TIM: The Transmission Intensity Monitor

The transmission intensity monitor (TIM) is used to detect the intensity after the sample. It is used either for normalization or to measure x-ray absorption scans.

The TIM detects primarily the x-ray fluorescence coming out a CVD diamond film or a YAG crystal that is used as active instrument beam stop. For low intensities, the fluorescence detection could change into photoelectron detection. Furthermore, for particular experiments with very intense beams when the IBS would be damaged by each shot resulting in a non-linear fluorescence intensity measurement, a gas will be used as source of fluorescence or photoelectron. The measuring principle is displayed in tim_scheme below. The x-ray beam with sizes varying between the blue and green patterns impinges in the diamond film at 45° and ejects both electrons and fluorescence photons that can be detected with the MCP.

TIM measurement scheme using a diamond film or YAG mounted at 45°.

The detection system is a MCP and its charge is integrated, amplified, digitized and measured with a micro-TCA. The possibility of detecting both fluorescence and electrons can be realized by means of the potentials to apply to the MCP and anode. Alternativly a grid can be put in front of the MCP to repel charged particles in the fluorescence detection mode. This allows using always the same MCP electric configuration.

Several MCPs can be installed in the chamber, as well as imagers to detect the beam on the diamond or YAG screen. The MCPs and imagers look at the screen at different angles, as shown in tim_chamber.

A 3D drawing of the vacuum chamber.

The CVD and the YAG are mounted on a translation stage that allows to move them in and out and change from one to the other. At the exit port of the chamber, a B₄C beam stop is installed.

• MPOD MCPs
  • Ramping up:
  • Ramping down:
  • General things:

Interlocks

The roughing vacuum is shared with the FCCD, so we better do not open the roughing vacuum while they are pumping down.

No	Property	Op	Value
c1	SCS_CDIDET_FCCD2M/GAUGE/P9.Value	GT	1e-2
c2	SCS_CDIDET_FCCD2M/VALVE/SRV2.state.IsClosed	EQ	FALSE
c3	SCS_BLD_TIM/TPUMP/TP1.state.RPM80	EQ	FALSE

No	Logic	Action
a1	c1 or (c2 and c3)	CClose

No	Logic	Action
a1	c1 or (c2 and c3)	COff

Like always, if the turbo pumps fail, their gate valve should close. Unless, well, the vacuum is really bad and the pump is not running then we probably want to pump down, as described for the DPS.

No	Property	Op	Value	Hyst	Timeout
c1	SCS_BLD_TIM/TPUMP/TP1.state.RPM80	EQ	FALSE
c2	SCS_BLD_TIM/GAUGE/PCC22	LT	1e-4	1	100000

No	Logic	Action
a1	c1 and c2	CClose