2.3.5 Calibration Sources

  ACIS external calibration sources will consist of three radioactive sources rigidly mounted to expose the entire focal plane from a distance of roughly 18 inches (46 cm), while ACIS is out of the HRMA imaging point. ACIS will not have a direct line of sight to the sources during ACIS observations.

Suitable sources have been provided by Isotope Products in convenient packages (the A-2, a 0.5" diameter cylinder, 0.25" high). All three sources will be composed of radioactive ⁵⁵Fe. One source will directly illuminate the focal plane with Mn K$\alpha$ and K$\beta$ X-rays from the ⁵⁵Fe. In each of the other two sources, X-rays from the ⁵⁵Fe will illuminate a metallic target. The source geometry is such that fluorescent characteristic X-rays from the target illuminate the focal plane. Viewed from the source, each CCD subtends $2.1 \times 10^{-3}$ ster. Details of the sources are shown in Table  2.6. Note that the output of each source will decay with a half-life of 2.7 years.

 

Serial No. ⁵⁵Fe Activity Target Principal Line Flux Other lines

mCi(date) Material (Energy) ct s^-1 CCD^-1 (keV)

(measured) (August 1998)

SN301 0.055 (8/96) none Mn K (5.9,6.4) 45 Mn L(.64-.67),Au M(2.2)

SN802 11 (5/97) Ti Ti K (4.5,4.9) 25(est) Mn K(6.4)

SN702 93 (5/97) Al Al K (1.5) 22(est) Mn K(6.4)

 

⁵⁵Fe is a highly compatible calibration source for the ACIS. It causes no CCD background problems and is routinely used in lab calibration. A 500 $\mu$Ci source will provide 160 cts/sec over a single CCD chip. The needed collimation will reduce this slightly. Tests were conducted at the MIT CCD lab by exposing chips to this flux level to search for long term radiation damage effects on the CCD chip. The total dose at the CCD was found to be less than 10 rads for the entire mission life. The low Z material collimator (aluminum) is sufficient to ensure that little fluorescence is caused by the source. (Low Z reduces the collimator fluorescent yield.)

Two other sources of X-ray calibration lines are available only early in the mission. A contamination monitor source for the HRMA is mounted on the front cover of the mirror assembly. This source will not be visible to ACIS after the sunshade (and front mirror cover) is opened during the Orbital Verification phase, within the first 30 days of on-orbit operations. ACIS will, however, make a measurement of this source in order to measure any mirror contamination up to that point.

The HRMA contamination sources consist of 16 separate sources (one on each of four mirror shells and one on each mirror quadrant). Each source will be a combination of ¹⁰⁹Cd and ⁵⁵Fe, producing the characteristic L lines of Ag (near 3.0 keV), K lines of Mn (near 5.9 and 6.5 keV) and L lines of Mn (near 0.64 keV). Spectra were collected by ACIS from this source during XRCF HRMA/ACIS calibration.

The ACIS Internal Contamination Monitor source is mounted on the interior surface of the detector assembly door. The primary purpose of the source is to check for gross contamination of the ACIS blocking filter and focal plane during initial on-orbit operations. The 10 $\mu$Ci ⁵⁵Fe source (measured in August, 1996) emits Mn L (645-670 eV) and Mn K (5.9 and 6.4 keV) X-rays in a fixed flux ratio (about 300:1, as detected with the ACIS UV optical blocking filter in place.) In order to minimize scattering of the emitted X-rays when the door is open, the ICM is equipped with a collimator. The collimator allows only a small fraction of detectors S2 and S3 to be illuminated when the door is closed. The ACIS door can in principle be closed to utilize this source, but the operational risk of failure to reopen is likely to prevent this use in any situation except for extreme emergencies.