# Presentation: 687161

## HBD Transmission MonitorUpdate VII:  Systematics & HBD CF4

- B.Azmoun, S.Stoll
- Brookhaven National Lab

**HBD Working Group Meeting: Jan. 9, 2007**

## Set up

- Fixed “pick-off”
-  mirrors

- moveable mirror

- 1

- Gas cell

- Monitor
- pmt

- windows

- Cell pmt

- 2

- 3

- Beam
-  X-sect.

## I.  Systematics:  Stability of Instrument 

-  Back to back Vac Scans should exhibit a double ratio of 1.0, however a
- systematic deviation from 1.0 is observed for almost all scans.

## Reproducibility of back to back Vac Scans

-  Each Trans. curve shows the same characteristic shape, which is reproducible using two completely different data sets.
-  This characteristic shape is in “phase” with the scan, either wrt time or wavelength.
-  The amplitude of this structure is modulated in subsequent scans (randomly?).

## Double Ratio Decomposed

-  These ratios do not perfectly cancel out the wavelength
- dependent structure
-  Using the Lamp Mon. in the trans calc. yields better results
- than simply taking a direct ratio of cell currents.

- T =

- T =

- or

## Systematic Checks: Dark Current Subtraction

-  DC is ~ Const.
-  The small changes in the baseline (DC) as measured before and after
- the scan can only account for a fraction of a percent deviation from 1.0
- in the double ratio.

- Baseline
- (Dark Current)

## Systematic Checks: Reproducibility of mirror and Grating Position 

-  At 160nm, deviations up to 5 -10% have been observed away from
- 1.0, however, we cannot reproduce such results here by simply moving
- the moveable mirror or by rotating the grating.

## Systematic Checks: Constancy of PMT Current (double ratio @ fixed wavelength Vs time)

-  The lamp and/or the pmt current appear to be constant to within 2% at
- a given wavelength.
-  Interestingly, we do not see the characteristic structure here (seen in
- previous trans. curves), which implies that this structure is not correlated
- with time.

## Effect of Misaligning the Moving Mirror

-  Slightly misaligning the moveable mirror seems to exacerbate the “characteristic structure”, and also seems to shift it in wavelength.
-  As part of the alignment process, the mirror position is finely tuned (usually @ 160nm) by translating the moving mirror back and forth slightly until the pmt current is maximized.  (The mirror position is then fixed for all scans). Interestingly, the current in each pmt is maximized at different mirror positions for different wavelengths  chromatic aberration/astigmatism.

## Keeping Lamp ON Vs Cycling power to Lamp

- Although the scans with the lamp left on continuously exhibit smaller deviations from 1.0, than when the lamp power is cycled on/off, the results are not significantly different.

## Lamp Intensity Vs Time

- Intensity drops at more moderate rate compared to initial measurements: 0.5%/scan

## Summary: Instrument Stability

- The instrument appears to suffer from a significant (systematic) instability. The following appear not to be responsible:
- Mirror/Grating Position
- Baseline Current (dark Current + Noise)
- Cycling power to the lamp
- Observations
- Instability is in phase with each scan (likely wrt to wavelength)
- The amplitude of the deviations change in subsequent scans.
- Optimizing the moveable mirror position at different wavelengths appears to impact the systematic trend away from 1.0.
- Eliminating the Mon. in the double ratio results in further deviations from 1.0
- Possible Causes for instability: relationship between Mon. and Cell changes from scan to scan
- The spatial distribution of the beam (i.e., the beam shape) is changing with wavelength from scan to scan in a reproducible way when a scan is started
- Fraction of beam impinging on the Cell and/or Mon. pmt aperture changes from scan to scan in a reproducible way when a scan is started.
- Possible Solution: Replace “clipping” mirror with slow “chopper” mirror, such that the Mon. and the cell see the same exact beam cross sect. (modulo beam divergence).

## HBD CF4 Transmittance (Finally!)

-  Although the transmittance curve is convoluted with the funny wavelength dep. structure, it clearly exhibits about ~20% loss of light @ 170nm in the HBD output gas, compared to bypass gas.  This is reproducible for both cells containing the output gas from the HBD, but the input gas looks the same as previous scans, as expected.
-  HBD PA’s turned on 2/3 of the way into the scan.

- H2O = 13.7ppm
- O2 = 6.1ppm

- H2O = 12.6ppm
- O2 = 8.5ppm

- H2O = 1.0ppm
- O2 = 0.6ppm

## H2O/O2 Log

- Scan Start/Stop

- PA’s ON

## PPM’s @ 80% Trans. @ 170nm 

- T(170nm) = exp(-****H2O170nmN(T=20C, P=atm.)*L*ppm) =
- = exp(-3.37E-18cm2*2.59E19part./cm3 * 50cm*ppm) ~ 80%
-  ppm = 51.1ppm H20  Discrepancy wrt Hygrometers (reading ~13ppm) ???

- Can almost see water absorption bands in last two plots of previous slide (peaking at ~125nm & 170nm)

## Back Up Slides

## Earlier Results in Lab