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/

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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/

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ce

ll'/m

on

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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

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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.