UV detectors in Space
John Vallerga
Space Sciences Laboratory
University of California, Berkeley
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
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)
0 (Aug 08)
1
+ still operating
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Anode Types (at SSL)
Cross Delayline
(XDL)
Cross Strip
(XS)
Medipix ASIC
Intensified CCD
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Design/Performance Flexibility
Input aperture
Window/door
Photocathode
KBr - CsI - CsTe - GaN
(QE, bandpass)
(xray to optical)
MCPs
d < 160mm, r > 7cm,
pore spacing > 3µm
(format, resolution)
Anode
(resolution, gain, rate)
Delayline (4 amps)
Cross Strip (128 amps)
Medipix ASIC (65k amps)
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
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
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
COS
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
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)
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
COS resolution
Resolution mask image (12 x 10 mm subsection )
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
COS spatial linearity
Residual to linear fit
After correction
Pixel size
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
COS local gain sag
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
COS QE
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
COS flat field
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
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
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
MCP Fixed pattern noise
COS flat field
16 x 10 mm
Optical tube flat field
25 mm
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Cross Strip Anode
Anode strips
Resolution < 8 µm FWHM
at gain of 500000
Count rates > MHz
8µm
Y amps
X amps (ASIC)
Charge cloud from MCP
Resolution mask,
5 µm pores
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Medipix/Timepix ASIC readout
Previous Pixel
• 256 x 256 array of 55 µm pixels
Shutter
Mask bit
Mux .
Clock out
• Integrates
counts,
not charge
Polarity
Lower Thre sh.
Disc.
Disc.
• 100 kHz/pxl
Input
log ic
Preamp
Mux .
• Disc.
Frame rate: 1 kHz
13 bit
coun ter –
Shift
Register
• Low
noise (100e-) = low gain
Mask bit
operation (10 ke-)
Next Pixel
Upper Thresh .
• GHz global count rate
Analog
Digital
• ~1 W watt/chip, abuttable
• Developed at CERN
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Original Medipix mode readout
256 x 256
Zoom
(14 mm)
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Timepix version of Medipix
Amplitude rather than counts
using “time over threshold’
technique
If charge clouds are large, can
determine centroid to subpixel accuracy
Tradeoff is count rate as event
collisions in frame destroy
centroid information
Single UV photon events
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Factor of 8 improved resolution!
256 x 256 converted to 4096x4096 pixels (3.4µm pixels)
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Factor of 8 improved resolution!
256 x 256 converted to 4096x4096 pixels (3.4µm pixels)
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Zoomed
5-6
5-6 pattern resolved = 57 lp/mm
Linewidth = 8.8 µm
The MCP pore spacing of 8µm limits further improvement
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Standard specifications
• Format size
• Spatial resolution
• Quantum Efficiency
• Linearity (integral and differential)
• Deadtime per event
• Maximum rate (global and local)
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Additional specifications
•
•
•
•
•
•
•
Pixel size (bits per event, X,Y,T,P etc)
Lifetime
Thermal stability
Fixed pattern distortions
Response uniformity
Livetime characteristics
Calibration accuracy
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
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
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
FUSE MCP gain loss
(Sahnow, SPIE 5488, 2004)
Total fluence map (through 2004)
Gain vs. fluence
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
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
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
COS Test Scrub
Post-Scrub Lifetest on COS sample plates (10-10-10µm)
Full gain. Nov. 24-28, 1999
Scrub Current
1 vs. Time
COS test plates (10-10-10µm)
Relative Gain
1
5
Scrub of COS Test plates (10-10-10µm)
10
Scrub gain of ~ 50,000 . Nov. 16-23, 1999
10 5
1.5
Relative Gain
0.01
1000
1000
0.001
100
Gain Sag
0.5
0.001
0.05
0
0
50
Time (hours)
100
0.1
0.15
Extracted Charge (C cm
150
0.2
100
)
-2
Gain Sag
0.0001
0
0.1
0.2
0.3
0.4
10
0.5
Extracted Charge (C cm-2)
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
)
0
-2
0.0001
10
0.25
)
0.01
10 4
104
-2
1
0.1
Gain Sag (%/C cm
Relative Gain
0.1
Gain sag (%/C cm
Current (µA)
2
Relative Gain
2.5
COS Flight Scrub
Initial Scrub
Post Rework
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Detector walk due to channel bias
Bias
Bias
Three different rear fields,
fixed gain
Three different gains,
fixed rear field
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
COS flat field
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Deadtime
Digital Event Counter (cps)
100000
FUV-01 Segment A
80000
60000
Non-paralyzable fit:
DEC = R / (1 + R*t)
FEC= R * exp(-R*330ns)
Best fit: t = 7.4µs
40000
20000
0
0
50000
100000
Fast Event Counter (cps)
FUSE
COS
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
150000
200000
Detector “Oddities” in Space
GALEX (sealed tube)
EUVE
Filter pinholes
Deadspot
The “smudge”
Current transients
FUV “Blob”
Current “anomaly”
FUV “flash”
FUSE
The “Worm”
Current spikes and event
bursts
Y blooming
Thermal drift
Walk correction
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
EUVE
Smudge
Cosmic rays saturating
charge amps
Deadspot
Best focus gain sag
Pinholes in Al/B filter
After launch, vibration?
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
FUSE
(Sahnow, SPIE 4854, 610,2003)
The “Worm” focus near QE grid
Current spikes and event bursts
associated with Space Weather
Y blooming
function of input count rate - anode
charging?
Thermal drift
poorly monitored
Walk distortion in X
set pre-launch for nominal gain
correctable in list (X,Y,P) mode
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
GALEX
(Morrissey, SPIE 6266, 2006)
Current transients
associated with space weather
sealed tube concern
FUV “Blob”
window charging
Current “anomaly”
MCP short that was annealed
FUV “flash”
significant breakdown event
that cured itself
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Lessons Learned
•
•
•
•
•
Start early
Build Prototypes
Stress Prototypes
Build Flight Spares
Include lots of “knobs” to adjust in orbit
– Hardware
– Firmware
– Software
jvv@ssl.berkeley.edu
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga