Operation of a single-photon–counting x-ray charge-coupled device camera
spectrometer in a petawatt environment
C. Stoeckl,
a)
W. Theobald, and T. C. Sangster
Laboratory for Laser Energetics, University of Rochester, 250 East River Road, Rochester, New York
14623-1299
M. H. Key, P. Patel, and B. B. Zhang
Lawrence Livermore National Laboratory, Livermore, California 94550
R. Clarke, S. Karsch, and P. Norreys
Rutherford Appleton Laboratory, Didcot, OX11 0QX Oxon, England
(Presented on 20 April 2004; published 5 October 2004)
The use of a single-photon–counting x-ray charge-coupled device (CCD) camera as an x-ray
spectrometer is a well-established technique in ultrashort-pulse laser experiments. In single-photon–
counting mode, the pixel value of each readout pixel is proportional to the energy deposited from
the incident x-ray photon. For photons below 100 keV, a significant fraction of the events deposits
all the energy in a single pixel. A histogram of the pixel readout values gives a good approximation
of the x-ray spectrum. This technique requires almost no alignment, but it is very sensitive to
signal-to-background issues, especially in a high-energy petawatt environment. Shielding the direct
line of sight to the target was not sufficient to obtain a high-quality spectrum, for the experiments
reported here the CCD camera had to be shielded from all sides with up to 10 cm of lead. © 2004
American Institute of Physics. [DOI: 10.1063/1.1788867]
I. INTRODUCTION
Single-photon–counting x-ray charge-coupled device
(CCD) spectrometers are frequently used in ultrashort-pulse
laser experiments, mostly for K-shell spectroscopy.
1–3
For
single-photon counting, the incident x-ray flux is attenuated
such that the probability that two x-ray photons hit a single
pixel is small. Consequently, the pixel value of each readout
pixel is proportional to the deposited energy from the inci-
dent x-ray photon. If the x-ray energy is not too high
͑Ͻ100 keV͒, a significant fraction of the x-ray photons de-
posit all their energy in one pixel. In this case, a histogram of
the pixel values provides a good approximation of the inci-
dent x-ray spectrum. This technique has the advantage of
requiring almost no alignment and the potential of providing
absolute x-ray flux information. Consequently single-
photon–counting x-ray CCD spectrometers are also used in
astronomical satellites,
4,5
where an extensive set of calibra-
tion and characterization data exists. For the satellite data,
the low number of incident photons is the biggest issue,
whereas in ultrashort-pulse laser experiments, photon counts
are generally very high. Signal-to-background issues, espe-
cially in a high-energy petawatt environment, become
dominant.
3
Shielding strategies against background x rays
must be carefully chosen to obtain high-quality spectra. In
this article, results from a recent experimental campaign at
the petawatt facility of the Rutherford Appleton Laboratory
(RAL) are presented showing successful strategies to im-
prove the signal-to-background ratio.
II. EXPERIMENTAL SETUP
The single-photon–counting x-ray spectrometer consists
of a Spectral Instruments Series 800 Camera using a 2k
ϫ2k-pixel, backthinned CCD chip with a pixel size of
13.5
m.
6
The CCD was cooled to −35°C to reduce the dark
current, and the images were recorded with 16-bit resolution.
The camera was mounted 3.8 m from the target outside
the target chamber on a 1 m vacuum tube. Mounting the
camera in air and using thin vacuum windows was not pos-
sible because the x rays of interest, Cu K-shell radiation at
ϳ8 keV, are strongly absorbed in air. The RAL petawatt
target chamber is very well shielded with 10 cm of lead on
three sides and on top. The side where the access doors are
located is unshielded but backed by a curtain shield of 10 cm
of lead and 60 cm of concrete (Fig. 1). The CCD camera was
shielded against x rays scattered from structures close to the
target with up to four lead collimators of 10 cm length inside
the target chamber and the vacuum tube (inner shielding).
The CCD camera housing was surrounded by up to 10 cm of
lead to shield against x rays from the sides and the back of
the CCD (outer shielding). A matched K-edge filter was used
to attenuate the K-shell signal to maintain single-photon
counting. Figure 2 shows the transmission of the 150
mof
Cu filter used for Cu K-shell spectroscopy. Compared to a
simple high-pass filter against the thermal radiation from the
target, a K-edge filter attenuates the spectrum above the lines
of interest, thus improving the signal-to-background ratio.
The targets were irradiated with 1053 nm pulses from
the RAL Vulcan petawatt laser, which delivers up to ϳ500 J
in ϳ1 ps in a 60 cm beam.
7,8
These pulses are focused with
a f /3 off-axis parabola to a focal spot of ϳ10
m full width
a)
Electronic mail: csto@lle.rochester.edu
REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 75, NUMBER 10 OCTOBER 2004
0034-6748/2004/75(10)/3705/3/$22.00 3705 © 2004 American Institute of Physics