The CAST X-ray detectors

Three type of x-ray detectors have taken data in CAST during data taking periods: a Charge-Coupled Device (CCD), a Time Projection Chamber (TPC), and several MICRO pattern GAseous Structure (MICROMEGAS) detectors. Currently, four X-ray detectors are installed at both ends of the 10 m long magnet in order to search for photons from Primakoff conversion. Two Micromegas detectors [1] follow the sunset. They have replaced the formerly used TPC [2] and show a better performance in terms of background discrimination than the former detector. The sunrise is covered by another Micromegas detector and a combination of a X-ray mirror optics with a Charge Coupled Device (CCD) [3].

The x-ray telescope and the p-CCD detector

An x-ray telescope device is placed at the end of one of the magnet bores at the sunrise side focusing the expected signal in a small region of a pn-CCD chip specially designed to cover the x-ray energy range 1-10 keV. The delicate telescope system is kept in vacuum (10-5 mbar) to avoid contamination and any adsorption on its reflective mirror surface, which would result in a degradation of the telescope system efficiency. The system has additional gate valves which separate the magnet from the mirror optics, and the optics from the CCD detector to insulate the mirrors system atmosphere from the other systems in safety mode.

The x-ray mirror system

The Wolter I type x-ray mirror telescope installed in CAST is a prototype of the x-ray satellite mission ABRIXAS, which finished in 1999. It consists of a combination of 27 nested and gold coated parabolic and hyperbolic mirror shells with a focal length of 1600 nm. The maximum aperture of the outermost shell is 163 nm, while the smallest shell has a diameter of only 76 nm. The front side of the x-ray mirrors shell is divided into six sectors, from which only one is used given that the magnet bore diameter (43 mm) is smaller than the sector size. The telescope efficiency of each sector was characterized at PANTER facility, and the sector which showed the best performance was chosen.

The overall performance of the x-ray optics depends on the transmission efficiency and the spot size on the pn-CCD chip. The use of a telescope mirror system entails a loss in signal efficiency, which is counteracted by an significant increase of signal-to-background ratio by more than a factor 100 as signals are concentrated to a spot of 9 mm2 of an area of 1452 mm2.

The pn-CCD detector

A CCD detector is placed at the focal length of the x-ray optics. The pn-CCD detector for CAST is a prototype, 280 μm thick, developed for the ESA’s XMM-NEWTON mission. It has a sensitive area of 2.88 cm2 divided into 200×64 pixels, each

Micromegas detectors

During the first phase of CAST and 4He data-taking periods, a conventional Micromegas was used, covering one of the four magnet bore exits. Althought it was the only detector without any shielding, it showed the lowest background level of the three detectors: 5 x 10-5 s-1 keV-1 cm-2. This fact motivated the installation of a shielding in 2008, consisting of a 25 mm thick archeological lead layer and an external 15-20 cm thick polyethylene layer. Between these two parts, there is a cadmium foil to absorb the neutrons thermalized in the polyethylene. A 5 mm-thick copper layer is placed inside the lead which works also as Faraday cage. The whole setup is flushed with nitrogen in order to remove radon.

The new shielding reduced the background level by a factor ~3, reaching the level of ~1 x 10-5 s-1 keV-1 cm-2 during the data-taking period between April and September of 2008. At the end of 2008, two of the Micromegas detectors were replaced by two new microbulk [4] type models. These detectors are built with very low-background materials: kapton and copper for the readout and Plexiglas and aluminum for the chamber.

Figure 1

Left: The Micromegas detector used in CAST. The active area is situated at the front part and is covered with a stainless steel window and a Plexiglas piece. The strips are read by four Gassiplex cards situated at the rear part. Center: The circular lead shielding that surrounds the readout and the stainless steel tube that comes out from the magnet bore. Right: The Faraday cage and the external polyethylene shielding that covers the lead shielding. The electrical signals and the gas tubes are extracted from the copper box via feedthroughs.

The data acquisition system registers the analogue signal induced in the mesh for each event with a 1 GHz FADC and the integrated charge on each strip using four Gassiplex cards. The noise level of both signals is less than 1% for x-rays of 6 keV because the detector is operated at a gain around 5 x 103. In the analysis [5], several parameters are extracted like the cluster size, multiplicity and width from the strips and the risetime, width, amplitude and integral from the mesh pulses. Some of these variables are used in the analysis routines to discriminate x-rays from background events, considering the physics case. An x-ray of less than 10 keV produces a primary ionization localized in a spatial range less than 1 mm. The amplification of this charge gives a narrow pulse with a fixed risetime and a mean strip multiplicity which corresponds to around 5 mm. In contrast, cosmic muons and high energy gammas produce a spatially extended ionization, resulting to broader pulses and higher multiplicities.

For each calibration run (x-rays of the 55Fe source), a selection area containing the 95% of the events is generated for the following parameters: width and number of strips activated in each cluster, for the strips; risetime, pulse width and baseline fluctuation, for the mesh pulse. These areas are used as selection criteria in background runs for rejecting cosmic muons and high energy gammas. As an example, the selection area generated by calibration for the number of strips activated in each direction and the background events’ distribution is shown in figure \ref{fig:CASTcuts} (respectively center and right). In the same figure (left), the active area of the CAST Sunrise detector is also shown. Three other criteria are also used in the analysis: the number of clusters and the mesh pile-up (complementary to the other ones) and the baseline fluctuation (used for rejecting noisy events).

Left: Active surface of the CAST Sunrise detector, generated with the calibration events in 2010. The horizontal and vertical lines are the images of the drift frame. No event appears at the corners due to border effects of the detector. Center and right: Number of strips activated in each direction for X-rays generated by a $^{55}$Fe calibration source (center) and by background events (right) in the CAST Sunrise detector in 2010.

The raw trigger rate of Micromegas detectors in CAST is around 1 Hz. Most of the background events (mainly muons) are rejected in the offline analysis, remaining only those compatible with X-rays. As shown in figure \ref{CASTAnalysis} (left), the background level is reduced at least by one order of magnitude for all energies. Meanwhile, the signal efficiency is respectively 60 and 80% at 3 and 6 keV. This reduction in efficiency at low energies is due to noise effects, resulting in a detector energy threshold of 1.5 keV. The final background spectrum of the detector is shown in figure \ref{CASTAnalysis} (right). It mainly consists in three peaks at 3, 6 and 8 keV, generated by the fluorescence lines of the Micromegas’ copper, and the stainless steel of the window and the magnet’s bore.

Left: Background energy spectrum of the CAST Sunrise detector between 0 and 20 keV before (black line) and after the application of the selection criteria (blue line). Right: A zoom of the final background spectrum. The three peaks are generated by the fluorescence lines of the near materials: iron at 3 and 6 keV, copper at 8 keV.

2012: The update of the shielding of Sunset detectors

The main goal of Sunset upgrade was to reduce the background contribution of the 6 keV peak, originated in the vacuum pipes and the cathode. For this purpose, the lead thickness and the shielding closure was increased and steel was replaced by copper in the setup.

The copper innermost layer of the shielding was increased from 0.5 to 1 cm, to attenuate the natural radiation originated in the lead. Polyethylene was removed to leave space to the extension of the lead shielding up to 10 cm. Moreover many lead bricks were machined to assure a better enclosure than in previous setup, mainly at the region where the electronics is plugged and around the connection to cold bores. Apart from that, the cathode, the gas connections and the pipe were replaced by new copper components. The inner of the last piece was also coated with Teflon to suppress the copper fluorescence.