HBD Transmission Monitor: Update on Assembly & Testing

HBD Working Group Meeting 8/29/06B. AzmounBrookhaven National Lab

8/29/06 B. Azmoun, BNL

General Layout

Mirror SteppingMotor

Gas cells

Sample Chamber

Slits (Recip. Disp. = 40A/mm Band-pass = 40A)

Collimation Box

Light focusing Optic

D2 Lamp

McPherson 234/302VUV Spectrometer

Monochrometer

8/29/06 B. Azmoun, BNL

Exterior

Gas Cell Stabilizers (adjustable)

Gas In/Out ports

Lamp Monitor (PMT: CsI PC) Turbo Pump/Full Range

Vac Gauge (P = 4x10-5mbar)

Gas Cell PMT(CsI PC)

Electronics• 2 Picoammeters (ref, Sample)• Switch• Lamp Power Supply• 2 Stepping Motor Controllers (Monochrometer, Moveable Mirror)• Laptop PC (Labview Control panel)• RS232 – USB Controller

PMT Aperture

8/29/06 B. Azmoun, BNL

InteriorUncorrected Diffraction Grating: beam position doesn’t change w/ wavelength

Cylindrical Mirrors: light collection & collimation

Reference Beam Splitter: two mirrors spaced 5mm apart clipthe top and bot. edge of the beamand focus each onto Ref. PMT.

Moveable Mirror: As mirror is translated, the beam is successivelysent through each gas cell

The Central portion of the beam (sweet spot) continues through the

gas cells

ALIGNMENT (Grating @ zero-order)

• Monochromater• Light collimating Optics• Reference Mirror• Moveable Mirror• Gas Cell Stabilizer

Beam Size @ PMT aperture (19mm diam.)

8/29/06 B. Azmoun, BNL

Labview Control Panel

Select wavelength range Select wavelength step size Scan which gas cells Select number of data points for each current measurement Set path for file (tab delimited text, includes wavelength, mean

current, sigma)

8/29/06 B. Azmoun, BNL

PMT Current OutputPMT pe Raw Current Vs Wavelength

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

1100 1200 1300 1400 1500 1600 1700 1800 1900

Wavelength [Angstroms]

Cu

rren

t [n

A]

Lamp Mon.PMT

Gas Cell PMT

Lamp Monitor: Dark Current and Sigma Vs Wavelength

0

5

10

15

20

25

30

35

1100 1200 1300 1400 1500 1600 1700 1800 1900

Wavelength [Angstroms]

Sig

nal

/DC

0

200

400

600

800

1000

1200

1400

1600

1800

2000 Signal/DC

Signal/Sigma

Sig

nal

/Sig

ma

Gas Cell PMT: Dark Current and Sigma Vs Wavelength

0

2

4

6

8

10

12

14

16

18

20

1100 1200 1300 1400 1500 1600 1700 1800 1900

Wavelength [Angstroms]

Sig

nal

/DC

0

50

100

150

200

250

300

350

400

450 Signal/DC

Signal/Sigma

Sig

nal

/Sig

ma

• Reference and Sample Spectra have different shape?…possibly due to difference in reflectivity of mirrors and/or additional MgF2 window• Just discovered late last night: lamp grossly misaligned in its mount (removed for polish)…once aligned, we’ll get a much bigger signal!

8/29/06 B. Azmoun, BNL

Beam Ray Tracing (very similar to what is observed)

Output of Collimation Optics

@ Moveable Mirror

@ Exit Flange

@ PMT

After aperture

D2 Lamp

Monochrom.

Light Coll. Optics

8/29/06 B. Azmoun, BNL

Beam Astigmatism The divergence in the beam leads to a beam spot

that is slightly larger than the PMT aperture and/or the cell diam. (this is including the halo…the sweet spot is well contained, however it is preferable to eliminate all stray/peripheral rays and produce a well defined beam spot on the PC)

Due to the inherent astigmatism of the beam, any attempt to reduce the size of the beam results in significant attenuation (i.e. in attempting to eliminate peripheral rays only (@ the PMT), it is more likely to eliminate a significant number of central rays)

Weak solution: Vertical Slits placed right after the ref. mirror and Horizontal slits placed at the exit port to the cell provide some collimation, but there is still significant attenuation.

Better Solution: Place long collimator tube (with opaque surface) inside gas cell and eliminate all rays that hit the surface. By definition, all unwanted rays are eliminated, while all of the central rays make it through the aperture, and produce a well defined spot.

Were we too cautious???

Ratio: PMT Current w/ Collimator : PMT Current w/o Collimator

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1100 1200 1300 1400 1500 1600 1700 1800 1900

Wavelength [Angstroms]

Curr

ent w

/ Col

l./Cu

rren

t w/o

Col

l

8/29/06 B. Azmoun, BNL

Work to be Done PMT Protective “Fence”

After all, the PMT seal on the gas cell is contiguous with the interior of the HBD.

Must Perform real transmission measurement to test this seal, and perform a full dry run Finishing up plumbing for (in lab) gas system (Gas

in/out + Analysis) Labview program debugging almost complete Finally will install in Hutch (in ~2 weeks) …

Assuming all goes well