the contribution from 12C∗ projectiles with different flight angles can be obtained if the stopping power of the target medium is exactly know. In other words, the functionG(ϑ, ϕ) can be converted to a convolution functionC(Eγ, Eγ0)on the γ–energy axis: The problem is, however, that the function
g(u) = m τ S
is not known at this stage as the stopping powerS is just the quantity to be determined.
Although C(Eγ, Eγ0) could be determined iteratively, it was approximately determined using the stopping power of water for carbon ions calculated by means of SRIM2013 [99].
As the termC(Eγ, Eγ0) itself is a correction term, the effect of the approximation on the uncertainty of the stopping power used in this approach is only of second order.
Once the function C(Eγ, Eγ0) is known, the measured attenuated γ–energy spectra
d ˜Na/dEγ can be expressed by:
As theγ–energy spectra were measured in discrete channels, equation 6.2 can be expressed by
yu =MA
B ·xu (6.5)
where the vectors xu and y
u represent dNu/dEγ and d ˜Nu/dEγ, respectively. The matrix MA
B =MT
A×M
B contains the matrices M
A andM
B which stand for the system response functionA(Eγ−Eγ0)and the angular resolution functionB(Eγ, Eγ0)of the detector system, respectively.
Analogous to the unattenuatedγ–energy spectrum, the vectorial form of equation 6.4 can be written as:
ya =MA
C·xa, (6.6)
where ya and xa are the discrete representation of the measured attenuated γ–energy spectrumd ˜Na/dEγ and of the true attenuatedγ–energy spectrumdNa/dEγ, respectively.
If the matricesMA
B andMA
Cwere regular so that their inverses exist, the trueγ–energy spectra could be obtained by:
xu = However, these matrices are usually not regular and cannot be inverted uniquely. The determination of the so–called pseudo–inverse often leads to singularities and therefore to large errors when involving high–dimensional matrices.
For the deconvolution of spectra involving a matrix with a deficient rank, there are several approaches available. One approach is the boosted Gold–Deconvolution which is based on the algorithm of Raymond Gold published in 1964 [36]. The resulting vector y of a measurement can be written as:
y =M x , (6.8)
where x represents the true spectrum and M is the matrix representation of the system response.
The boosted Gold–deconvolution technique is an iterative method where the vector elements of the k–th iteration can be calculated according to [36]:
x(k+1)i = yi0
Another common method is the minimisation ofχ2 whereχfor thek–iteration is given by [8]:
χ(k)=y−M ·x(k). (6.11)
The minimisation of χ2 can be solved by an iterative method:
x(k+1) =x(k)− ∇f(x(k)), (6.12)
where ∇f(x(k))is the gradient at the k–th iteration:
∇f(x(k)) =−2M y−x(k)
. (6.13)
The iteration process is terminated if the minimum of χ2 is reached.
Due to the uncertainties of the experimental data, it is not reasonable to continue the iteration until χ2 has reached the absolute minimum. Instead, the iteration process is
6.3 DECONVOLUTION 65 usually terminated when the iteration converges to χr= 1 that is defined by
χr= χ2
N −1 +u (6.14)
with the number N of the data points and the degree of freedom u.
Chapter 7
Results and discussion
In the following, the determination of the stopping power of water for carbon ions in the energy interval between1 MeV and6 MeVusing the aforementioned methods and data is explained and presented.
The subsequent analysis of the uncertainty using analytical methods as well as Monte–
Carlo based approaches is given in detail with a listing of the overall uncertainty of the stopping power including the individual contributions.
Furthermore, the obtained stopping power including the determined uncertainty is compared to available data, semi–empirical formulas and theory. The discrepancy in the data between the stopping cross section of water and that of water vapour is investigated on the basis of the mean excitation energy and the mean charge state of the projectile.
7.1 Stopping power
Analogous to section 6.1 and section 6.2, equation 2.25 can be written in its vectorial form:
xa =D·xu, (7.1)
where xa and xu are the true attenuated and unattenuated γ–energy spectrum, respec-tively. The elements of the matrixD are given by:
dij = m τ S(vi)exp
vi
ˆ
vj
m τ S(v0)dv0
(7.2)
and contain the unknown stopping power S. As an analytical solution of equation 7.1 is infeasible, the stopping power was determined numerically by solving equation 7.1 by means of minimisation of χ2. For this purpose the stopping power is represented as an energy dependent function with a set of parameters. The parameters were varied untilχ2 reaches the minimum.
67
For the representation of the stopping power, the approach of Paul and Shinner [66, 67]
was employed. They used a parametrized Weibull function [60] and the stopping power for helium [45] to fit experimental data for heavier ions:
S=yw(x(T, ζ)Zp2
ZHe2 SHe. (7.3)
The function yw is given by yw(T, ζ) = ζ1+ (1.01−ζ1) where T is the kinetic energy of the projectile in MeV and Ap is the mass number of the projectile. ζ1, ζ2, and ζ3 are free parameters for the fit function. The quantity κ depends on the atomic number Zp of the projectile and is given by κ = 17.18−0.657Zp. The functionyw can be considered as a model for the energy dependent effective charge of the projectile Zeff =f(T) and was taken from references [69, 70].
As in the case of the unfolding procedure, the iteration in solving equation 7.1 was stopped if the square of the difference between the calculated and measured γ–energy spectrum converged to a minimum. In the ideal case, the relative χ2 is then close to 1.
The degree of freedomuin equation 6.14 is given by the number of the free parameters.
The global minimum ofχr was determined by varying the values of the three parameters ζ1, ζ2 and ζ3 over a range that results in physically reasonable energy dependence of the stopping power.
Eγ
4460 4480 4500 4520 4540 4560 4580
-1/ keVγE/ daNd
4350 4400 4450 4500 4550 4600
-1/ keVγE/ daN~d
Figure 7.1: (a) Best fit (−) of the attenuatedγ–energy spectrum in comparison to the measured attenuated γ–energy spectrum (◦) restricted to the energy interval defined by the kinematics.
(b) Calculated attenuatedγ–energy spectrum convoluted with the energy and angular resolution function of the detector in comparison to the measured attenuated γ–energy spectrum.
Due to lateral straggling, the minimisation process has to be restricted to carbon
pro-7.1 STOPPING POWER 69 jectile energies higher than that of the lower kinematic boundary. Figure 7.1(a) shows the γ–energy spectra with restriction to the energy interval defined by the kinematics which was obtained after the minimisation of χ2 using the carbon projectile energy restriction of T ≥ 1 MeV. The convolution of the calculated attenuated γ–energy spectrum with the energy and angular resolution function of the detector is shown in figure 7.1(b). The measured attenuated γ–energy spectrum was corrected for the background as described in section 5.1.
Figure 7.2 shows the stopping power determined in this work along with the data predicted by SRIM2013 [99], MSTAR [68], CASP [82] and as recommended by the ICRU report 73 [11].
/ MeV T
2 2.5 3 3.5 4 4.5 5 5.5 6
mµ / keV / S
500 550 600 650 700 750 800 850 900 950 1000
This work MSTAR ICRU 73 errata SRIM2013 CASP
Figure 7.2: Stopping power of water for carbon ions determined in the present work in com-parison to the stopping power as recommended by the ICRU [11], calculated with MSTAR [68], calculated with CASP [82] and predicted by SRIM2013 [99] for water.