Problem P7 Stéphanie Ménard Dosimetry Department 92262 Fontenay-aux aux-roses FRANCE
What are the applications of Gamma-Ray Spectrometry in Radiological Protection and in Safety? In the environment: after accidental releases of radionuclides (contamination), for surveys over large areas In the control of nuclear materials In workplaces in the nuclear fuel cycle
What do we measure with a spectrometer? Pulse height distributions => Energy Peak count rates => Fluence
What physical information is essential? To determine radionuclide activity levels The activities are derived from measured full-energy peak count rates Knowledge of the detector peak response is essential but complex: Peak response depends on the photon energy and angle of incidence Assumptions are made to make measurements in the environment
What physical information is essential? To calculate dose quantities These quantities are derived from the energy distribution of incident radiation field To determine the energy spectrum: response matrix is needed Unfolding method is used
How to determine the influence of parameters on the response? Two methods Experimental method Numerical method Limits of the methods - experimental : irradiation conditions (energy, angle), cost - Numerical : assumptions on the geometry, detector noise
Example of a GeHP detector simulated by A.L. Weber (/DSMR/SATE) -Ray photograph of Ge 2D plot
Description of the geometry - Al cryostat Al holder γ Ge detector (core) Ge dead layer Be window
Proposed Tasks 1) Determination of peak efficiencies and PHD at 8 energies ( 15, 30, 60, 100, 250, 500, 750, 1000 kev) 2) Estimation of the influence of the following parameters: - dead layer thickness - Source distance - Angle 2 3) Estimation of the influence of the Al holder (optional) 4) Influence of the incidence angle on PHD (optional)
Participant Task 1 Task 2 Task 3 Task 4 Comment (optional) (optional) P7-A Doppler B. P7-B (peak) P7-C Doppler (peak) (peak) Broadening P7-D P7-E P7-F P7-G P7-H Doppler B. P7-I
Participant Task 1 Task 2 Task 3 Task 4 Comment (optional) (optional) P7-J P7-K (peak) (peak) P7-L (peak) P7-M P7-N P7-O P7-P Doppler B. P7-Q (peak) P7-R
Monte Carlo Codes used MCNP 4B, 4C, 4C2, 4C3 : P7-B, P7-D, P7-E, P7-F, P7-I, P7-L, P7-N, P7-O, P7-Q, P7-R MCNP 2.4.j : P7-G EGS4-UCDOD : P7-A EGS4 + KEK improvement: P7-H EGSNrc (v2): P7-M GEANT 3: P7-K PENELOPE v2001: P7-J Beta version of MCNP 5 : P7-P
Method used to analyze the results How to compare pulse height distributions? 100keV, Normal Pulse Height Dist rib. / source part icle 10-2 10-3 10-4 10-5 10-6 10-7 10-8 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 Energy (MeV)
Method used to analyze the results Pulse height distribution divided into several areas Energy limits of the areas depend on the incident photon energy
Method used to analyze the results Counts per source particle in the energy binnings of these areas are summed The results of the sums are compared to the author s sums. The number of areas depend on the photon energy ( ex: 6 areas at 30 kev, 9 at 100 kev, 5 at 1 MeV) 6 areas for 30 kev: 1-4.2,4.2-9,9-14.8,14.8-22.4,22.4-29 and 29-31.6 kev
Task 1 Determination of peak efficiencies and pulse height distributions at 8 energies (15, 30, 60, 100, 250, 500, 750, 1000 kev). Peak efficiency is defined as the number of events in the full-energy peak per emitted particle
Task 1: Ratios of the 9 areas obtained for photons of 100 kev P7- A P7-C P7- D P7- E P7-F P7-G P7-H Ratio (participant/author) 1.2 1.1 1 0.9 0.8 En (kev) 0 20 40 60 80 100
Task 1: Ratios of the 9 areas obtained for photons of 100 kev P7-A and P7-H: The effect of Doppler Broadening to response is simulated. The version of MCNP ( 4C2) used by the author does not take into account this effect. So the ratios obtained for the region 29.2-44 kev are about 3. Results corresponding to Compton edge area are lower than the author s result. This effect is explained in the presentation of Dr Namito. The participant P7-C presents this effect in his report but his results are without Doppler Broadening
Doppler Broadening effect This effect changes the shape of pulse height distribution in these 3 Areas.
Task 1:100 kev P7-I P7-J P7-K P7-L P7-M P7-N P7-R Ratio (participant/author) 1.25 1.2 1.15 1.1 1.05 1 0.95 0.9 0.85 0.8 0.75 0 20 40 60 80 100 En (kev)
Task 1: 100 kev 5 P7-B P7-D P7-E P7-H P7-Q Ratio (participant/author) 4 3 2 1 0 0 20 40 60 80 100 En (kev)
Task 1: Ratios of the 9 areas obtained for photons of 100 kev P7-B: The source used is not isotropic and it is an annular ring source. P7-D: The participant simulated the 8 energies in 1 simulation using specific options of MCNP. But the results obtained were not multiplied by this factor 8. P7-E: MCNP code use default parameters for a surface source. The source emission is not isotropic P7-H: This participant forgot to normalize the results ( source emission in an hemi-sphere) P7-D, P7-E and P7-H computed simulations without these errors: their results are presented on the other figures
Task 1: Do the results depend on incident E? Ratio (participant/author) 1.1 1.09 1.08 1.07 1.06 1.05 1.04 1.03 1.02 1.01 1 0.99 0.98 0.97 0.96 0.95 0 100 200 300 400 500 600 700 800 Energy: 750 kev P7-A P7-C P7-D P7-E P7-F P7-G P7-H En (kev) 5 areas: 2-168, 168-332, 332-488, 488-495, 495-504 kev Results are close Effect of Doppler Broadening: smaller
Do the results depend on incident E? Energy : 30 kev At 30 kev, results are close only for the last area Ratio (participant/author) 1.2 1.1 1 0.9 0.8 P7-A P7-C P7-D P7-E P7-F P7-H 0 10 20 30 En (kev)
Task 1: Do the results depend on incident E? Ratio (participant/author) 1.1 1.09 1.08 1.07 1.06 1.05 1.04 1.03 1.02 1.01 1 0.99 0.98 0.97 0.96 0.95 0 100 200 300 400 500 600 700 800 Energy: 750 kev P7-I P7-J P7-K P7-L P7-M P7-N P7-R En (kev) Ratio (participant/author) 1.2 1.1 1 0.9 0.8 Energy : 30 kev P7-I P7-J P7-K P7-L P7-M P7-N 0 10 20 30 En (kev)
Task 1: Results relative to incident E? Ratio (participant/author) 4 3.5 3 2.5 2 1.5 1 Energy: 750 kev P7-B P7-D P7-E P7-H P7-M P7-P P7-Q Ratio (participant/author) 5 4 3 2 Energy: 30 KeV P7-B P7-D P7-E P7-H P7-P P7-Q 0.5 1 0 0 100 200 300 400 500 600 700 800 En (kev) 0 0 10 20 30 En (kev)
Task 1: Results relative to incident E? Energy: 750 kev Energy: 30 KeV 10 0 10 0 Ratio (participant/author) 10-1 10-2 10-3 P7-B P7-D P7-E P7-H P7-M P7-0 P7-P P7-Q Ratio (participant/author) 10-1 10-2 10-3 P7-B P7-D P7-E P7-H P7-O P7-P P7-Q 10-4 0 100 200 300 400 500 600 700 800 En (kev) 0 10 20 30 En (kev)
Task 1: peak efficiencies Ratio (participant/author) 1.04 1.02 1 0.98 0.96 0.94 0.92 P7-A P7-C P7-D P7-E P7-F P7-G P7-H Ratio (participant/author) 1.06 1.04 1.02 1 0.98 0.96 P7-I P7-J P7-K P7-L P7-N P7-R 0.9 10 1 10 2 10 3 Photon energy (kev) 0.94 10 1 10 2 10 3 Photon energy (kev)
Task 1: peak efficiencies 4 3.5 10 0 thor) Ratio (participant/au 3 2.5 2 1.5 1 P7-B P7-D P7-E P7-H P7-M P7-P P7-Q uthor) Ratio (participant/a 10-1 10-2 10-3 P7-B P7-D P7-E P7-H P7-M P7-0 P7-P P7-Q 0.5 10-4 0 10 1 10 2 10 3 Photon energy (kev) 10 1 10 2 10 3 Photon energy (kev)
Task 1: Peak efficiencies P7-O: There is a problem with the description of the source (volume source and emission ) P7-P: this participant made an error on the distance between the detector and the source: 20 cm instead of 2 cm P7-Q: there is a little shift of energy on the PHD of the participant at 100 kev
What can we learn from this? Differences observed with the variation of incident E in the areas can be explained by physics model inside Monte Carlo codes = > Doppler Broadening feature ( presentation of Y. Namito) = > Author s results without this effect Factors of under-estimation or over-estimation are due to the source distribution ( annular ring, parallel beam, distance, volume source )
Task 2 Estimation of the influence of the following parameters: - dead layer thickness ( +/- 50%, 50 and 150 µm) - Source distance (+/- 50%, 1.95 and 2.05 cm) - Angle uncertainty (2 ) The influence of the dead layer thickness ( +/- 50%, 50 and 150 µm) depend on incident energy. At lower energies, this variation could give a factor 30. The presentation of Dr Achouri will focus on this point.
Task 2: Influence of the dead layer thickness on the peak efficiencies 14 0.07 encies areas Ratio of the peak effici 10 1 Influence of DL: 50/100 Influence of DL : 150/100 10 0 10-1 1.5 % 10 1 10 2 10 3 Photon energy ( kev )
Task2: dead layer thickness uncertainty Dead layer uncertainty: - 50 % Dead layer : + 50 % Ratio (participant/author) 1.05 1 0.95 P7-A P7-C P7-D P7-E P7-H P7-I P7-N P7-R Ratio (participant/author) 1.05 1 0.95 0.9 0.85 P7-A P7-C P7-D P7-E P7-H P7-I P7-N P7-R 10 1 10 2 10 3 Photon energy (kev) 0.8 10 1 10 2 10 3 Photon energy (kev)
Task 2: Influence of the distance between the source and the Ge detector 1.04 3% on peak efficiencies Ratio of the peak efficiencies areas 1.03 1.02 1.01 1 0.99 0.98 Influence of distance: 1.95 cm/ 2.00 cm Influence of distance: 2.05 cm / 2cm 0.97 10 1 10 2 10 3 Photon energy ( kev )
Task 2: source distance uncertainty 1.3 Distance: 1.95 cm 1.1 Distance: 2.05 cm 1.25 Ratio (participant/author) 1.2 1.15 1.1 1.05 1 P7-A P7-C P7-D P7-E P7-H P7-N P7-R Ratio (participant/author) 1.05 1 0.95 0.9 0.85 P7-A P7-C P7-D P7-E P7-H P7-N P7-R 0.95 0.9 10 1 10 2 10 3 Photon energy (kev) 0.8 10 1 10 2 10 3 Photon energy (kev)
Task 2: angle uncertainty 1.1 Angle uncertainty: 2 thor) Ratio (participant/au 1.05 1 0.95 P7-A P7-C P7-H P7-N 0.9 10 1 10 2 10 3 Photon energy (kev)
Task 2 Results of the participants compared to the author s results have the same trends than the results of task 1.
Task 3 (optional) Influence of the Al Holder Any change on peak efficiencies P7- A P7-C P7-D P7-E P7-N
Influence of the Al Holder at 100 kev 1.1 Without Al Ratio (without Al Holder/with Al Holder) 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Factor 5 0 0 20 40 60 80 100 En (kev)
Task 3 (optional) Important influence of the Al Holder observed only in the computations without Doppler broadening
What can we learn from this problem? Factors of under-estimation or over-estimation are mainly due to the source distribution ( annular ring, parallel beam, distance, volume source ) Users must be aware of that. At lower energies, physics model of the code could introduce important factors of under-estimation of numerical PHD compared to experimental ones.
Check-list for a Ge detector simulation Be careful of the dead layers thickness, Al holder, Source ( incident angle, distance ) Window Physics at lower energies