K2 Summit and K2 Base Direct Detection Camera User's Guide
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Linear Accumulation Setup Mode
Linear mode is the traditional energy-integrating read out mode that captures the total
integrated signal level in each pixel just like a CCD camera does. Individual frames can be
summed for an effectively unlimited dynamic range.
In K2 Base and K2 Summit linear mode charge is collected, integrated during the exposure,
and read out to provide the image. Sensor electron charge proportional to the total energy
deposited by the incoming primary electron is accumulated. This mode benefits from Direct
Detection’s improved DQE arising from the transmission detector and inherent higher
conversion efficiency. When not counting, there is no requirement to keep charges separated.
This mode is good for higher dose images and allows for dose rates up to 400 times higher
than in Counted mode, and somewhat higher than is practical even in traditional CCD
cameras. Of course, if higher doses are used, the user must remain aware of the lifetime total
dose capability of the camera and avoid unnecessary exposure of the camera to the beam.
Radiation Hardness and Expected Lifetime
Minimize exposing the CMOS sensor to the electron beam when the camera is not in
use.
Based on the results to date, the lifetime of the K2 detector is estimated at 5 billion
e
-
/pixel. Considering exposure rates of 100 electrons/pixel/second, if the sensor were exposed
continuously for 24 hours a day every day, the expected lifetime would be well over 1 year.
The camera employs a pixel design that is 10 times more resistant to incident electron
damage as compared with other direct-detection sensors. The extremely short exposure time
coupled with the event discrimination of Counted mode confers an additional approximately
10 times immunity to radiation.
The sensor can be damaged and marked by a bright beam.
Use caution with intense
beams and bright spots. Do not leave the sensor exposed to the beam when changing
magnification levels or spot sizes. These can create brief intense spots. Annealing may be
able to reduce or remove minor beam spots.
Direct Detection cameras receive the incoming electrons directly to the imaging sensor, and
in the case of K2™ can detect them in several modes. This Direct Detection eliminates the
need for scintillator and fiber optics or lenses and greatly reduces Point Spread Function
(PSF). Furthermore, in Gatan’s Counted modes, noise can be essentially eliminated,
particularly because with Gatan’s transmission sensor, there are no scattered electrons re-
entering the detector from below.
