1. Microdosimetric Distributions for a Mini-TEPC due to Photon Radiation
Elisabetta Gargioni, Bernd Grosswendt
Physikalisch-Technische Bundesanstalt, Braunschweig, Germany

2. Main characteristics of a mini-TEPC
They are reliable detectors which
can be used in unknown fields
can work in highly intense fluxes
can be calibrated to generally accepted radiation quantities

3. Characterization of a mini-TEPC
Use in clinical dosimetry
Appropriate field
Appropriate energy range

4. Simplified geometry of the Mini-TEPC developed at LNL
C3H8-TE gas, r = rgas
A-150, r = g / cm3
2.70 mm
1.31 mm
Rexolite, r = g / cm3
Titanium, r = g / cm3

5. Current Monte Carlo study
Determination of pulse-height and microdosimetric spectra due to
250-kV X-rays, 137Cs, 60Co

6. Spectral distribution of 250-kV X-rays
100
150
200
250
300
5000
10000
15000 N ( E )
/ keV
(Courtesy of Gianfranco Gualdrini, ENEA, Bologna)

7. MC code: “MINI-TEPC.FOR" (developed by B. Grosswendt)
event-by-event simulation for electrons
ad-hoc geometry specification
ad-hoc application of variance
reduction techniques

8. Detector model - first approximation1 3 2 4 r
rgas
simplified geometry with one material (propane-TE)
scaling according to the density

9. Main structure yes no Data INPUT Database reading for CS-data Data
OUTPUT
yes
N =NMAX? no
START NMAX photons
interaction
Management of
e- transport,
possible var. red. tech.
photon transport
→ secondary e- (IZAHL)
Analysis of cluster data
Simulation of e- transport
start of e- transport

10. Effect of material approximation on photon physics10 -3 -2 -1 1 2 3 4
μ(E)tot/ρ in cm2/g
E / MeV
Titanium
Propane based TE-Gas
Rexolite
A-150

11. Effect of material approximation on electron physics
Titanium
Propane based TE-Gas
Rexolite
A-150 10 -3 -2 -1 1 2
2x10
S / ρ in MeV cm2/g-1
E / MeV

12. Electron CSDA-range in propane-based TE-gas
need of variance-reduction techniques! 10 -7 -6 -5 -4 -3 -2 -1 1 2 3 4
E / keV
(Rρ) in g/cm2
D = 2 mm

13. total particle weight is exactly conserved in splitting
Electron splitting
I = importance
r = IB / IA
n = integer ( r )
Volume B B IB A
IB > IA
Volume A IA
if n>1, split into n particles of weight (wgt / n)
total particle weight is exactly conserved in splitting

14. total particle weight is only statistically conserved
Russian roulette
If r < 1 play Russian roulette:
Volume B B
with probability r, keep the particle and alter its weight to (wgt / r) ? IB A
IB < IA
Volume A IA
with probability (1-r), kill the particle
 weight set to 0
total particle weight is only statistically conserved

15. Truncation
Volume B
energy? B A
Volume A
Region of interest
energy?
Remove particle from part of phase space that do not contribute to the final results

16. ST = total scattering cross section
Forced collisions
Particles entering specified cells are split into
collided and uncollided parts:
wgt × e-ΣTd d
wgt × (1-e-ΣTd)
For distance-to-boundary d:
probability(no-collision) = exp(-ST)d
probability(collision) = 1-exp(-ST)d
ST = total scattering cross section

17. Sampling the flight distance in forced collisions
if s is the flight distance and d the max. flight distance: ) ( 1 s F e f T d S = - ) ( s F = x ( ) { } T d e s S - = x 1 ln

18. Secondary electron distribution for 60Co-photons
Greatest contribution from the wall! 1 2 4 3 10 -2 -1 1 2 3
-10 -9 -8 -7 -6 -5 -4 -3
T / keV
ΔN / ΔE in keV-1
Auger e-
IBODY=1
IBODY=2
IBODY=3
IBODY=4

19. Pulse-height spectrum for 60Co-photons in the gas cavity
electrons
deposited energy 10 -2 -1 1 2 3
-10 -9 -8 -7 -6 -5 -4 -3
ΔN / ΔE in keV-1
T / keV

20. Secondary electron distribution for 250-kV photons
Greatest contribution from the wall! 1 2 4 3 10 -3 -2 -1 1 2 3
-10 -9 -8 -7 -6 -5 -4
ΔN / ΔE in keV-1
T / keV
Auger e-
IBODY=1
IBODY=2
IBODY=3
IBODY=4

21. Pulse-height spectrum for 250-kV photons in the gas cavity10 -2 -1 1 2 3
-10 -9 -8 -7 -6 -5 -4 -3
ΔN / ΔE in keV-1
T / keV
electrons
deposited energy

22. Cluster-size distribution for 60Co-photons10 1 2 3 -9 -8 -7 -6 -5 -4 -3 Pν
cluster size ν

23. Cluster-size distribution for 250-kV photons10 1 2 3
-11
-10 -9 -8 -7 -6 -5 Pν
cluster size ν

24. “Microdosimetric” spectrum
Mean cluster size: å = 1 n P M W: n E W D =
ΔEν = energy absorbed for generating an ionization cluster of size ν

25. Microdosimetric spectra for Dequiv = 2 mm
250 kV x-ray
137 Cesium
60 Cobalt 10 -2 -1 1
3x10
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
fit ν 2 Pν / M1
νW / D in keV/μm

26. Microdosimetric spectra at different Dequiv10 -2 -1 1 2
0.0
0.1
0.2
0.3
0.4
0.5
fit ν 2 Pν / M1
νW / D in keV/µm

27. With / without variance reduction techniques: test10 1 2 3
-10 -9 -8 -7 -6 -5 -4 Pν
cluster size ν
Russian roulette and electron splitting (106 photon)
without variance reduction techniques (107 photon)

28. Future work
Material definition
 CS data!
Geometry improvement
(10B, twin cavities, …)
Coupling with MCNPX

29. Coupling with MCNPX PTRAC MCNPX: Condensed history down to 1 keV
use of CS-data and other capabilities
PTRAC
Output of particle-track data
mini-tepc:
Event-by-event
down to ~10 eV

30. Test calculation - I 60Co, energy deposited in the gas cavity10 -6 -5 -4 -3 -2
5x10
-11
-10 -9 -8 -7
dN / dE in MeV-1
E / MeV
mini-tepc
mcnpx

31. Test calculation - II RX 250 kV, energy deposited in the gas cavity10 -6 -5 -4 -3 -2
5x10
-12
-11
-10 -9 -8 -7
dN / dE in MeV-1 -1
E / MeV
mini-tepc
mcnpx

32. Thank you for your attention!

33. Concept of cluster size distribution
Pn(T) is the probability of forming an ionization cluster of size n
Definition: The cluster size is exactly the number ν of ionizations produced by a particle in a specified piece of matter