Radiation in the Natural World W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 1.

Radiation in the Natural World

W. Udo Schröder, 2010

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ANSEL EXPERIMENT 1

PHOTON SPECTROSCOPY

Scope of ANSEL Experiment 1

• Ubiquitous presence of radiation on Earth, e.g., g-ray photons

• Concepts of absorption coefficient and cross section

• Introduction to g-interactions with matterPhoto electric effectCompton scatteringPair production

• Operational principles of inorganic scintillation detectors

• Examples of energy spectra with NaI(Tl) detectors• Experimental setup with a 3”x3” NaI(Tl) detector• Lab measurements in Expt. 1


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W. U

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ANSEL Expt 1: Gamma Spectroscopy

4

Probability and Cross Section

Absorption upon intersection of nuclear cross section area s

j beam current density (#part.time x area)A area illuminated by beamL= 6.022 1023/mol Loschmidt# NT # target nuclei in beamMT target molar weightrT target densityx target thickness[s]=1barn = 10-24cm2

Target

x

Beam

0N j A

Transmitted

0xN N e

absorption

TT

P

per nucl

#nuclei

in eus

A

beam

LA x

M

x

Mass absorption coefficient m

0 0 1 xabsN N N N e

00abs

T

T

T

NN N x

A

L Ax

j current density j

M

N

Thin target approximation

abs

nucl

NN j

elementary absorption cross

section per nucleus

Beam area A

Nucleus area s

Differential Cross Section/Probability

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5

b 0 T

d

b0 T

Flux of particles b

d ,dN , j N d

d

dN , d ,j N

d d

A( a ,b )B

Reaction A a B b

Projectile current j0

d Ejectile numbers

TN

bdN ,

Projectile current j0

DetectorDWdet

b detbd

,dN

N,,

dd

Ejectile numbers measured

A( a,b )B

Reaction cross section :

Spherical Coordinates

W. U

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6

q

f

dq

df

rrsinq

rsin q df

r dq

x

y

z

Spherical Coordinates

x r sin cos

y r sin sin

z r cos

dr

dA

Volume Element dV=dA·dr

3

2

2

2

0 0

Volume Element

d r dx dy dz

r dr d sin d

Solid angle element

dAd d sin d

rIntegral :

d d sin d 4

2

2 2sphere

Total Solid Angle :

dA 4 rd 4

r r Unit of s.a. = sr (steradian)








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g-Induced Processes in Matter

g-rays (photons): from electromagnetic transitions between different nuclear energy states detect indirectly (charged particles, e-, e+)Detection of secondary particles from: 1. Photo-electric

absorption2. Compton scattering3. Pair production4. g-induced reactions

1. Photo-electric absorption (Photo-effect)

ħw photon is completely absorbed by e-, which is kicked out of atom

2

2

;

'

13.6

3, 5,

kin n n

n

K L

E E E binding energy

ZE Rhc Moseley s Law

nRhc eV Rydberg constant

screening constants

different subshells

ħw A

Electronic vacancies are filled by low-energy “Auger” transitions of electrons from higher orbits


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1. Photo-Absorption Coefficient

5 7 4

5 1 2

( , )

( , )

PE

PE

E Z Z E low E

E Z Z E high E

g g g

g g g

Absorption coefficient m (1/cm)

“Mass absorption” is measured per density r

/m r (cm2/g)

“Cross section” is measured per atom

s (cm2/atom)

Abso

rpti

on C

oeffi

cient

/m

r (c

m2/g

)

Pt

Wave Length l (Å)

Absorption of light is quantal resonance phenomenon: Strongest when photon energy coincides with transition energy (at K,L,… “edges”)

Probabilities for independent processes are additive:

mPE = mPE(K)+mPE(L)+…


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2. Photon Scattering (Compton Effect)2 2 2 2

0 0( ) ( ) : 0Relativistic E pc m c photons m m

E p c

g

g g g

22

2 2 2 2

222 2

2

2

:

2 cos

:

0.511

1 1 cos

g g g g

g g g g

g g

gg

g

e e

e

e e e

e

e

Momentum balance

p p p p c p p c

p c E E E E

Energy balance

E m c E p c m c

m c MeV

EE

E m c

q

f

l l’

1 cos

" "

22.426

C

C

Ce

Compton wave length

pmm c

Compton Electron Spectrum

0 0.2 0.4 0.6 0.8 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7Compton Energy Spectrum

Energy (MeV)

Cro

ss S

ectio

n (b

)

0.644

0

Nexp E( )

ddE E( )

10 E

0

2

2

2

2

:

: 1

(

80

1 1 cos

1 cos

1 1 cos

(" ")1 2

kin

e

e

e

e

Maximum electron energy Compton Ed

Scattered photon energy

Scattered recoil electron energy

E E E

Minimum photon energy

EBackscatter E

EE

E m c

E E m c

m

E

c

m

E

c

g g

gg

g

g g

g

gg

g

2

2

) :

2

1 2kin CE

e

e

ge

EE EE m c

E m cg

g

g

Actually, not photons but recoil-electrons are detected

Recoil-e-

spectrum

true

finite resolution

Com

pto

n E

dge


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3. Pair Creation by High-Energy g-rays

{e+, e-,e-} triplet and one doublet in H bubble chamber

Magnetic field provides momentum/charge analysis

Event A) g-ray (photon) hits atomic electron and produces {e-,e+} pair

Event B) one photon converts into a {e-,e+} pair

In each case, the photon leaves no trace in the bubble chamber, before a first interaction with a charged particle (electron or nucleus).

Magnetic field

e-

e-

e-

e+

e+

g-rays

A

B


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Dipping into the Fermi Sea: Pair Production

22 1.022Threshold eE E m c MeVg

Dirac theory of electrons and holes:

World of normal particles has positive energies, E ≥ +mc2 > 0

Fermi Sea is normally filled with particles of negative energy, E ≤-mc2 < 0

Electromagnetic interactions can lift a particle from the Fermi Sea across the energy gap DE=2 mc2 into the normal world particle-antiparticle pair

Holes in Fermi Sea: Antiparticles

Minimum energy needed for pair production (for electron/positron)

Energy

0

-mec2

+mec2

normally filled Fermi Sea

normally empty

e-hole

e-particle

Eg

-[mec2+Eki

n]

+[mec2+Eki

n]


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The Nucleus as Collision Partner

2

2

2

: 2 ....

Threshold e

e kin kin

E E m c

Actually converted E m c E E

g

g

28 2 2

222

2 2

5.8 10 2

( , )1

137 2

e

PP

kin e e

cm E m c

P Z Ed eZ

dE m c E m c

P slowly varyingg

g

g

Increase with Eg because interaction sufficient at larger distance from nucleus

Eventual saturation because of screening of charge at larger distances

Excess momentum requires presence of nucleus as additional charged body.

e+

e-

recoil nucleus

g

Pb

1barn = 10-

24cm2


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4. g-Induced Nuclear Reactions

Real photons or “virtual” elm field quanta of high energies can induce reactions in a nucleus:

(g, g’ ), (g, n), (g, p), (g, a), (g, f)

Nucleus can emit directly a high-energy secondary particle or, usually sequentially, several low-energy particles or g-rays.

Can heat nucleus with (one) g-ray to boiling point, nucleus thermalizes, then “evaporates” particles and g-rays.

n

a

nucleus

g

g

secondary radiation

p

incoming

g-induced nuclear reactions are most important for high energies, Eg (5 - 8)MeV


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Efficiencies of g-Induced Processes

Different processes are dominant at different g energies:

Photo absorption at low Eg

Pair production at high Eg > 5 MeV

Compton scattering at intermediate Eg.

Z dependence important: Ge(Z=32) has higher efficiency for all processes than Si(Z=14). Take high-Z for large photo-absorption coefficient

Response of detector depends on

• detector material

• detector shape

• Eg








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Sci

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Scintillation Mechanism: Inorganic Scintillators

Primary ionization and excitations of solid-state crystal lattice: free (CB) e- or excitons (e-,h+)sequential de-excitation with different Eph and time constant.

Advantage of inorganic scintillator: high density, stopping power good efficiency

Disadvantage: slow response – ms decay time, “after glow”,some are hygroscopic

Electronic excitation: VB CB (or below)Trapping of e- in activator states (Tl) doping material, in gs of activator band e transition emits lower Eg, not absorbed.

18

Sci

ntill

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nDet


Environment of g Scintillation Measurement

19








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Shapes of Low-Energy g Spectra

Photons/g-rays are measured only via their interactions with charged particles, mainly with the electrons of the detector material. The energies of these e- are measured by a detector.

The energy Eg of an incoming photon can be completely converted into charged particles which are all absorbed by the detector, measured energy spectrum shows only the full-energy peak (FE, red) Example: photo effect with absorption of struck e-

The incoming photon may only scatter off an atomic e- and then leave the detector Compton-e- energy spectrum (CE, dark blue)An incoming g-ray may come from back-scattering off

materials outside the detector backscatter bump (BSc)

measured energy

measu

red

in

ten

sit

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measured energy (MeV)

measu

red

in

ten

sit

yShapes of High-Energy g Spectra

High-Eg can lead to e+/e- pair production,

e-: stopped in the detector

e+: annihilates with another e- producing 2 g-rays, each with Eg = 511 keV.

One of the 511 keV can escape detector single escape peak (SE) at FE-511 keV

Both of them can escape detector double escape peak (DE) at FE-1.022 MeV

The energy spectra of high-energy g-rays have all of the features of low-energy g-ray spectra

e+/e- annihilation in detector or its vicinity produces 511keV g-rays

FE


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Quiz

• Try to identify the various features of the g spectrum shown next (well, it is really the spectrum of electrons hit or created by the incoming or secondary photons), as measured with a highly efficient detector and a radio-active AZ source in a Pb housing.

• The g spectrum is the result of a decay in cascade of the radio-active daughter isotope A(Z-1) with the photons g1 and g2 emitted (practically) together

• Start looking for the full-energy peaks for g1, g2,…; then identify Compton edges, single- and double-escape peaks, followed by other spectral features to be expected.

• The individual answers are given in sequence on the following slides.


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Spectrum of g Rays from Nuclear Decay


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g1


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g2

g1


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g2

g1

CE g2


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g2

g1

SE g2

CE g2


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g2

g1

SE g2

DE g2 C

E g2


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g2

g1

SE g2

DE g2

511 keV

CE g2


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g2

g1

BSc

SE g2

DE g2

511 keV

CE g2


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g2

g1

BSc

SE g2

DE g2

511 keV

CE g2

Pb X-rays


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g1+g

2

g2

g1

BSc

SE g2

DE g2

511 keV

CE g2

Pb X-rays








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Prin

cipl

es M

eas


35

Slow

Fast

Produce logical signal

Principle of Fast-Slow Signal Processing

DiscrimCFTD

PreAmp

Amp

Gate Generator

Data Acquisition System

Energy

Gate

0

0t

t

CFTD Output

CFTD Internal

t

CFTD Input

Principle of a Constant-Fraction Timing Discriminator:

t independent of E

here f = 0.5

Source

Produce analog signal Binary

data to computer

NaI Det.








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Lab Measurements Expt. 1

With the help of the TA set up detector, electronics and data acquisition:

• Power up the NaI detector (+1750 V) and the electronics NIM and CAMAC bins.• Place a 22Na g source close (5 cm) to the face of the NaI. • On the scope, follow the analog pulse along the slow circuit.

– Check the effects of the settings of gain and time constant controls at the TC 248 main amplifier.

• On the scope, inspect the output of the fast (lower) part of the TC248 and feed it to a discriminator used to derive a digital signal for strobing the ADC in the CAMAC crate.

• Trigger the scope Ch 1 with the discriminator output signal, view on Ch 2 the analog signal and ascertain a proper (low) setting of the discriminator threshold.

• Feed analog signal to the ADC (Ch 7)• Feed the digital signal to the CC-USB CAMAC controller Input 1 and use the

signal appearing at Gate 1 output to strobe the ADC.• Check on the scope the proper relative timing of analog and strobe signals.• Start the EZDAQ data acquisition according to the EZDAQ setup checklist.

• Accumulate, display and save a 22Na g energy spectrum in histogram form.


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Source Info


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Counts

/keV

Set of typical Calibration g-ray sources

Semi-log plot of a calibrated 22Na g-ray spectrum taken with a 3”x3’ NaI(Tl) detector

Lab Measurements Expt. 1 (cont’d)

• Check the appearance of the NaI spectrum for the 22Na g source and place the dominant structure in the middle of the spectrum by adjusting fine and coarse gain of the main amplifier.

• After the above choice of gain (and previous integration) parameters, do not change the amplifier settings for any of the additional measurements.

• Take a final measurement for the Na source (5 min). Then remove this source and place it far away from the detector (in the cabinet).

• Based on the 22Na g energy spectrum, perform a coarse calibration of the ADC channel numbers in g-ray energy. In this task utilize the well measured channel # positions of the full-energy peak (1.275 MeV), of the associated Compton edge (ECE = ?? MeV) and of the 0.511 MeV annihilation peak.

• Perform similar, individual measurements for the 60Co and 54Mn sources. • Verify that the main g lines for these sources appear in the spectrum

approximately at the expected locations.• Measure the g-ray energy spectrum for the unknown source. • Remove all sources from the vicinity of the NaI detector and perform a

measurement of g-ray energy spectrum of the room background. To accumulate sufficient intensity, accumulate data for at least several hours (possibly overnight).


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Data Analysis Expt. 1

• Identify in the measured spectra for the three known sources the prominent spectral features and correlate their channel positions (ch#) with the known energies (Eg or ECE). In the fits keep track of experimental errors. Use Gaussians for g lines and half-Gaussians for Compton edges.

• Generate a calibration table and a plot of energies of the positively identified prominent spectral features from the three known sources (22Na, 60Co, 54Mn) vs. the experimental channel numbers for these features.

• Perform a least-squares fit for the calibration data E g (ch#) and include the best-fit line in the calibration table and plot.

• Generate plots of all measured energy spectra as Counts/keV vs. Energy/MeV. • Identify the g-ray energies of prominent features in the spectrum for the

unknown source. Based on the provided search table, suggest the identity of the unknown source (or source mix).

• Identify the g-ray energies of prominent features in the spectrum for the room background. Based on the provided search table, suggest the identities of the various components.


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Sample Spectrum


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Counts

/channel

Radiation in the Natural World W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 1.

Documents

ansel expt

gamma spectroscopy

udo schrder

cross section absorption

atom absorption coefficient

nucleus area slide

scope of ansel experiment

target x