HBD Transmission Monitor Update VIII: HBD Gas Transmittance + Systematic Error HBD Working Group Meeting 1/23/06 B.Azmoun, S.Stoll Brookhaven National Lab

Part I: HBD Gas Transmittance

Transmittance: HBD Gas PA ON  H2O=17.5ppm PA OFF  H2O=13.0ppm 40% loss 30% loss Flat Baseline PA ON  H2O=8.5ppm PA ON  H2O=8.0ppm N2 abs. edge.? 20% Loss N2 contribution to Loss? CF4 N2

Expected H2O ppm’s based on %Light Loss

HBD Gas PPM’s % Light LossH2O PPM (X-sect.) H2O PPM (Hygro.) 20% (N2) % (CF4) % (CF4) Absorption of light depicted in transmittance curve may be due to the presence of an additional absorbent substance other than water (a bit far fetched, considering the shape of the trans. curve looks like water alone) Discrepancy btwn Panametrics Hygrometer and X-sect Calc.:

PPM’s Vs pe Loss

PartII: Instrument Stability

D2 Lamp Window Haze Appearance of Film over Time

Lamp Aging Intensity drop due to film build-up Relative Transmittance of Film

Instrument Stability Earlier we thought of the loss in lamp intensity over time as an ancillary problem. Now we believe that it is directly responsible for the observed “instability” in the baseline of our transmittance measurements. The build-up of the hazy film on the lamp window behaves like a spatial filter, a time dependent filter, and a wavelength dependent filter. This complex filter introduces instability in our baseline measurements because the Mon. and Cell currents do not diminish at the same rate. Dissimilar SlopesSimilar Slopes

Propagation of VUV beam Wavelength [Angstroms] (Cell/Mon) / (Cell’/Mon.’) Cell PMT Mon. PMT Film on lamp window = Filter Beam Splitter (Spatially separates Beam) Beam X-sect. (Non-uniform) Similar 160nm  Ratio ~1.0 Dissimilar 135nm  Ratio  1.0

Conclusions Constancy in intensity drop (slope) at each wavelength leads to the reproducible bipolar shape (systematic error) we’ve been observing. The longer we wait between successive scans, the more the two beams “walk” away from each other, leading to a larger shift in the baseline. Go for quality, not quantity. If we spend more time taking a Vac scan for every gas scan, we have a much better chance for producing a quality transmittance measurement. As pointed out last time, we can avoid these systematic errors if we replace the present beam splitter with a “slow chopper”. While we’re at it, we should also replace the planar moving mirror with a focusing mirror in order to reduce beam divergence.