Chandra Cal Workshop - HRC Background

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Characteristics of the HRC Background

Michael Juda

Outline of Talk

HRC Background Reduction Measures
Background Components
Temporal Behavior
HRC-I Spatial Distribution
HRC-S Spatial Distribution
Future Directions

For more details see "Characteristics of the On-Orbit Background of the Chandra X-ray Observatory High Resolution Camera", 2002, SPIE Proc. 4851

HRC Background Reduction Measures

HRC MCPs made from ``low-noise'' glass

The HRC detector housing is surround on five sides by plastic scintillator for use as an active anti-coincidence shield.

Signals from shield are used to flag MCP events caused by penetrating particles
Flagging works for the HRC-I but not for the HRC-S

For each MCP event, telemetry contains info on the shape of the charge distribution from the MCP as well as state of on-board hardware checks. Particle event signature is different from x-ray event. (see poster by C. Wilton)

HRC telemetry also contains scalars to report rates: MCP triggers (Total rate), triggers that pass on-board validity checks (Valid rate), and antico-shield rate.

Background Components

Internal ( ~ 10 counts/s)
- radioactive isotopes within instrument materials
- detector noise
External ( ~ 150 - a few ×1000 counts/s)
- cosmic rays
- solar flares
- coronal mass ejections (CMEs)
- Earth's trapped radiation belts

CXO particle detector (EPHIN) measures the local radiation envirnoment.

electrons: four energy channels in the range 0.25-10 MeV
protons: four energy channels in the range 5.0-53 MeV
Integral channel: electrons with E > 8.7 MeV, protons and nuclei with E > 53 MeV/nucleon

Temporal Behavior

Variation in the background observed since mission start.

Flaring on time scales of seconds to hours observed on top of a quiescent level.

Reasons for variability:

solar flares and CMEs
transit of fringes of radiation belts
solar-cycle modulation of cosmic ray flux

HRC-I MCP trigger rate correlates with the EPHIN high-energy particle flux (ignoring X-rays and the flaring component)

Flaring Background

HRC-I Spatial Distribution

Reasons for spatial variation

``Hot-spots''
detector gain/sensitivity variations
antico-shield sensitivity variations
shielding by surrounding materials

Non-Sky-Viewing Background

Raw	Filtered

Combined AR Lac Fields Background

Raw	Filtered

Coincidence shield flagged Events

HRC-S Spatial Distribution

Reasons for spatial variation

``Hot-spots''
detector gain/sensitivity variations, including UV/ion shield
shielding by surrounding materials

HRC-S is operated in several different modes depending on the type of observation. Modes select different areas of the detector.

HRC-S Spectroscopy Mode (Default)

HRC-S Spectroscopy Mode (Default) - Filtered

HRC-S ``Timing'' Mode

Raw	Filtered

HRC-S Edge-Blank Center vs ``Timing'' Mode

Future Directions

Track temporal changes
Continue to refine characterization
- Changes in the ``quiescent'' distribution over the solar-cycle
- Are there differences in flaring correlated with:
  - Rate in flaring component
  - Characteristics observed in EPHIN or particle detectors on other spacecraft
  - Time since recent solar flare or CME
  - Proximity to the Earth's radiation belts
Develop models for estimating the background for a given observation

File translated from T_EX by T_TH, version 3.01.
On 22 Nov 2002, 11:55.

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