Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Gamma-Ray Spectroscopy Dr.Ir. Peter Bode Associate Professor Nuclear Science & Engineering.

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Gamma-Ray Spectroscopy Dr.Ir. Peter Bode Associate Professor Nuclear Science & Engineering

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 INAA: Semiconductor detectors RNAA: Semiconductor detectors Scintillation detectors

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Principle of a semiconductor detector Solid-state ionisation detectors

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Solid-state ionisation detectors Energy band structure of extrinsic semiconductors Impurities: P, As B, Al, Ga

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Solid-state ionisation detectors N-type Ge: Impurities such as P and As as electron donors P-type Ge: Impurities as B, Al, Ga as positive charge donors

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Solid-state ionisation detectors Semiconductor detector: Junction diode with P and N type impurities on either side Applying a reverse bias: A P-I-N structure is formed

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 n-type silicon e h -V0 qeqe dV 1 qhqh dV 2 dq = ( q e dV 1 + q h dV 2 )/V i = dq/dt n + contact p + n junction reverse bias, fully depleted silicon diode germanium detector Solid-state ionisation detectors

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Some properties of semiconductor materials Solid-state ionisation detectors

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Schematic representation of a Ge-semiconductor detector, Solid-state ionisation detector

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Solid-state ionisation detector

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Solid-state ionisation detectors Contacts: n + : diffusion of Li-atoms 700 – 1000  m (dead layer) p + : implantation of B-atoms 0.3  m

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Different types of Ge semiconductor detectors Solid-state ionisation detector

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Solid-state ionisation detector Channel number  pulse height No.of pulses (* 1000)

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Solid-state ionisation detector

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Solid-state ionisation detector Pulse height spectra obtained with Si(Li) detectors. Left: X-ray spectrum of 241 Am Right:  - spectrum of 241 Am

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Different types of cryostats for use with Ge-semiconductor detectors Solid-state ionisation detector

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Energy resolution Usually: Full Width on Half Maximum @ 1332 keV of 60 Co @ 122 keV of 57 Co @ 6 keV of 55 Fe

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Energy resolution State-of-the-art: 1332 keV: 1.58 – 2.0 keV, depending on crystal size 122 keV: 0.6 – 1 keV 5.9 keV: 0.2 – 0.5 keV

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Peak Shape Ratio of : FWHM/Full Width 0.1 M FWHM/Full Width (1/50) M

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Gamma-ray peak shape and tail parameters

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Peak Shape Ratio of : FWHM/Full Width 0.1 M theoretically: 1.83 FWHM/Full Width (1/50) M theoretically: 2.38 Importance: symmetry !!!

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 High energy (top) and low energy tail parameters

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 High energy tail of pulser peak

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Calibration source activity

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Peak-to-Compton ratio Defined as: Ratio of peak height at 1332 keV and average peak height in energy range between 1040 and 1096 keV

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Peak-to-Compton ratio State-of-the-art: p/C ~ 50-100, depending on size of crystal: pC = 34.75 + 1.068 (ε Co-60 ) - 4.96.10 -3 (ε Co-60 ) 2

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Efficiency Absolute efficiency defined as: Relative to the efficiency of a 3” x 3 ” NaI(Tl) detector, defined as 1.2.10 -3 counts/1332 keV photon, measured at a source-detector distance of 25 cm

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Determination of photopeak efficiency curve Absolute: Using calibrated sources with known gamma-ray emission rates and activity values, traceable to Bq Single gamma-ray emitting radionuclides Point sources Extended sources Problem: Many sources contain 60 Co and 88 Y; corrections for coincidence effects require also the p/T curve

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Determination of photopeak efficiency curve Relative: Using mix of sources with well-established gamma-ray intensity ratios 1 source for entire energy range, e.g. 152 Eu 2-5 sources, e.g. 182 Ta + 133 Ba + 75 Se + 24 Na + … Problem: Intensity ratios not always well established

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Determination of photopeak efficiency curve Relative: 1 source: advantage: simple disadvantage: do not always fully cover entire energy range; inter/extra-polation disputable in 80-150 keV range 3-5 sources: advantage: better coverage all energy ranges disadvantage: more cumbersome, problems with non- matching parts

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Determination of efficiency curves Relative: Using mix of sources with well-established gamma-ray intensity ratios 1 source for entire energy range, e.g. 152 Eu 2-5 sources, e.g. 182 Ta + 133 Ba + 75 Se + 24 Na + …

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 New Tools for Nuclear Spectroscopy Better and bigger Ge detectors High count rate electronics High-resolution scintillation detectors (LaBr 3 (Ce)) Position-sensitive (strip) detectors Monte Carlo modeling Image processing

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Bigger Ge- Detectors Absolute photopeak efficiency 75 cm 3 (17 %) 4 cm 560 cm 3 well Photon energy, keV 0.3 %  20 % 3 %  90 %

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Typical improvement in detection limits 20 % 100 % well 125 cm 3 Arbitrary units 0,01 0,1 1 0,01 0,1 0,25-0,3 1 0,01 0,1 0,15-0,25 1 Bigger Ge-Detectors well 560 cm 3 0,01 0,1 1 0,07 - 0,1

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 New Tools for Nuclear Spectroscopy LaBr 3 (Ce) scintillation spectra P.Doorenbos et.al., IEEE Transactions 51 (2004) 1289 Developed and Patented by T.U.Delft: produced by Saint Gobain, France

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Preamplifiers - Resistor feedback - Pulse optical feedback high resolutions (planar detectors) - Transistor feedback high count rates

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 NIM bin and power supply - Adequate capacity - standard: +/- 24 V +/- 12 V +/- 6 V

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 High Voltage supply Typically (+/-) 3-5 kV Different power supplies for Ge and NaI(Tl) detectors dV/dt networks LN2 switchoff option

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Spectroscopy Amplifiers Analogue systems Digital systems - Gaussian shaping - Triangular shaping - Gated-integrated shaping

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Baseline retoration and Pole-zero setting

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Spectroscopy Amplifiers Essential: - Amplifier and HV at opposite outermost places in NIM-BIN - Match amplifier’s output DC level to ADC’s input DC level - High time (shaping) constant: better resolution, lower throughput Often typically set at 3  s for most (coaxial) detectors 6  s for planar detectors

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 ADC - Linear Approximation or Wilkinson ADC - Successive Approximation ADC Wilkinson ADC’s : best linearity Wilkinson ADC: dead time depends on channel no. Succ. Appr. ADC: fixed dead time per channel. Settings: zero level, lower level, upper level, conversion gain

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Pulse generators - Used for dead time/pile-up correction - Fed into the test input of the preamplifier - Essential: input peak shape must match detector signal (rise time and fall time (1000-2000  s )

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Dead-time stabilizer Loss free counting Digital dead-time stabilizer

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Signals Use oscilloscope Track: - Output signal preamplifier - Output signal spectroscopy amplifier

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Bad signals

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Low energy tailing: - wrong baseline restoration - incomplete charge collection High energy tailing: - wrong baseline restoration Peak broadening: - increased noise - incomplete charge collection Shifts/instability: - proportional: gain - constant: DC problems

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Spectral shape

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Photoelectric effect (1) The photon energy is transferred to an atomic electron K L e-e- h Cross section Energy photo-electron: Photo-electron

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Compton effect (1) h h ’ E e-  Energy conservation: Energy Compton electron: Angular correlation: Partial energy transfer to a ‘free’ electron 

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 h h ’ E e-  Angular distribution of the scattered photons Compton effect (3)

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Only if E  > 1022 keV Mainly if high Z Pair production Annihilation

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Interactions of photons

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Photon interacts with crystal: Absorption of all energy by photoelectric effect Absorption of all energy by multiple scattering and subsequent absorption by photoelectric effect

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Photon interacts with crystal: 3. Absorption of part of the energy of the photon due to scattering effects and escape of scattered photon from the crystal

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Photon interacts with crystal: 3. If energy of photon > 1022 keV: pair production effects: -All energy absorbed - One of the electrons escapes from the crystal: only E- 511 keV deposited - both scattered electrons escape from the crystal: E-1022 keV deposited

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Photon interacts with crystal: 4. Al low energies: All energy transferred to Ge electrons, but scattered Ge electrons escape from crystal: E-(Ge-k  ) keV deposited in crystal

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Coincidence effects

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Coincidence effects Coincidence count rate: R c = D.  1.  2 Random coincidence count rate: R r = 2. .R 1.R 2 R r = 2. .D 2.  1.  2  1,  2 : total counting efficiency (= full energy photopeak efficiency x peak/Total ratio) Relevant: True coincidence effects: Absolute efficiencies Random coincidence effects: Count rate + absolute efficiencies In all cases needed: peak-to-Total ratio

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Measuring the p/T curve Simple: using e.g. 65 Zn or 137 Cs Complex: modeling using MCNP

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Coincidence effects

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Compton suppression systems

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Compton suppression spectometer Timing Electronics Anti- coincidence Spectroscopy electronics Analogue to Digital converter Multi-channel pulse-height analyzer Annular scintillation detector shield with plug detector Sample

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Gamma-ray spectrum of 137 Cs, recorded with and without Compton suppression. Compton suppression spectrometer

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Compton suppression spectrometer

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Selecting a detector

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 P.Bode, J.Radioanal.Nucl.Chem. 222 (1997) 117-125 + 127-132

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Errors in gamma-ray spectroscopy - Source-to-detector distance - Filling height and source-self attenuation effects - Coincidence summing effects

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Steps to be taken for an NAA laboratory Vial with comparator Vial with sample Neutron fluence rate Estimation neutron fluence rate at sample position Not necessarily the arithmetic mean of the comparator values !

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Source-detector distance Source-end-cap distance e.g. R cm End-cap thickness + end-cap-crystal distance e.g 0.6 cm Distance to average point of complete absorption of photon energy e.g. 2.5 cm

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Steps to be taken for an NAA laboratory Don’t correct just for difference in distance to end-cap! Include distance to average interaction point inside the Ge crystal (typically at ~ 1.5-2.5 cm (depends on size of detector; add 0.5 cm crystal-end cap distance) sample standard 10 cm 0.5 cm 0.2 cm 0.5 cm 2.5 cm Correction for count rates: {10+ (0.5/2)+0.5 + 2.5} 2 /{10+(0.2/2)+0.5 + 2.5} 2 = 1.023 Assuming uncertainty in interaction depth 1 cm: correction is then 1.025 Uncertainty of correction is difference between these corrections, viz. 0.2 %

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Source-detector distance

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 The mot optimal choice

Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Gamma-Ray Spectroscopy Dr.Ir. Peter Bode Associate Professor Nuclear Science & Engineering.

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Summer Course Instrumental Neutron Activation Analysis July 7-18, 2008 Gamma-Ray Spectroscopy Dr.Ir. Peter Bode Associate Professor Nuclear Science & Engineering.

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