Humidity control update

The new closet approach works very well. I topped up the humidifier on Thursday, and on Monday afternoon, there was plenty of water left in the humidifier. One contributing factor is the relatively warm weather over the weekend. If this trend continues, we will likely only need to top up the water twice per week, until the summer months.

Starcheck uniformity experiment

Back in January, we mounted Starcheck on a Velmex linear stage, again, to measure the response uniformity of all the detectors in a rigorous way.  By moving the profiler 9mm per step and taking a 300MU profile each step, we tie 22 detectors directly to the central chamber.  By measuring profilers at 3mm and 6mm offset, we managed to tie remaining two groups of 22 detectors to the central chamber, in an indirect way.  Note that the starcheck central chamber has a 6mm gap between it and its neighbours, rather than the 3mm spacing for other chambers.  This was a cause of grief in analysis, as we can't simply multiply the detector number by 3 to get the chamber position.  

Looking at the data, what we see is that all three groups follow the same shape, suggesting a systematic bias.  For now, I speculate that the bias is due to beam quality modification by the wedge.  Since we already know that the Starcheck central chamber show strong energy dependence, it is not surprising that we observe beam quality effect due to beam softening by the wedge.  Note that the detector at -78mm is an obvious outlier, showing a 2% deviation from the central chamber.  Given the 1%, 2SD uncertainty quoted by the manufacturer, this is possible.  Further characterization may be needed before any conclusion about this detector can be made.  The three colors represent the three groups of detectors, offset from each group by 3mm.  The red dots represent detectors tied directly to the central chamber, while the other two colors represent detectors tied through the 3mm and 6mm measurements.

New data on Jan 23rd Vickers experiment

A significant discrepancy between Al chamber measurement obtained by Claudiu and Al film measurement obtained in October was observed. Adding Monte-carlo data into the mix, I found an unusually large discrepancy between MC and chamber measurement. If we add Jan 23rd data of Aluminum into the mix, which we know is 2 sheets of aluminum(#32 and #33), the profile is very close to Monte-Carlo simulation. The question now is, what is the foil used in October's Vickers experiment?

February 07 update:  The foil used in October's Vickers experiment is the same as Jan 23rd.  The results from October seem to be 0.5% narrower than Jan 23rd film measurement, which can be due to different distance between exit window and film.  Also note that the green circle just shows the excellent circular symmetry of electron beam, allowing easy contouring and circular fitting in Matlab.  I'll try to submit a Vicker X-ray contour soon to compare.

Starcheck detector response

With the Friday Morning Talk out of the way, we can now focus on other stuff, such as Starcheck response characterization.  Previously we compared non-rotated and rotated Starcheck profiles to get a feeling of uniformity.  Now we put it in a more rigorous test, by scanning a dose wedge with Starcheck.  Mounting starcheck on a Velmex linear stage, we move the profiler 9mm between successive scans.  The result is 23 scans, which can be used to link detectors on a line to central detector, as they are measuring the same dose.  The data points are 3 detectors apart, except for the central detector, which is 6mm away from its neighbours.  This exception actually caused me fair bit of grief in trying to figure out what's wrong with the parsing code.  After adjusting the code, we have a comparison of 22 detectors with the central detector.  Note that a systematic trend can be observed, with the edge detectors reporting lower dose than central detectors.  This may be attributed to the fact that for edge detectors, when they are at the beam axis, the central detector is at the edge of field.  Since Starcheck uses only the central detector for absolute dose measurement, a change in beam quality as we move across the wedge profile would cause a change in charge to dose conversion coefficient.  With that in mind, we note that detectors in the central 10cm are within 1%, which is within the manufacturer's specification.

EBT film monitoring

After much procrastination, I finally got to the monitoring of EBT films. A TinyTag logger, the one used in the room next to Malcolm's office, was put inside the cardboard box of film cutter, along with two sheets of film. The two films were from a good batch, not the most recent batch that contains two boxes. The box was put into a

Gold foil and A16 measurements-finally some agreement.

This time around, with the beam structure resolved, we can finally begin to compare A16 measurements with film measurements. By applying 1% expansion correction to film, centering the A16 profile, and correcting the A16 profile for monitoring chamber effect and chamber size effect, we obtained a perfect match. Of course, the drawback of such approach is that we could be attempting to obtain a match by applying corrections. The monitoring chamber correction applied to A16 profiles is derived from window only profile, and may not be applicable. The size correction is also derived from window only profile, but we have more reasons to use that correction. Given the 1mm size of film pixel, we can approximate the film as a point detector, and the A16's relatively large detector size requires a -0.6% correction in X-axis. With all these corrections applied, and A16 centered, the gold profile matches perfectly, except for film non-uniformity issues near the peak.

New Vickers Electron work, gold foil #1

New experiment performed on Gold foil was scanned today. Gold foil #1, 30 micron thick, was film 27B. The curves look symmetric, without major features. A look at lateral and longitudinal profiles suggest that no difference between the two profiles can be detected beyond the uniformity level of the film. In this experiment, we performed a repeat film measurement without moving the gold foil, thus allowing the beam to hit the same spot of the foil. The results of the repeat confirmed the idea that differences between the two profiles are purely due to film non-uniformity.