1. VERITAS Upgrade Overview Jamie Holder (Delaware)
Rene Ong (UCLA)
Jamie Holder (Delaware)
VERITAS Operations Review, Tucson, December, 2009

2. VERITAS Status In summer 2009, VERITAS T1 was relocated.
A new mirror alignment system (McGill) also significantly improved the optical performance.
VERITAS is currently the most sensitive TeV Observatory in the world.

3. The Competition H.E.S.S Phase II MAGIC II HESS Phase II:
A single large (600m2 mirror area) telescope to be located at the center of the array.
Camera is complete.
Telescope structure still under construction in Namibia.
MAGIC II
MAGIC II:
MAGIC have installed a second 17m diameter telescope 80m from MAGIC I.
Stereo observations began this summer.
No report on sensitivity yet.

4. VERITAS Upgrade Overview
Many options were considered over a series of meetings
Key goals:
reduce energy threshold and improve sensitivity.
These goals impact essentially all science programs, and can be most easily achieved at the current site by:
Increasing the photon collection efficiency
Upgrading the trigger
Additionally, we plan to add a unique scientific capability, at low cost, by instrumenting the central pixel of each telescope with fast readout and continuous digitization.

5. Why Improve Sensitivity?
For a pointed instrument such as VERITAS, with a limited duty cycle, even a modest sensitivity improvement is worthwhile. For example:
M82 “Nature” result required 2 seasons, and 150 hours
The new configuration reduces this to ~90 hours
The proposed upgrade reduces this to ~60 hours: easily achievable in a single season.
The VERITAS Catalog now contains 27 sources.
Many require follow-up observations.
New sources are being added (>1/month since the summer).
VERITAS time is precious!
VERITAS Catalog

6. Why Lower the Threshold?
Lowering the energy thresholds impacts many areas of VERITAS science:
Complete broadband source spectra with Fermi.
Increased energy range for dark matter searches.
Energy dependent morphology studies of extended sources.
The gamma-ray horizon is not as restrictive as once thought - VERITAS continues to push the distance record (3C66A, 1ES ).
A lower threshold accesses more distant sources, and provides a more sensitive probe of the EBL.
Fermi shows that delayed high energy emission from GRBs is common – up to ksecs.
1ES
z = 0.341
GRB C

7. Increasing the photon collection Efficiency: High QE PMTs
‘Super Bialkali’ PMTs offer 50% improved photon collection efficiency.
Equivalent to increasing the mirror area by the same factor.
Dramatic improvement in the low-energy response.
Sensitivity improvement also expected (~30% reduction in the observing time required to detect a source).

8. High QE PMTs Preliminary Purdue results
Three potential Hamamatsu replacement tubes identified:
R HQE: Used for MAGIC II. Low gain (6 stages).
R : Best match in physical dimensions and gain (8 stages).
R : UV glass version of the R9800.
Samples of R10408 and R9800 are in hand. Testing is underway at Purdue and Wash U. One R10408 pixel also installed in T4.
Preliminary Purdue results
R HQE

9. High QE PMTs Preliminary Wash U results
Three potential Hamamatsu replacement tubes identified:
R HQE: Used for MAGIC II. Low gain (6 stages).
R : Best match in physical dimensions and gain (8 stages)
R : UV glass version of the R9800
Samples of R10408 and R9800 are in hand. Testing is underway at Purdue and Wash U. One R10408 pixel also installed in T4.
Preliminary Wash U results
R HQE

10. High QE PMTs Practicalities: PMT cost ~ $1.2M
Delivery timescales 12 months (R10408) – 18 months (R10560)
Also requires
New pre-amplifiers
Pixel assembly
Camera integration
New light cones
Total cost ~$2.0M

11. Trigger Upgrade Noise Triggers Individual telescopes
Each PMT signal channel has programmable Discriminators (CFDs)
CFD output sent to programmable pattern selection trigger (L2)
any 3 adjacentPMTs in a camera within ~7ns
A valid trigger is transmitted to a central array trigger (L3), which corrects for path length and triggers on telescope coincidence
any 2 out of 4 telescopes within 50ns
Trigger rate ~ 250 Hz
Noise Triggers
Individual telescopes
Cosmic Ray triggers
Array

12. Trigger Upgrade
Current L2 system is CAMAC-based, uses RAM memory lookup tables to check for valid patterns (e.g. any 3 adjacent pixels).
The design is old technology, slow, and provides little diagnostic monitoring.
Iowa State/Argonne have developed and field-tested a fast FPGA based L2 trigger system.
This system will allow to reduce the coincidence gate width by a factor of two, provides real-time diagnostic monitoring and channel-channel time skew adjustment.
The system also allows a fast parameterization of ‘hit’ pixels.
This information can be used as input for a future geometrical array trigger providing gamma-hadron discrimination at the hardware level (e.g. with parallaxwidth parameter).
Test of the prototype ISU/ANL trigger ( on T4 (April 4, 2009).
Bias curves obtained on T4 simultaneously with the VERITAS trigger and the ISU/ANL trigger.

13. Trigger Upgrade -ray proton/NSB
Current L2 system is CAMAC-based, uses RAM memory lookup tables to check for valid patterns (e.g. any 3 adjacent pixels).
The design is old technology, slow, and provides little diagnostic monitoring.
Iowa State/Argonne have developed and field-tested a fast FPGA based L2 trigger system.
This system will allow to reduce the coincidence gate width by a factor of two, provides real-time diagnostic monitoring and channel-channel time skew adjustment.
The system also allows a fast parameterization of ‘hit’ pixels
This information can be used as input for a future geometrical array trigger providing gamma-hadron discrimination at the hardware level (e.g. with parallaxwidth parameter).
Reject
background

14. Trigger Upgrade
Simulations indicate significant reductions in background, providing lower energy threshold.
Improved diagnostics, maintenance and support are also important
Total cost ~$300k.

15. Fast Optical Observations
ACTS are arrays of >10 m class optical telescopes, with relatively crude optics and very fast photon detectors.
This provides access to a largely unexplored parameter space, to study fast optical transient events, and perform optical intensity interferometry.
Modern off-the-shelf digitizers and high speed disks allow storage of a complete night’s waveforms for a single PMT in each telescope which can then be analysed and correlated offline.
The cost is ~$240k, and will allow useful scientific observations during bright moonlight.
National Instruments 200MS/s PXIe digitizer

16. Status of proposals
MRI-R2 proposal submitted to NSF through the University of Utah, with $544k matching funds from Utah.
Regular NSF proposal submitted through SAO.
This also includes a component to upgrade the VERITAS mirror coating facility:
hardware cost of only $38k.
constant high reflectivity is a significant advantage over e.g. HESS, whose mirrors have degraded to ~60%
the existing 25 year old pump is a “single point of failure”, whose replacement could take many months
The upgrade proposal was also submitted to DoE.
VERITAS Upgrade program was presented as an RFI to the ASTRO2010 decadal review, and to the DoE PASAG.
The PASAG report recommends support for the upgrade under all funding scenarios.

17. Upgrade Project Management (regular NSF proposal)

18. Summary
The VERITAS collaboration plan to continue to upgrade the observatory over the coming years.
Improved sensitivity, and a lower energy threshold will have the broadest impact on all VERITAS science goals.
These can best be achieved at the current site by improving the camera photon collection efficiency, and by upgrading the trigger
Technological developments (High QE PMTs and fast FPGAs) allow these goals to be met.
An optical monitoring/ intensity interferometry upgrade provides a novel way to increase the science yield from the observatory, at modest cost.

19. Backup Slides

20. Trigger Development
Hardware produced (I/O, L1.5, L2) and tested on one VERITAS telescope
VERITAS camera processed in three 192-pixel L1.5 boards for coincidences and merged in L2
Bias curve shows lower inflection point due to improved efficiency in rejecting NSB (slightly faster coincidence requirement)
20% higher cosmic-ray rate: larger trigger field-of-view, better timing alignment
Test of the prototype topological trigger (TT) on T4 (April 4, 2009).
Bias curves obtained on T4 simultaneously with the VERITAS trigger and the topological trigger (TT). The coincidence gate of the TT was slightly shorter than the standard trigger.

21. Channel Timing Alignment
•Compensate different delay times (HV and cables)
•Range: 0 to 5 ns
•Precision: 50 ps
•Measured with 30 min
laser run at nominal HV
•Intend to improve to better than 1 ns FWHM
•Working underway to understand delays of ~10 outliers
• Could fix with additional 6ns from FADC-output module
Would Loose
150 pixels
Without Alignment
at 3 ns Gate width
Not shown: ~25 dead CFD channels in T4

22. Background Suppression with Topological Array Trigger (L4)
proton
Calculate 1st moment of image in each camera
Use stereo view from multiple telescopes to project image back into the sky
Identify -ray images by tight correlation of projection
Do this in real time (10 MHz at L2, 10 kHz at L4)
Parallaxwidth <di> di
Reject
background
• Q-factor ~1.5
• Method verified with VERITAS data!