mini- H igh A ltitude W ater C herenkov experiment

October 20-21, 2005 “Towards the Future” Workshop Andrew Smith

mini-High Altitude Water Cherenkov experiment

e

145 meters

4 meters

Andrew SmithUniversity of Maryland


What is miniHAWC?

• Milagro – Existing water Cherenkov all-sky gamma-ray observatory.

• HAWC – A “science” driven effort to construct an all-sky observatory with point source sensitivity of the Whipple 10m.

• miniHAWC – Demonstrate HAWC technology at low cost with Milagro PMTs/instrumentation.


What is the Sensitivity of miniHAWC?

Answer: ~15x Milagro. 1y ~60mCrab source at 5

Interpretation 1: Designers of Milagro are Stupid.Interpretation 2: Designers of miniHAWC are Smart.

Designers of Milagro = Designers of miniHAWC

Interpretation 3: Designers of Milagro/miniHAWC were naive, but have wised up.


Critical Variables

• Size – Bigger is better until you reach sqrt(A) regime.

• Photocathode Density – More is better until you detect all the particles.

• Altitude – Higher is better until you can’t breathe. Diminishing returns.

Tools: PMTs (8” Hamamatsu), Water, black and white materials.


Detector Layout

Milagro:450 PMT (25x18) shallow (1.4m) layer273 PMT (19x13) deep (5.5m) layer175 PMT outriggers

Instrumented Area: ~40,000m2

PMT spacing: 2.8mShallow Area: 3500m2

Deep Area: 2200m2

HAWC:5625 or 11250 PMTs (75x75x1,2)Single layer at 4m depth or 2 layers at Milagro depths

Instrumented Area: 90,000m2

PMT spacing: 4.0mShallow Area: 90,000m2

Deep Area: 90,000m2

miniHAWC:841 PMTs (29x29)5.0m spacingSingle layer with 4m depth

Instrumented Area: 90,000m2

PMT spacing: 4.0mShallow Area: 90,000m2

Deep Area: 90,000m2


Equipment

• Milagro DAQ:– 898 8” Hamamatsu PMTs– Single data/HV cable ~150m length – Custom front end boards. Analog to level crossing

conversion. (Amplitude through time over threshold.)– FASTBUS TDC– VME – FASTBUS interface with VME readout– 2000Hz maximum readout with ave multiplicity ~20-30%


How do you make an EAS array more sensitive to gamma-ray sources?• Energy Threshold

– Altitude– Big, hermetic, sensitive

• Angular Resolution– Big Lever Arm

• gamma/hadron Separation– Must detect penetrating particles

Sensitivity increase is the product of the improvement made in each category.


4500m

2600m

Difference between 2600m (Milagro) and 4500m (Tibet):~ 6x number of particles~ 2x lower energy threshold

Altitude


Hadron induced cosmic ray showers contain 5-20x more energyin penetrating +/- and hadrons than EM particles.

High Pt hadronic interactions lead to wide lateral distributions.

Need mass! Need large Area!

Muon/hadron Detection

HAWCminiHAWC

Milagro


High Altitude Detector w/o /hadron separation

ARGO: ~6000m2 RPC detector.

Reported at ICRC expect sensitivity of 8-13/year on the Crab.

Milagro currently achieves~8/year on the Crab.


Curtains

• A high altitude version of Milagro would trigger at >10kHz. Need to control spurious triggers due to single muons.

• Install curtains to optically isolate the the PMTs.


Simulation Strategy

• Use Milagro Simulation/Reconstruction software.

• Use observed Milagro crab signal to anchor simulations to reality. Shared systematics with Milagro.

• Use new /hadron discrimination variable for HAWC/miniHAWC that excludes the core location.

CMilagro = (nPMTs above 2 PE)/(Max “muon layer” hit)

CminiHAWC = (nPMTs above 2 PE)/(Max “muon layer” hit > 20m from core)

• As an illustration, consider 2 trigger threshold: 50 PMTs, 200 PMTs.


= ~0.4 deg = ~0.25 deg

Angular Resolution

/hadron Separation

Cut: nTop/cxPE>5.0Eff = 34%Eff CR= 3%

Cut: nTop/cxPE>5.0Eff = 56%Eff CR= 1.5%


Triggering with Curtains

• Multiplicity trigger at ~80 PMTs gives same trigger rate as Milagro at 50 PMTs• Much higher Gamma area.


Effective Area

Detector size


Energy (Crab Spectrum, nTop/cxPE>5.0., <30O)Significance fromCrab Transit (~5 hr) 4

Crab signif/year 80

5 point source sensitivity reach ~60mCrabof 1 year survey

Energy Resolution ~30% above median

Angular Resoultion 0.25O-0.40O

S/B (hard cuts) ~ 1:1 for CrabTypical day 20 excess on 25 bkg

Q(Milagro -> miniHAWC) = 15!

Single layer doesn't limit sensitivity

miniHAWC Sensitivity


Curtains Test in Milagro


Site

– High Altitude.– Power– Internet– Don’t need darkness or

good weather…– YBJ very interested. – Chinese don’t have money for site prep.

(ARGO)– Investigating a site in Mexico.


Cost - Detector Elements

• Pond (0.2-2.0 M$)• Black Liner Material (~100k$ @ $1/m2 )• Pump/Recirculation System. (~$200k$)• PMTs – Reuse with base and encapsulation w/ new

connectors.• Cables – Purchase new. (~100$k)• Front End Electronics – keep as is.• TDC and DAQ

– 2kHz DAQ keep current electronics– Faster VME TDCs (~200$k)

• Online computing – A few computers can reconstruct in real time. (~10$k)

• Building – Assemble functional DAQ in a trailer and ship to site. (~200$k)

• Internet Access – Live with slow internet if necessary. Need prompt alert capability.


Issues – The Good/Bad• Simple Analysis- Event weighting could increase

sensitivity further. In Milagro Q=1.6.

• Can reconstruct showers down to 20 PMTs if we could trigger at 6kHz.

• Site.

• Calibration.

• Noise.

Milagro in 1.4y ~8/y


Summary

• 15x Sensitvity increase over Milagro~3x from Altitude,Area~3x from /hadron separation~1.5x from Angular resolution

• ~60mCrab sensitivity (5 in 1year)

• Mostly proven technology– Leverage $1.5M investment in Milagro equipment– Could construct rapidly if site available.


An Ideal Air Shower Gamma-Ray Detector:

1) Large Physical AreaCollection AreaContain CoreSample Lateral Tails

2) High Efficiency for 20 MeV Continuous DetectorEfficient e Converter

3) Calorimetry Hadron/Muon Identification

4) Altitude~40gm/cm2 increase in altitude double ground level particles.Lowers threshold.


1) Large Physical Area

(Diagram Showing Shower Curvature)

Core position reconstruction required for accurate angle reconstruction. --> Core must be contained within detector --> Effective area <~ Physical Area --> 104 to 105 m2 detector required to rival area of IACTs --> Long lever arm for angle reconstruction.

Shower front is curved.Without core position: Pointing error dominated by systematics ==>

PSF >~ 0.7O


2) High Efficiency for detection of 20MeV Gamma-Rays

In extended air showers 's out number e+/- by 5-10 to 1.

Mean energy of EAS 's is ~20 MeV. ( not strongly correlated with primary VHE energy.)

Plot showing spectrumof showering particles

Plot Showing de/dxfor electrons.


HAWC layout

75x75 grid of 8” PMTs - in 2 layers (depth =2m,6m, separation = 4m)

Angle reconstruction with top layer. Calorimetry with bottom layer

Opaque “curtains” separate PMT cells.Eliminate “cross talk” between counter.Limits trigger rate compared to Milagro


How does the HAWC design measure up?

1) Size: – 300mx300m = 90,000 m2

2) Efficiency: Water acts a both conversion medium and radiator. ~1 PE/25 MeV

3) Calorimetry: Deep (~15Xo) PMT layer for Muon/Hadron rejection.

4) Altitude: Select optimal site.


Simulation Strategy

Simulate various HAWC geometries at 2 altitudes.Compare results with Milagro. (Milagro sensitivity verified by observations of the Crab.)

2 altitudes considered: 4500m ( ~Tibet Lab altitude) 5200m ( ~Atacama Plateau)

2 sets of cuts considered: Std - 50 PMT multiplicity cut Hard - 200 PMT multiplicity cut

= ~0.4 deg = ~0.25 deg


Gamma-Hadron Separation:

Remove events with one or more large hits away from the core ---> muon/hadrons in lateral tails ---> “cxPE” is largest bottom layer hit with R

core>10m

---> “nTop” is number of PMTs hit in top layer

Cut: nTop/cxPE>10.0Eff = 40%Eff CR = 5%

Cut nTop/cxPE>10.0Eff g = 65%Eff CR = 2%


Energy of reconstructed events (Crab Spectrum)nTop/cxPE>10., <30O

Altitude = 4500m

Altitude = 5200m

Threshold Lower at zenith.


Point Source Sensitivity of HAWC:

Altitude 5200m 4500mCrab Transit (~4 hr) 25 10 Median Energy

Time to 5 10min 60min

5 point source sensitivity reach 10mCrab 25mCrabof 1 year survey

Energy Resolution ~30% above median

~1/4 sensitivity of HESS (40/hr) with>1000x the exposure!


miniHAWC:

HAWC is a potentially large complex project.

Consider a another possibility, the relocation of the Milagro apparatusto an optimized pond located at a high altitude site.

Milagro owns ~900 8” Hamamatsu PMTs and a DAQ capable of 2000Hz readout.

--> Single layer consisting of a 29x29 Grid of PMTs with 5m separation (150mx150m pond) and 4m depth.

--> Utilize same reconstruction and /hadron separation methods as HAWC.

--> Simulation altitude 4500m.


Summary:

Current EAS arrays are not detecting/utilizing the “whole” shower. Huge improvements in /hadron separation possible.

Excellent angular resolution possible.

miniHAWC/HAWC could survey the entire northern or southern skyto a sensitivity of ~60mCrab/10mCrab. (conservative estimates)

Limiting detector to a single layer doesn't seem to reduce sensitivity. Could reduce cost of HAWC instrumentation by factor of ~2-3

Wide field of view and high duty cycle:SurveysPrompt VHE GRB emission?AGN monitor.Ideal for study of diffuse/extended sources.???


The Diffuse Galactic Plane in miniHAWC and HAWC

Use Neutral H map to trace out VHE Gamma-Ray flux. Normalize to Milagroobserved TeV diffuse emission from the Galactic plane.


Conclusions

• Milagro and other air-shower arrays play an important and complementary roll in VHE astronomy.– Survey

– GRBs

– Extended/Diffuse Sources

– Monitoring Variable Sources

– Solar Activity Monitoring

– Water Cherenkov Method has not been exploited efficently

– Design improbments (Size, Altitude, …) lead to much better than sqrt(N) sensitivity improvents.


1) Size: – 150m x 150m = 22,500 m**2

2) Efficiency: Water acts a both conversion medium and radiator (Cherenkov) ~1 PE/40 MeV

3) Calorimetry: Deep (~15Xo) PMT layer for Muon/Hadron rejection.

4) Altitude: Select an optimal site.

mini- H igh A ltitude W ater C herenkov experiment

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