Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Subject: Re: clarification (Mass spec)
Date: Aug 14, 0:40 am
Poster: Dave Cooper

On Aug 14, 0:40 am, Dave Cooper wrote:

>Without a lot of thought as to feasibility, I am wondering whether a CCD array or a dynamic ram, with the lid off, might not make a reasonable detector. Kind of depends on what the structure of the IC is, and whether there is a passivating layer, and how good the focussing of your filter is. I know there is some guy at JPL who is making very sensitive charge detectors in this way (using them to do things like weigh individual viruses, with each virus charged with 1 e- ), but his chips are custom made, and somwhat expensive, I should think.


I think you are right... the only problem I could see, would be the build up of the "detected" species on the CCD element.(Gases probably would still tend to adsorb..) The elements would have to be completely open.. and the likelkhood that SIO2 will form on the exposed silicon surface..even in a vacuum...is real.

We encounter the oxide formation on Si disk detectors used for ebeam detection in air. For us however it is beneficial since it gives several volts insulation against the local plasma. Was about to say.. even a monolayer might prevent charge detection, but...actually the capacitive current would still registered. Big question would be sensitivity, which is proportional to capacitance.

In the quadrupole MS.. the ion beam undulates in response to the AC field so it probably doesn't come out in a real well defined place. A beam director could work.. Actually there is a technique that involves post filter acceleration to add ion energy, up to a few kv's worth..which is used with very heavy ions which go sooo slowly that they can't kick off secondaries at the electron multiplier. The target site in multipliers is a spot a couple mm in diameter, so that might indicate how the ion beam is normally localized upon exit.

Using a CMOS device with exposed gate would also work, I think.. But...I think it needs to be reasonably large, so the beam finds it. The standard smoke alarm detector comes to mind...I believe the currents from the artificial radiaoactive source are in the sub pico amp ranges. The MOSFET detector is obviously very cheap...Maybe you could take out the source and stick the whole thing at the end of the quadrupole.for a poor man's detector.

There is a shop in Boulder CO, Silicon Sensors Inc.,that makes ultra thin PIN type photodiodes, among other sensors. Not sure if reversed biased jct at the threshold of avalanching.. could be pushed over by a few ions.. but it is a fascinating concept. Going to think about this whole topic a bit more. Thank you for the nice ideas!!

Dave C.