Directional Detectors and Digital Calorimeters

Ed Norbeck and Yasar Onel University of Iowa

For the 25th Winter Workshop on Nuclear Dynamics

Big Sky, Montana 1-7 February 2009

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First a brief report on the performance of an actual directional detector.

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The QuarkNet group from Bettendorf HS have constructed and tested a directional particle detector that makes use of light produced by the Čerenkov effect.

Much of the work was done by high school students Mitch Miller and Nathan Premo at the U of Iowa in 2008.

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A charged particle moving at almost the speed of light through a transparent material makes light that goes off at a angle with respect to its direction. This angle is given by cos (θ) = 1/n where n is the index of refraction of the material.

For n = 1.414, θ = 45º

Particle

θ

Čerenkov light

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Muon angle (relative to vertical) allowed by QuarkNet scintillator paddles is ± 24º

HV Signal

Lucite cylinder 3” OD 4” long

PMT1

PMT2

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Each point shows the size of the signal in PMT1 and PMT2. Blue points are with the same system turned upside down. With no exception the PMT looking up has the larger signal.

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Here the 4 inch long lucite cylinder is replaced with a 2” long cylinder. The looking down PMT has the larger signal 3% of the time.

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In these detectors the PMTs did not show the usual dark current pulses.

Why?

Is a bare PMT a directional Čerenkov detector?

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A PMT by itself is a directional Čerenkov detector

UpFacing the incoming muons

Side

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Up, Down and Side

1

11

21

31

41

51

61

71

0 200 400 600

Signal Size

Co

un

ts Up (2048 Counts)

Down (1863 Counts)

Side (1108 Counts)

The down system is the same as the up but turned upside down.

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What part of the PMT sees muons?

Face only

All counts

Tail only

No counts

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Conclusions

• Directional Čerenkov detectors are simple and effective.

• A high-energy test beam may be available at your local high school.(There are over 300 high schools in the QuarkNet program)

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Ed Norbeck, Burak Bilki,Yasar Onel and José Repond

What is a digital calorimeter and why would anyone want one ?

May get improved energy resolution by not measuring the energy!

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Sampling Calorimeter

Heavy metal absorber Detectors

For good energy resolution need large sampling fraction

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Common “thick” detectors

•Čerenkov

•Scintillator

•Semiconductor

Each has its own set of problems.

All have low density, makes calorimeter longer

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Some excellent gas detectors

• RPC (Resistive Plate Chamber)• GEM (Gas Electron Multiplier)• MICROMEGAS (MICRO-Mesh-Gaseous Structure)

All are “thin”, at most a few mm of gas.

Sampling ratios of 10-5

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Amplitude

Num

ber o

f cou

nts

Threshold

Noise

Signal

Signal from MIP in a thin detector

If single MIP, energy lost in absorber known.

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To measure energy in a jeteach particle must go into separate pixel.

Requires excellent position resolution

Many pixels

Cubic meter detector

40 planes each with 10,000 1.0 cm2 pixels

400,000 single-bit channels

Does not require A to D conversions!

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Problems!

•What if a MIP is recorded in two pixels?

•May be below threshold in one plane

•May collide and make several new MIPs

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Extrapolate tracks back to find number of MIPs near junction

Need elaborate computer programs

“Particle flow algorithms”

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With analog calorimeter calibration is a major problem.

With digital calorimeter drift in sensitivity can cause

•Missed point in a track or

•spurious point (noise)

Both of these easily dealt with by

Particle Flow Algorithm

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A few events… μ Calibration Runs120 GeV protons with 1 m Fe beam block no μ momentum selection

One of many perfect μ event

μ event with double hits in x

μ at an angle or multiple scattering

μ event with δ ray or π punch through

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A few events…e+ Run 1 - 16 GeV secondary beam Čerenkov signal required

8 GeV e+ event

8 GeV e+ event with satellites 8 GeV e+ event

8 GeV e+ event

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A few events…π+ Run 1 – 16 GeV secondary beam Veto on Čerenkov signal

8 GeV π+ event (typical)

8 GeV π+ event (early shower)8 GeV π+ event (early shower)

8 GeV π+ event (early shower)

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A few events…Multiple particles

120 GeV protons without beam block

2-π event (upstream shower?) 3-π event (upstream shower?)

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This technique allows detailed tracking for penetrating particles.

(It is difficult to implement ten nuclear reaction lengths of silicon.)

Digital calorimetry is a rapidly developing field.

Cosmic ray studies used lead plates and emulsion 50 years ago.Spark chambers used camera readout.

Concluding comments