Chapter 4 Simulation and Discussion
4.2 Modification
4.2.4 Predictions by this model
What if the stimulated light source changes from Green light to UV light?
There suppose to have possibilities that UV light could pump deep trap electron n2. Therefore, using UV light as a stimulated light source not only pump shallow trap electrons (dosimetric trap n1), but also pump deep trap n2 as well, as shown in Fig. 4-13.
For UV light source has different impact factors for dosimetric trap and deep trap, an impact factor “d” was set to deal with this question. Impact factor “d” was set from zero to one. When “d” is zero or one that means stimulated light source can only pump electrons in dosimetric trap or deep trap respectively. Therefore, OSL intensity only attribute to one trap state. When “d” is equal to “0.5”, in the middle of zero and one, which means stimulated light source can pump electrons in both traps and it is a pumping competition. Therefore, stimulating light source impact factor f1 and f2 (in the Figure 3-24) are set (-f)(1-d) and (-f)(d), in the Eq. 3-13.&Eq. 3-14. “-f” is correlated with stimulating light source and “d” is associated with one trap stimulated or two trap stimulated.
The simulation results are shown in Fig. 4-14. When “d=0%”, the result is again back to R. Chen’s paper (or Fig. 4-4). When “d=50%”, two trap stimulated competition occur, the OSL intensity is still similar to one trap stimulation. This is an important prediction for POSL using UV light source. Because POSL only consume little amount of
Valence Band
Fig. 4-13. Two trap stimulated competition band diagram [30].
4.2 Modification
electrons in the traps and two traps concentration will not run out completely, thus pumping probabilities will follow with two trap concentration ratio condition. In 2009, We proposed a quantum selection rule for OSL and indicated pumping probabilities depend on two traps energy level ratio with concentration ratio [30]. For long time stimulation, the pumping probability of two traps should reduce to 1:1 which is the same function in this simulation “d=50%”. In the extreme case “d=100%”, shallow trap ran out of electron concentration and only deep trap can do the function. In reality, this phenomenon can only take place in the pre-bleach process. Dosimetric trap (shallow trap) had been already bleached out by pre-heat or optical bleach before UV-Pulse OSL measurement.
According to this mathematical model prediction, pre-bleach shallows process seems to enlarge linear dose dependence from 10Gy to 50 Gy.
0 20 40 60 80 100
-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Relative OSL Intensity (104 count)
Dose (Gy)
d = 0%
d = 50%
d = 80%
d = 90%
d = 100%
Fig. 4-14. OSL intensity for two traps stimulates competition.
Chapter 5 Conclusions
Chapter 5 Conclusions
Although maximum precision measurement of Landauer POSL Reader with Luxel Al2O3: C badge is 10Gy, the high dose exceeds 10Gy measurement still can finish roughly by the decay slope of optical bleach process and standard deviation calculation in our experiment (Figure 5-1). We also find out the modification model to explain this bleach decay phenomena successfully. Those two methods enlarge the estimate scale for the Landauer reader with Luxel Al2O3: C badge in the case of nuclear event or nuclear emergency.
Suspected Badge
Fig. 5-1 The proposed flowchart to distinguish the over dose and under safety margin situation.
Chapter 5 Conclusions
Using the UV lamp as bleach light source has not only bleach function, but also pump effect [20]. Therefore, we use commercial visible light source with UV filter instead of UV lamp to avoid pump effect [16]. Moreover, in our optical bleach simulation, the saturation point with UV filter and without UV filter is different. These simulations totally match experimental evidence. That is the saturation point for maximum bleach could be attributed to balance of the bleach effect and pump effect.
So that we could make a prediction that the higher energy photon incidents will cause the pumping effect and bleaching effect simultaneously. When the competition approaches the equilibrium, the higher residue of the electrons will be staying the trapped level so that we could make a fine tune to set the electrons quantitatively trapped in the desired energy level.
When F-center accepts 206nm UV light, it will convert back to F+ center (F-center + 206 nm Æ F+-center + e-) [20]. Therefore, we assumed that F-center converts to F+-center (F-center + high energy (beta-rays) Æ F+-center + e-) should occur in Luxel Al2O3: C badges. This assumption successfully describes the phenomena of OSL intensity saturation in high dose in our simulation.
By modified simulation result, changing light source energy from green LED to higher energy light UV lamp in POSL system seems no apparent signal change and maintains its original OSL curve. Therefore, using two trap model by changing light source energy seems provide another idea for new type of OSL reader. Moreover, according to simulation result in UV stimulation, clean shallow trap electron concentration left deep trap dose the function seems to enlarge linearity dose dependence from 10Gy to 60Gy in Al2O3: C. This phenomena provided us an idea in deep trap usage to retrospect nuclear emergency event.
Chapter 5 Conclusions
In the future, a prototype UV-light-source OSL reader should be set up to verify the simulation result of two traps competition model (as shown in Fig). For further study, the flexible light source design in Fig. 5-2 can do the OSL light source modulation experiment. Meanwhile, a preliminary medical image reconstruction experiment can use the optical fiber design. Hoping those works will be helpful for evolution in radiation dosimetry technique in Taiwan.
Light Source
This module could be designed in reflection mode.
This module could be designed in reflection mode.
This module could be designed in reflection mode.
This module could be designed in reflection mode.
Transmission mode Grating
Transmitting Mode
Fig. 5-2. Schematic diagram of prototype design for UV-light-source OSL reader.
References
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Appendix I
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Appendix I
Number Dose (Gy) Case Serial#
1 0 XA01131183T
2 500 XA00302618L
3 background XA00805731N
4 background XA008379254
5 background XA00000570J
6 1 XA008375385
7 1 XA000695848
8 1 XA008638270
9 5 XA008352888
10 5 XA00836603H
11 5 XA008389609
12 10 XA00227099F
13 10 XA00691630F
14 10 XA00867925Y
15 50 XA00061705I
16 50 XA00838950A
17 50 XA00031544Q
18 100 XA00867979L
19 100 XA00254080V
20 100 XA008685817
21 background XA008077692
22 background XA00834036M
23 background XA00669640C
Appendix II
Appendix II
% Bombard stage
function dy = vdp1000(t,y)
dm1=-3000*m1*nc+5000*(800000-m1)* nv+25*m2*nc;
dm2=-5*m2*nc+400*(240000-m2 )* nv-25*m2*nc;
dn1=800*(70000-n1)* nc;
dn2=200*(30000-n2)* nc;
dnv=17000-400*(240000-m2) * nv - 1000*(1000000-m1 )*nv ; dnc=dm1+dm2+dnv-dn1-dn2;
% relaxion balance (Thermal equilibrium) function dx = vdp2(t,x)
dm1=-3000*m1*nc+5000*(800000-m1 )*nv;
dm2=-5*m2*nc+400*(240000-m2) * nv;
dn1=800*(70000-n1)* nc;
dn2=200*(30000-n2)* nc;
dnv=-400*(240000-m2) * nv - 1000*(1000000-m1 )*nv ; dnc=dm1+dm2+dnv-dn1-dn2;
% OSL Read
dm1=-5000*m1*nc;
dm2=-5*m2*nc;
dn1=(-f)*(1-d)*n1+2000*(7000-n1)*nc;
dn2=(-f)*d*n2+200*(20000-n2)*nc;
dnv=0;
dnc=dm1+dm2-dn1-dn2;
% bleach
function dz = bleach(t,z) dm1=-4000*m1*nc;
dm2=-4000*m2*nc;
dn1=7000*(-VB)*n1+2000*(7000-n1)*nc;
dn2=200*(20000-n2)*nc;
dnv=0;
dnc=dm1+dm2-dn1-dn2;