WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga UV detectors in Space John Vallerga Space Sciences Laboratory University of California,

WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga

UV detectors in Space

John Vallerga

Space Sciences Laboratory

University of California, Berkeley


200+ Detector Years in Orbit!Mission # of Detectors Years

EUVE 7 8.3

ALEXIS 6 12

SUMER (SOHO) 2 4

UVCS (SOHO) 2 12.0+

FUSE 2 7.9

IMAGE 2 5.7

ALICE (Rosetta) 1 3.7+

GALEX 2 4.7+

ALICE (New Horizons to Pluto) 1 1.8+

COS (Hubble) 1 0 (Aug 08)

+ still operating


Anode Types (at SSL)

Cross Delayline (XDL)

Cross Strip (XS)

Medipix ASICIntensified CCD


Design/Performance Flexibility

Input aperture Window/door

Photocathode KBr - CsI - CsTe - GaN(QE, bandpass) (xray to optical)

MCPs d < 160mm, r > 7cm,(format, resolution) pore spacing > 3µm

Anode Delayline (4 amps)(resolution, gain, rate) Cross Strip (128 amps)

Medipix ASIC (65k amps)


Case study of two UV spectrometer detectors

Cosmic Origins Spectrograph

“Hubble class” instrument

Large, reliable, stable, well calibrated and tested.

Extreme QA

ALICE on New Horizons

Pluto mission

Low power, mass, telemetry

“Moderate” resolution, calibration, testing, QA


COS


Case study of two UV spectrometer detectors (cont.)

Parameter COS ALICE

Size (mm) 178 x 10 38 x 20

Format (pxls) 32768 x 1024 1024 x 32

Resolution (µm) 25 80

Mass (kg) 33.4 0.66

Power (W) 37 1.1

Cost $$$$$$$$$$ $

Both have: curved MCPs, vacuum doors, CsI

photocathodes, 20 kHz ct rates (10% deadtime)


COS resolution

Resolution mask image (12 x 10 mm subsection )


COS spatial linearity

Residual to linear fit

Pixel size

After correction


COS local gain sag


COS QE


COS flat field


Improvements in MCP Detectors since COS

• MCP fixed pattern noise improved

• Electronics (Time to Digital Converters)– Faster (250 kHz at 10% deadtime)– FPGA logic– Lower power

• Readout technologies– Crossed Strip– Medipix and Timepix ASICs


MCP Fixed pattern noise

COS flat field 16 x 10 mm

Optical tube flat field 25 mm


Cross Strip Anode

Resolution mask, 5 µm pores

Anode strips

Y amps

X amps (ASIC)Charge cloud from MCP

Resolution < 8 µm FWHM at gain of 500000

Count rates > MHz

8µm


Medipix/Timepix ASIC readout

• 256 x 256 array of 55 µm pixels

• Integrates counts, not charge

• 100 kHz/pxl

• Frame rate: 1 kHz

• Low noise (100e-) = low gain operation (10 ke-)

• GHz global count rate

• ~1 W watt/chip, abuttable

• Developed at CERN

Input

Preamp

Disc.

Disc. logic Mux. 13 bit

counter –ShiftRegister

Clock out

Shutter

Lower Thresh.

Disc.

Mux.

Previous Pixel

Mask bit

Analog Digital

Upper Thresh.

Next Pixel

Mask bit

Polarity


Original Medipix mode readout

Zoom 256 x 256

(14 mm)


Timepix version of Medipix

Amplitude rather than counts using “time over threshold’ technique

If charge clouds are large, can determine centroid to sub-pixel accuracy

Tradeoff is count rate as event collisions in frame destroy centroid information

Single UV photon events


Factor of 8 improved resolution!

256 x 256 converted to 4096x4096 pixels (3.4µm pixels)


Factor of 8 improved resolution!

256 x 256 converted to 4096x4096 pixels (3.4µm pixels)


Zoomed

5-6 pattern resolved = 57 lp/mmLinewidth = 8.8 µm

5-6

The MCP pore spacing of 8µm limits further improvement


Standard specifications

• Format size

• Spatial resolution

• Quantum Efficiency

• Linearity (integral and differential)

• Deadtime per event

• Maximum rate (global and local)


Additional specifications

• Pixel size (bits per event, X,Y,T,P etc)• Lifetime• Thermal stability• Fixed pattern distortions• Response uniformity• Livetime characteristics• Calibration accuracy


Lifetime (MCP gain loss)

• High-fluence missions concern

• Gain loss can affect response due to threshold

• Can also affect imaging performance

• Somewhat ameliorated by HV increase

– Localized loss a problem


FUSE MCP gain loss(Sahnow, SPIE 5488, 2004)

Total fluence map (through 2004)

Gain vs. fluence


Scrub to stabilize gain loss

MCP gain initially decreases rapidly with extracted charge as adsorbed gasses are removed

Rate of gain loss vs. extracted charge eventually decreases

Scrub “end point” dependent upon expected fluence of misison

Exposure to gas at atmospheric pressures resets gain to initial higher levels - hence the need for sealed tubes or closable doors


0

0.5

1

1.5

2

2.5

0 50 100 150

Scrub Current vs. Time

COS test plates (10-10-10µm)

Current (µA)

Time (hours)

0.0001

0.001

0.01

0.1

1

10

100

1000

10

4

10

5

0 0.05 0.1 0.15 0.2 0.25

Scrub of COS Test plates (10-10-10µm)

Scrub gain of ~ 50,000 . Nov. 16-23, 1999

Relative GainGain Sag (%/C cm

-2

)

Extracted Charge (C cm

-2

)

Relative Gain

Gain Sag

0.0001

0.001

0.01

0.1

1

10

100

1000

10

4

10

5

0 0.1 0.2 0.3 0.4 0.5

Post-Scrub Lifetest on COS sample plates (10-10-10µm)

Full gain. Nov. 24-28, 1999

Relative Gain

Gain sag (%/C cm

-2

)

Extracted Charge (C cm

-2

)

Relative Gain

Gain Sag

COS Test Scrub


COS Flight Scrub

Initial Scrub Post Rework


Detector walk due to channel bias

Three different rear fields,fixed gain

BiasBias

Three different gains,fixed rear field


COS flat field


Deadtime

0

20000

40000

60000

80000

100000

0 50000 100000 150000 200000

Digital Event Counter (cps)

Fast Event Counter (cps)

Non-paralyzable fit:

DEC = R / (1 + R*t)

FEC= R * exp(-R*330ns)

Best fit: t = 7.4µs

FUV-01 Segment A

FUSE COS


Detector “Oddities” in Space

EUVE

Filter pinholesDeadspotThe “smudge”

FUSEThe “Worm”Current spikes and event burstsY bloomingThermal driftWalk correction

GALEX (sealed tube)Current transientsFUV “Blob”Current “anomaly”FUV “flash”


EUVE

Smudge

Cosmic rays saturating charge amps

DeadspotBest focus gain sag

Pinholes in Al/B filterAfter launch, vibration?


FUSE (Sahnow, SPIE 4854, 610,2003)

The “Worm” - focus near QE grid

Current spikes and event burstsassociated with Space Weather

Y bloomingfunction of input count rate - anode charging?

Thermal driftpoorly monitored

Walk distortion in Xset pre-launch for nominal gaincorrectable in list (X,Y,P) mode


GALEX (Morrissey, SPIE 6266, 2006)

Current transientsassociated with space weathersealed tube concern

FUV “Blob”window charging

Current “anomaly”MCP short that was annealed

FUV “flash”significant breakdown event that cured itself


Lessons Learned

• Start early• Build Prototypes• Stress Prototypes• Build Flight Spares• Include lots of “knobs” to adjust in orbit

– Hardware– Firmware– Software

[email protected]

Documents

WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga UV detectors in Space John Vallerga Space Sciences Laboratory University of California,