CXO Project Science: Chandra X-ray Shielding Study

1997 Nov 25 18:00 GMT

Removal of SIM focus-structure tantalum increases the contribution of the cosmic x-ray background by only about 25%, for any HRC configuration. In consultation with the HRC PI (SAO/ Steve Murray), we conclude that this is acceptable. Consequently, there is no science requirement for tantalum shielding on the SIM focus structure.

Summary

During HRC observations, the focus-structure tantalum does fill small gaps in shielding at the vertices of the HRC entrance aperture (snout); however, much of this shielding is unneccessary because it lies outside the HRC entrance aperture, viewed from any location in the HRC focal plane. In particular, Ta shielding outside the focus-structure stray-light baffles provides little additional x-ray shielding. Based upon detailed simulations, we conclude that removal of all focus-structure Ta would effect an increase in the background of only 0.002 ct cm^-2 s^-1 for HRC-I and 0.004 ct cm^-2 s^-1 for HRC-S, due to the cosmic x-ray background. Furthermore, from analysis of opacity maps viewed from the HRC focal-plane, we estimate that removal of this shielding would add more than 0.01 ct cm^-2 s^-1 due to nonimaged bright sources in only about 1% of pointed observations.

During Next-in-Line (NiL) HRC background measurements, tantalum shields only of order half the HRC entrance aperture (snout), with or without tantalum on the focus structure. Based upon detailed simulations, we conclude that removal of all focus-structure Ta would effect an increase in the background of only 0.009 ct cm^-2 s^-1 for HRC-I NiL and 0.004 ct cm^-2 s^-1 for HRC-S NiL, due to the cosmic x-ray background. Furthermore, from analysis of opacity maps viewed from the HRC in the NiL position, we estimate that the HRC background would increase at least 0.01 ct cm^-2 s^-1 due to nonimaged bright sources in about 15% of pointed observations.

Visualization of tantalum shielding

In order to visualize the role of tantalum in shielding the focal-plane instruments, we built a simple 3-D model incorporating key components --- the magnet baffle (blue), the stovepipe and brim (green), the focus-structure baffle (yellow) and the limiting aperture of the HRC snout (red). The figure also shows the HRMA's exit aperture (magenta).

To inspect the location of tantalum shielding during an HRC pointed observation, as viewed from HRC-I focal plane, click on left image; from HRC-S focal plane, click on right image.

To inspect the location of tantalum shielding during an ACIS-S pointed observation (the more benign HRC Next-in-Line condition), as viewed from HRC-I focal plane, click on left image; from HRC-S focal plane, click on right image.

X-ray opacity maps

In order to evaluate the role of all materials in shielding the focal-plane instruments, we modified our detailed 3-D model incorporating nearly 500 components. To inspect the x-ray opacity maps, as viewed from HRC-I +Z vertex, click on left image; from HRC-S +Y,-Z vertex, click on right image.

Simulation of cosmic x-ray background

In order to evaluate the effect of the focus-structure tantalum on the contribution of the cosmic x-ray background, we ran a full-scale (10^9 photon) simulation, using our detailed (500 component) 3-D model. To view the results of the simulation for the HRC-I, click on left image; for HRC-S, click on right image.

Acknowledgements

MSFC/ Kurt Dietz, Ron Elsner, and Steve O'Dell are conducting this study. We thank SAO/ Steve Murray, Bill Podgorski, and John Polizotti, TRW/ Scott Texter, and MSFC/ Tony Lavoie for their discussions and assistance.

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