Frame Store pnCCDs for Basic and Applied Science

Future missions and other applications require pnCCDs (see Fig. 7.14) with smaller pixels and even faster readout. Two potential applications are the German/Russian eROSITA mission and ESA's XEUS mission. The eROSITA mission shall be launched at the beginning of the next decade, and the XEUS satellite around 2020.

As in conventional CCDs, pnCCDs equally can be designed in a frame store format. This optimizes the ratio of exposure to transfer time, but requires more space on a chip because the store area does not serve as active area but as an analog storage region (Fig. 7.15).

Fig. 7.14 The tested prototype of frame store pnCCD has a format of 256 x 256 pixels with a size of 75 x 75 |im2 in the image area and 256 x 256 pixels with a size of 75 x 51 |im2 in the frame store. The number of pixels will be increased for the eROSITA flight devices to 384 x 384 in image area and frame store each

Table 7.1 Properties of CCD detectors used in current and future X-ray missions

Property

XMM-Newton

eROSITA

XEUS

Status

Operating

Prototyping

Research

Füll frame

Frame store

Frame store pnCCD or

Type

Active Pixel Sensor

pnCCD

pnCCD

(DEPFET APS)

Format

400 x 384

384 x 384

1024 x1024

Pixel size (|im2)

150x150

75 x 75

50 x 50 or 75 x 75

Readout noise

5 electrons

2 electrons

2 electrons

Sensitive thickness (|im)

295

450

450

Frame rate (fr. s-1)

14

20-1 000

200-1000

Output nodes per CCD

12

3

32

FWHM(Mn-Ka) (eV)

150

130

125

FWHM(O-K) (eV)

90

60

50

Energy range (keV)

0.3-15

0.2-20

0.1-20

The energy resolution (FWHM) refers to incident X-rays of the Mn-Ka line at 5.9 keV and O-Ka at 525 eV measured at temperatures around -60° C

For the pnCCD aboard the XMM-Newton satellite, the signals of one row (64 pixels) are processed in parallel in 23 |is. The extension to 128 channels on the CAMEX amplifiers, to match the smaller pixel pitch, was already realized for first prototype devices. In addition, the signal processing time must be shortened by a factor of two to obtain the same readout time per row. The increased readout speed will certainly have an impact on the power consumption, which is actually below 1W for the 36 cm2 large image area.

Row Row

Row Row

0 20 4D Column

Fig. 7.15 Operation of a frame store pnCCD in fullframe mode (left figure) and in frame store mode with 125 rows for the image area and the other 125 rows as frame store (right figure). A mask with three different pinhole sizes was mounted in front of the detector. The same frame rate (20 images per second) and the same Al-K X-ray flux were used for both measurements. The out-of-time events appear outside the illuminated spots distributed all over the transfer channels. The fast transfer of the image into the frame store (frame store mode) instead of transfer and read out row by row as performed in the full frame mode reduces the out-of-time event occurrence by a factor of 40 to a value of 0.35% [12]

0 20 4D Column

Fig. 7.15 Operation of a frame store pnCCD in fullframe mode (left figure) and in frame store mode with 125 rows for the image area and the other 125 rows as frame store (right figure). A mask with three different pinhole sizes was mounted in front of the detector. The same frame rate (20 images per second) and the same Al-K X-ray flux were used for both measurements. The out-of-time events appear outside the illuminated spots distributed all over the transfer channels. The fast transfer of the image into the frame store (frame store mode) instead of transfer and read out row by row as performed in the full frame mode reduces the out-of-time event occurrence by a factor of 40 to a value of 0.35% [12]

If the CCD is read out with 12.8 MHz, 10 |is would be required for the parallel readout of one pixel line comprising 128 channels. For the parallel transfer from the image to the storage area, 100 ns are needed per line transfer. A device of 1000 x 1000 pixels would be divided (as in the XMM-EPIC case) in two identical halves of the image area, i.e., 500 x 1000 pixels each. For the parallel 500 shifts thus 50 |s would be needed for the entire transfer from the image to the shielded storage area. The readout time for the storage area while integrating X-rays in the image part would then be 500 x 10 |is = 5 ms. That means, within 5 ms the whole focal plane would be read out. The out-of-time probability for X-ray events will then be 1:100. In this operation mode, 200 image frames can be taken in 1 s with a full frame time resolution of 5 ms. The key parameters of future X-ray detector systems are summarized in Table 7.1.

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