 16 bit integer to 65,535 ct s^-1

PMT HVPS Codes (Applied voltage vs step number)

Parameters of the shield

Scintillator area: 4300 cm²
Solid angle: 4pi steradians

Photomultiplier characteristics

Type: EMR/Schlumberger 541E-05M-13; trialkali (S-20) photocathode
Window: 1" diam end-on, sapphire
Sensitivity: 140 nm - 850 nm
QE at maximum sensitivity (410 nm): 20%
number of dynodes: 13
operational gain: 10⁶
Gain vs Voltage
dark current at 10⁶ gain: 6 x 10^-10 A
estimated average current at 10,000 ct s^-1 and 10⁶ gain: 2 x 10^-8 A

Component limitations to life

scintillator degradation after 10⁶ rad
BC-600 epoxy degradation after 10⁸ rad and only at wavelengths near 300 nm
electronics: meets mission radiation dosage specifications
Dow Corning 93-500 space grade encapsulant: radiation tolerance - "usable after 200 megarad exposure; hard and brittle after 500 megarad exposure".
photomultipliers: radiation hard; for currents < 10^-6 A lifetime is shelf life - approx. years

Telelemetry description

Shield MSIDs available to the OBC:
2SHLDART (ShldEvtRateA)
2SHLDBRT (ShldEvtRateB)

These are engineering SHIELD RATES A|B - Upper 8 bits (high order byte) of the shield rates reported in the HRC secondary science stream (2SHEV1RT and 2SHEV2RT). The secondary science MSIDs are 16 bit variables. The secondary science shield rates are not visible to OBC.

At any one time, only one rate is active and being reported 2SHLDART OR 2SHLDBRT. Currently 2SHLDART (ShldEvtRateA) is the active reporting side.

MSID Location and size:

2SHLDART (ShldEvtRateA) Size 8 bits max 255 min 0 In all formats: mf bytepos (270 + 4 sync) -- ------- 11 274 43 274 75 274 107 274 ----------------------------- 2SHLDBRT (ShldEvtRateB) Size 8 bits max 255 min 0 In all formats: mf bytepos (270 + 4 sync) -- ------- 12 274 44 274 76 274 108 274

HRC trend data

The HRC Team maintains a Web site at http://hea-www.harvard.edu/HRC/trends/index.html which among other information, has time series plots of HRC secondary science data, and GOES, ACE, and EPHIN data.

The anticoincidence shield as a back-up to the EPHIN

In order to prepare for the possibility of a performance degradation or failure of the EPHIN as a radiation monitor the HRC team and others have been looking at the HRC anticoincidence shield as a radiation monitor. In order to evaluate this possibility we have been investigating the correlation between the shield rates and EPHIN fluxes during the occurence of solar events that produce radiation above the P41 threshold of 8.47 ct cm^-2 s^-1 ster^-1. Since the shield has been off a large fraction of the time, overlapping solar events exceeding this threshold and shield data are sparse. We have identified about six times over the course of the mission when they are coincident. However, given the properties of the shield, this appears to be sufficient to come to some general conclusions about how the shield can be used as a radiation monitor. Firstly, the shield correlates qualitatively well with P41 and the other EPHIN channels as will be seen below. However, there is a large dispersion in the quantitative correlations. Secondly, the shield count rate saturates at 65,535 ct s^-1, well before the P41 threshold is passed. Thus using the shield as a monitor and triggering SCS107 near count rate saturation will provide conservative radiation protection but produce many false positives. Further investigation of the past history of all solar events, large and small, will provide an estimate of the magnitude of this problem.

One issue of concern with respect to false positives is a "glitch" in which the shield rate saturates during a format change. We see this in the R_shield data, However, the latter is not available to the OBC. Only 2SHLDART and 2SHLDBRT can be polled by the OBC. Currently, only 2SHLDART is meaningful. The reported value for 2SHLDART (ShldEvtRateA) was checked from 1999 thru 2003 for all telemetry transitions. The 2SHLDART (ShldEvtRateA) DN value was printed out before and after a telemetry change. DN changes were constant or smoothly changing in all cases. No spikes or saturated values occurred during a telemetry transition.

We have looked at the shield-EPHIN correlation after recovery from radiation events since a decision will have to be made when to re-initiate observations. Again, since shield rates below saturation are also well below the P41 threshold, recovery will be very conservative with respect to the radiation environment.

We'll illustrate the above with some examples of radiation events. The shield rates and EPHIN fluxes are shown after recovery. In order to illustrate the quantitative correlation between shield rates and EPHIN fluxes, the EPHIN fluxes have been scaled by a constant numeric factor and overlayed with the shield rate (after subtracting a quiescent background of 4000 ct s^-1).

December 26, 2001 event

P41:

P25:

P8:

P4:

INT:

More examples (in postscript)

April 3, 2001
P41, INT

November 23, 2001
P41, INT

April 20, 2002
P41, INT

The table below illustrates the variation in the P41 with event. The scale factor is the contant multiplier for the EPHIN P41 flux required to bring the flux into coincidence with the shield rate.

Event Date	P41
Dec 26, 2001	60000
Apr 3, 2001	110000
Nov 23, 2001	90000
Apr 20, 2002	40000

We observe a variation of about a factor of three in the scale factor in these events alone.

Conclusions and recommendations

If EPHIN degrades or fails, the shield is all we've got, poor dynamic range and all. SCS107 can be initiated just before the shield rate reaches saturation.

Recovery decisions less conservative than requiring a shield rate below saturation will have to be based upon GOES, ACE, and other external data.

Further collection of data by operating the shield differently then it is operated today is probably not necessary.

Lasdt updated: February 6, 2003