Gertsch Group

Institute of Biochemistry and Molecular Medicine, University of Bern, Switzerland

Black or white - which way to catch the photons

Black or white - which way to catch the photons
Jurg Gertsch - Tue Nov 03, 2009 @ 07:42AM
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What a happy day in our lab - for weeks we have tried hard to increase the sensitivity of our TopCount plate reader to make it good enough to measure scintillation proximity in the 96-well format - we tried normal and black plates, fiddeled around and sweared. I phoned Perkin Elmer and before they wanted to give me an advice they asked for my details to be able to charge me - PE was NOT very helpful at all and I didn't want to pay for an advice, so we just continued and insisted. Peter came up with the final decision that the detector is damaged because the sensitivity remained bad regardless of the program and background settings used. We had used black plates because of background and because with fluorescence black works best. Moreover, PE says black plates are good. What a mistake! Then Martina Blunder from Austria told us that white plates are much better - and to all our surprise they are about 1000 times better - When a charged particle strikes the scintillator, a flash of light is produced, which may or may not be in the visible region of the spectrum. Each charged particle produces a flash of photons. The association of a scintillator (the fluid you add) and photomultiplier with the counter circuits forms the basis of the scintillation counter apparatus. When a charged particle passes through the phosphor, some of the phosphor's atoms get excited and emit photons. The intensity of the light flash depends on the energy of the charged particles. Cesium iodide (CsI) in crystalline form is used as the scintillator for the detection of protons and alpha particles; sodium iodide (NaI) containing a small amount of thallium is used as a scintillator for the detection of gamma waves. It seems that in the polysterene plates not only absorb the photons but are also quickly degraded by the scintillator, thus producing millions of micro or nanoparticles which float around and also absorb the photons - with the white plates it's the opposite - photons are reflected and the probability that a photons strikes the photomultiplier is dramatically increased - or, alternatively, the particles from the plates have an impact on the ionization process. Or again, it's all due to Compton scattering, smaller escape peaks at energies 0.511 and 1.022 MeV below the photopeak for the creation of electron-positron pairs when one or both annihilation photons escape, and a backscatter peak. To be honest, I don't know but empirically it now works - white catches the photons and black eats them all. Thanks Peter, thanks Martina.
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