Lecture 2-Building a Detector

George K. Parks

Space Sciences Laboratory

UC Berkeley, Berkeley, CA

Brief summary of Lecture 1

Brief summary of Lecture 1 (cont’d)

• A detector is a device that converts incident particles and photons into signals without distorting the original information.

• Two major physics discoveries led to important development of detectors: photoelectric effect and that secondary electrons can be produced.

• Detector components include Photomultiplier Tubes (PMT) and Channel Electron Multipliers (CEM).

- PMTs multiply the number of electrons by discreet dynodes whereas CEMs multiply electrons continuously.

• Assemble a million of CEMs in a geometrical array and form Micro Channel Plates (MCP).

- Each channel is a pixel, so MCPs can form Images.

Schematic of Earth’s Magnetosphere

Density of Major Constituents in Earth’s atmosphere

Differential Energy Fluxes

Typical Oxygen spectra in the heliosphere

Measurement Requirements

Requirements

Detectors and Components

Detectors for Space

Measurement and Instrument Requirements

A Simple Detector for Photon Measurement

Imaging DetectorCollimator

Scintillators

Common Inorganic Scintillators

Light transmission

• Scintillators must be ableto transmit the light it generates.

• Generally not a problem withmost scintillators.

CsI Scintillator

Emission Spectrum of scintillators

• Scintillators produce different amount of light.

• NaI (Tl) more efficient than CsI (Na)

• It’s better if there is more light.

• Why? Directly affects the energy resolution of the detection system.

• How? Affects Statistics.

Absorption in material

€

I = Ioe−μx

Io = # incident h through xx = thickness = attenuation coefficient

• X-and gamma rays are penetrating.• Need high Z material to stop them.• Inorganic scintillators have higher density that organic scintillators.

NaI(Tl)

Temperature Dependence of NaI(Tl)

Entrance Window Material

• NaI(Tl) is hydroscopic, sealed in vacuum.

• Transmission of X-rays through various material in front of sealed NaI (Tl).

X-ray Absorption in NaI(Tl)

€

I = Ioe−μx

• 2 mm70% @ 100 keV

• 1/4 in (6.35 mm)~95% @ 100 keV

X-ray Absorption in CsI(Tl)

• Density = 4.51 g/cm3

• 2 mm83% @ 100 keV

• ¼ in (6.35 mm)~100% at 100 keV

€

I = Ioe−μx

X-ray Absorption in BGO

•Density = 7.13 g/cm3

• % of incident X-rays stopped in BGO. €

I = Ioe−μx

• 1 mm95% @ 100 keV

• 1.5 mm~100% @ 100 keV

X-ray Absorption in Plastic

• Density = 1.03 g/cm3

• Plastic scintillator often used in anti-conincidence part of an experiment to reduce cosmic ray contribution.

• 10 mm20% @ 20 keV• 130 mm82% @ 100 keV98% @ 20 keV

€

I = Ioe−μx

Properties of Scintillators (Room T)

Maximize photon collection

Plastic Scintillator (NE 102)

• Light emission by various particles• Sufficient for A/C application

• Range of various particles• Few mm to stop 2 MeV p+

Light emission of Inorganic Scintillators

Desired Properties of Scintillators

Conversion Efficiency Calculation (cont’d)

More Worries!

Conversion Efficiency Calculation

• To compute DE for different energies, use

different radioactive sources.

• Half-life of Sources. How to correct?

where A = activity level now

Ao = original activity level

t = time interval since the source calibrated

= mean half-life of the source

1 Curie = 3.7x1010 dps

€

A=Aoe−t /τ

Summary of important factors

Reminder-A simple Photon Detector

Reminder-Photomultiplier Tube

PMTs

Operating principle of PMTs

Photomultiplier tubes (PMTs)

• Hamamatsu listsmore than 300 different types

of PMTs.

• Different shapes, size, gain, etc..

• So many different parameters!

• What do they mean?

• How does one choose which PMTs to use?

Reminder-Buiding Detectors