On Tuesday, July 19, 2022 at 11:02:22 PM UTC-7, bill....@ieee.org wrote:> Some of it is. The anode isn't and none of the dynodes are, and every > last one of them it is tied to a metal pin coming out of the base.Right, I'm talking about the metal frame that would be the other electrode in any ion-migration scenario involving the photocathode. You can see it in the photo at http://www.ke5fx.com/tube.png where the seller sacrificed one of the units for disassembly. That metal frame is at the same potential as the photocathode, so nothing is going to migrate across the faceplate. -- john, KE5FX
Low Level Gamma Radiation
Started by ●June 5, 2022
Reply by ●July 20, 20222022-07-20
Reply by ●July 20, 20222022-07-20
John Miles, KE5FX wrote:> On Monday, June 27, 2022 at 4:42:33 AM UTC-7, Phil Hobbs wrote: >> If you're going to use an ionic scintillator in contact with an end-on >> PMT (one where the photocathode is deposited directly on the faceplate) >> you'll need to keep the cathode near ground to avoid ions migrating >> through the glass and corroding the PC. That means running the anode at >> high voltage and coupling the pulses out with a cap or transformer or >> something. > > Phil, can you elaborate on this point? If the crystal and PMT are well- > insulated from their surroundings, which they are, what mechanism > would cause noticeable amounts of ion migration? > > It's definitely more convenient to run these with negative HV than it would be > with positive HV, where you not only have to worry about DC blocking but > also ripple. The latter seems to be a big deal, going by the Theremino > docs and other sources. > > -- john, KE5FX >It's just ion migration through the glass envelope. I don't know if folks have found a good solution to it, but BITD this was a serious limitation. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●July 20, 20222022-07-20
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:> John Miles, KE5FX wrote: >> On Monday, June 27, 2022 at 4:42:33 AM UTC-7, Phil Hobbs wrote: >>> If you're going to use an ionic scintillator in contact with an end-on >>> PMT (one where the photocathode is deposited directly on the >>> faceplate) you'll need to keep the cathode near ground to avoid ions >>> migrating through the glass and corroding the PC. That means running >>> the anode at high voltage and coupling the pulses out with a cap or >>> transformer or something. >> >> Phil, can you elaborate on this point? If the crystal and PMT are >> well- insulated from their surroundings, which they are, what mechanism >> would cause noticeable amounts of ion migration? >> >> It's definitely more convenient to run these with negative HV than it >> would be with positive HV, where you not only have to worry about DC >> blocking but also ripple. The latter seems to be a big deal, going by >> the Theremino docs and other sources. >> >> -- john, KE5FX >> > > It's just ion migration through the glass envelope. I don't know if > folks have found a good solution to it, but BITD this was a serious > limitation. > > Cheers > > Phil HobbsRH Electronics doesn't seem worried: https://www.rhelectronics.store/diy-pic18-mca-kit-for-gamma-spectroscopy Here they show the output of the PMT and the pulse stretcher: https://static.wixstatic.com/media/e43988 _a63520f4ed07436a843ddc1be0fda46a~mv2.jpg Here they show the pulses from random gamma rays: https://video.wixstatic.com/video/e43988_ 8006efb319884c72b9d7f7418abdfa97/720p/mp4/file.mp4 I guess it may take years for ion migration to show up. It probably depends on the type of glass, the thickness, the type of ion, any separating material, and the applied voltage. 800 Volts is not much. It should be easy to add a simple inverter and ground the cathode. Hamamatsu shows several types of cathodes: What is a photocathode? A thin photosensitive film formed on the inner side (vacuum) of the light input window. It converts light entering through the light input window into photoelectrons. - I didn't know that. Near infrared is 0.75 to 1.3 microns, so the file doesn't have to be very thick. Types of photocathode Bialkali photocathode for visible light region Multialkali photocathode with sensitivity extending to near infrared region Alkali-halide photocathode for UV light detection GaAs, InGaAs (group III-V compound semiconductors) with sensitivity from UV to near infrared region https://www.hamamatsu.com/us/en/product/optical-sensors/pmt/about_pmts.html -- MRM
Reply by ●July 20, 20222022-07-20
Mike Monett <spamme@not.com> wrote: [...]> It should be easy to add a simple inverter and ground the cathode.Oops - it is already grounded. -- MRM
Reply by ●July 20, 20222022-07-20
Mike Monett <spamme@not.com> wrote:> Mike Monett <spamme@not.com> wrote: > > [...] > >> It should be easy to add a simple inverter and ground the cathode. > > Oops - it is already grounded.And the positive anode should eliminate any ion migration through the glass. -- MRM
Reply by ●July 20, 20222022-07-20
On Wednesday, 20 July 2022 at 15:11:13 UTC+1, Mike Monett wrote:> Mike Monett <spa...@not.com> wrote: > > > Mike Monett <spa...@not.com> wrote: > > > > [...] > > > >> It should be easy to add a simple inverter and ground the cathode. > > > > Oops - it is already grounded. > And the positive anode should eliminate any ion migration through the glass. > >Well, there are both positive and negative ions in the crystal, so reversing the polarity would just change the nature of what migrates. John
Reply by ●July 20, 20222022-07-20
On Wed, 20 Jul 2022 07:45:08 -0700 (PDT), John Walliker <jrwalliker@gmail.com> wrote:>On Wednesday, 20 July 2022 at 15:11:13 UTC+1, Mike Monett wrote: >> Mike Monett <spa...@not.com> wrote: >> >> > Mike Monett <spa...@not.com> wrote: >> > >> > [...] >> > >> >> It should be easy to add a simple inverter and ground the cathode. >> > >> > Oops - it is already grounded. >> And the positive anode should eliminate any ion migration through the glass. >> >> >Well, there are both positive and negative ions in the crystal, so reversing the >polarity would just change the nature of what migrates. > >JohnMigration after 10 dynodes of gain is better than migration at the photocathode where every electron literally counts.
Reply by ●July 20, 20222022-07-20
Mike Monett wrote:> Mike Monett <spamme@not.com> wrote: > > [...] > >> It should be easy to add a simple inverter and ground the cathode. > > Oops - it is already grounded. >Yup. Grounded-cathode is the usual method with scintillators. You couple the pulses out with a capacitor, so it's not that big a deal. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●July 20, 20222022-07-20
Mike Monett wrote:> Mike Monett <spamme@not.com> wrote: > >> Mike Monett <spamme@not.com> wrote: >> >> [...] >> >>> It should be easy to add a simple inverter and ground the cathode. >> >> Oops - it is already grounded. > > And the positive anode should eliminate any ion migration through the glass. >The anode isn't vulnerable to corrosion because it's not deposited on the glass, and it's a nice beefy piece of metal, compared with the very thin transparent photocathode. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●July 20, 20222022-07-20
On Wednesday, July 20, 2022 at 12:51:46 PM UTC-7, Phil Hobbs wrote:> Yup. Grounded-cathode is the usual method with scintillators. You > couple the pulses out with a capacitor, so it's not that big a deal.I seriously do not understand this. With a grounded cathode, the signal you're extracting at the anode end is exposed to ripple from the PMT supply, without benefit of a multi-megohm divider chain. There's also the need to use a DC restorer of some sort to figure out where the baseline is. Both of these problems go away with a grounded anode. Seems like a no-brainer. -- john, KE5FX
