The Cyclotron and the Irradiation Setup
The machine accelerates H-. The two electrons are stripped at a foil, which provides the first measurement of the (double) beam current. The last beam stop is just 20cm before the exit window which is a 7µm Havar® foil. 50cm from the exit window we place a thermally and electrically insulated box, which holds the samples. Since the beam spot has a diameter of only 4-8mm we have to scan the samples. Therefore the box is mounted on a controlled XY-stage.
The objects (diodes, mini sensors, small electronic devices,...) are fixed on an aluminium frame by Kapton® tape. The frame can be slid in a thermally insulated box with a graphite absorber at the back. The front window is made out of a GFK frame with two thin Kapton® foils.
The following table summarises the main parameters of the installation:
|Proton Energy||~23 MeV (25.3MeV at extraction)|
|Proton Current||~2µA (100nA - 20µA)|
|Max. Object Width||44cm|
|Max. Object Height||17cm|
Because of heating effects we restrict the proton current to 2µA. E.g., irradiating one sensor of 20mm x 20mm to 5x1015n1MeV/cm2 takes about 90 minutes.
Fluence Estimation and Measurement
To estimate the fluence for given irradiation parameters we use this formula (from IEKP-KA/2003-23 ):
with the number of scans n, the proton beam current I, the electron charge qel, the horizontal scan velocity vx and the step width in vertical direction Δz.
To minimise heating effects and to get a nice multiplier we choose the default scan speed vx to be 115 mm/s. The step size Δz is in general 1 mm. Only for irradiations with very small flux, for which we use a collimator producing a beam spot with a diameter of 1 mm, the step size is reduced to 0.2 mm. So, in general the formula reduces to the handy form:
with I in μA, vx in mm/s and N as number of double scans, which means scanning down and up (this is the number used in the control program). I.e., for a scan speed of 115 mm/s we get a fluence of 0.2 1014n1MeV/cm2 per double scan. We take the current on the last beam stop as the beam current. For our 23MeV protons we suggest a hardness factor of 2.0 (see talk at 16. RD50 workshop).
Due to the scanning procedure we have to avoid that the increased fluence at the turning points hit the structure. Therefore the area to be scanned is larger than the structure (12.5mm on both sides and 10mm to the top and bottom).
Post-irradiation dosimetry is done by Ni57-activity measurement in Ni-foils being attached to the structures during irradiation. Here we rely on a Germanium detector system provided by ZAG.