Next: Analysis Up: Absolute Calibration of Reference Previous: Absolute Calibration of Reference
Figure 4.45 illustrates the experimental set-up at the PTB laboratory. A standard MIT vacuum chamber, modified to hold two CCDs simultaneously was mounted to the PTB beamline via a ceramic electro-isolator to eliminate electrical interference between the CCD electronics and the BESSY facility. A gate valve and turbo pump located between the CCDs and the storage ring allowed the chamber to be connected and pumped down to the requisite vacuum without compromising the integrity of the storage ring. The CCDs were operated at the nominal flight temperature of -120 C.
Even with just a single electron in the storage ring, the incident photon flux would have caused significant pileup (defined as multiple photons landing in the same pixel or neighboring pixels during one integration time) in the CCDs and corrupted the absolute calibration. Two measures were taken to reduce the flux to an acceptable level. First, a chopper wheel with a 2.00 % transmission cycle was inserted into the beam line to reduce the incident flux. Second, only 256 rows of the CCD were read out. This readout mode reduced the integration time by a factor of four. The storage ring was usually operated with currents between 10 and 30 electrons, although measurements with as few as 5 electrons and as many as 50 electrons were performed to determine pileup effects.
The process of reading out 256 rows of the CCD limited the amount of the detector that could be calibrated during one measurement. To ensure that all the incident photons would fall on an active area of the detector (a necessary requirement for the determination of absolute quantum efficiency) a five mm high aperture was placed in the beam line and carefully centered on the electron orbital plane. The five mm slit produced an illumination pattern 208 pixels tall, and the CCD columns were nominally aligned perpendicular to the orbital plane. A two dimensional translation stage was incorporated into the section of the PTB beamline that the MIT chamber mounted to. To calibrate an entire chip, the chamber was moved an appropriate distance in the y direction, a 256-row swath of the CCD was read out, and the image was visually inspected to check that all the photons hit the active area. This procedure was repeated four or five times to calibrate the entire chip. The chamber was then moved in the horizontal direction to illuminate a second CCD inside the chamber. By placing two chips inside the chamber, the overhead associated with thermally cycling the CCD, venting the chamber, switching CCDs, re-evacuating the chamber and finally cooling the CCDs was eliminated. This configuration allowed calibration of as many as four chips in a single 48 hour user shift.
Given a well located electron beam, the synchrotron radiation from a
storage ring can be derived from Schwinger's equation [Riehle and Wende1986,Schwinger1949]:
W, e and m0 are the energy, charge and rest mass of the electrons and I is the current of the electrons in the storage ring. B is the magnetic induction of the bending magnets tangent to the observation point. a is the measure of the height of the beam, dSR is the distance from the beam to the observation point, and is the opening angle between the orbital plane and the observation point. c and are fundamental constants, and Kx is the modified Bessel function, order x, of the the second kind. Thus, the spectral photon flux can be expressed in terms of seven measurable quantities:
Similar measurements were also performed using the PTB Wavelength Shifter (WLS) beamline. Additional magnets are introduced into the normal storage ring configuration, thus boosting the energy of the electrons and shifting the energy of the synchrotron radiation. Figure 4.46 also shows how the WLS spectrum changes as a function of height above the orbital plane. Although the spectrum extends beyond 20 keV, the low high energy quantum efficiency of the devices limits the detection of photons to below 14 keV. The WLS experiments will be discussed in greater detail in Section 22.214.171.124.