HBD Transmission Monitor Update VIII: HBD Gas Transmittance +
Systematic ErrorHBD Working Group Meeting 1/23/06
B.Azmoun, S.StollBrookhaven National Lab
Part I: HBD Gas Transmittance
Transmittance: HBD Gascell1-input to HBD (vacuum)
cell/
mo
n /
ce
ll'/m
on
'
0.4
0.6
0.8
1.0
1.2
normalized to cell 1 pmt (vacuum)normalized to monitor (vacuum)preamps off (HBD-CF4)preamps on (HBD-CF4)
cell2- West HBD output
cell/
mo
n /
ce
ll'/m
on
'
0.4
0.6
0.8
1.0
1.2
1.4
cell3- East HBD output
wavelength (nm)
120 130 140 150 160 170 180
cell/
mo
n /
ce
ll'/m
on
'
0.4
0.6
0.8
1.0
1.2
1.4
Vacuum and Gas Scans from HBD Gas Transparency Monitor. 1/10
cell1-input to HBD
cell/
mo
n /
ce
ll'/m
on
'
0.4
0.6
0.8
1.0
1.2
fri / thurs (vacuum)fri / thurs (HBD N2)sat / thurs (HBD N2)
cell2- West HBD output
cell/
mo
n /
ce
ll'/m
on
'
0.4
0.6
0.8
1.0
1.2
cell3- East HBD output
wavelength (nm)
120 130 140 150 160 170 180
cell/
mo
n /
ce
ll'/m
on
'
0.4
0.6
0.8
1.0
1.2
Nitrogen Scans from HBD Gas Transparency Monitor. 1/18-1/20
PA ONH2O=17.5ppmPA OFFH2O=13.0ppm
40% loss
30% loss
Flat Baseline
PA ONH2O=8.5ppmPA ONH2O=8.0ppm
N2 abs. edge.?
20% Loss
N2 contribution to Loss?
CF4
N2
Expected H2O ppm’s based on %Light Loss
Transmittance in 36cm of Ar Vs PPM 's of H2O
0
10
20
30
40
50
60
70
80
90
100
110
1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
Wavelength [Angstroms]
% T
ran
smit
tan
ce [
%]
[O2]=0.43ppm [H2O]=9.8ppm
[O2] = 0.63ppm [H2O]=11.32ppm
[O2]=0.48ppm [H2O]=22.45ppm
[O2]=0.03ppm [H2O]=38.25ppm
[O2]=0.60ppm [H2O]=57.5ppm
[O2]=2.5ppm [H2O]=118.0ppm
[O2]=0.35ppm [H2O]=206.80ppm
[O2]=0.06ppm [H2O]=317.5ppm
HBD Gas PPM’s
% Light Loss H2O PPM (X-sect.)
H2O PPM (Hygro.)
20% (N2) 50 8.0
30% (CF4) 80 13.0
40% (CF4) 120 17.5
• 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
Ratio of spectra taken before and after lamp cleaning.(~ transmission spectra of lamp residue)
WAVELENGTH (NM)
120 130 140 150 160 170 180
ratio
0.0
0.1
0.2
0.3
0.4
0.5
0.6
monitor cell1
HBD Gas monitoring system- Monitoring pmt history (Lamp Intensity)
measurement # (~ lamp "on" time)
0 50 100 150 200
mo
nito
r p
mt
cu
rre
nt
(nA
)
0.0
0.5
1.0
1.5
2.0
2.5
HBD Gas monitoring system- Monitoring pmt history (Lamp Intensity)
measurement # (~ lamp "on" time)
100 120 140 160 180 200
mo
nito
r p
mt
cu
rre
nt
(nA
)
0.0
0.2
0.4
0.6
0.8
1.0
Lamp left on(2.5 days)
12/15
swap in lab system lamp
repolishlamp 1/12
repolishlamp 1/12
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 Slopes Similar Slopes
Propagation of VUV beam
Wavelength [Angstroms]
(Cel
l/M
on) /
(Cel
l’/M
on.’)
Cell PMT
Mon. PMT
Film on lamp window = Filter
Beam Splitter(Spatially separates Beam)
Beam X-sect.(Non-uniform)
Similar Slopes@ 160nm Ratio ~1.0
Dissimilar Slopes @ 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.