vol. 5 1/2016 Inżynier i Fizyk Medyczny
36
radioterapia
\
radiotherapy
artykuł naukowy
\
scientific paper
Secondary standard thimble chamber measurements
10cm set up
The centre of the chamber was positioned 10cm from the centre of the source.
21,00 ºC
752,00 mmHg
1,014
Dwell Position (cm) Current (nA)
144,0
7,0040
146,0
7,0110
148,0
7,0180
150,0
7,0190
152,0
7,0280
Sweet spot
154,0
7,0250
156,0
7,0240
158,0
7,0170
160,0
6,9910
162,0
6,9560
Sweet spot
152,00
All subsequent chamber measurements performed at 152
Measurements at 10cm distance at 152 dwell position
POSITION A
POSITION B
Ion recombination for Farmer chamber:
Measurement
Current (nA)
Measurement Current (nA)
Geometric mean:
12,00
1
7,042
1
7,137
7,09482146
Full voltage
200
2
7,046
2
7,149
Half voltage
100
12,30
3
7,046
3
7,15
Full readings
7,145
12,15
average
7,04
average
7,15
Half readings
7,107333 7,097 7,111 7,114
Ion recombination 1,001785
Time difference between well and Farmer chamber measurements =
1,00 hour
or
0,04167 days
Therefore, decay correction (Kdecay) =
0,9996
So, to correct Farmer readings to source strength at time of well chamber readings:
1,0004
10cm calibration factor for secondarystandard thimble chamber
AKR well = AKR Farmer
CF = chamber correction factor
ISL = inverse square law correction ebcause the reference is done at 1m but these measurements are at 10cm
= 3,32E+11
Ctp =
Temperature =
Atmospheric pressure =
ISL
f
K C M CF
elec
delay
TP
6,9500
6,9600
6,9700
6,9800
6,9900
7,0000
7,0100
7,0200
7,0300
7,0400
140,0
145,0
150,0
155,0
160,0
165,0
Reading (nA)
Dwell Position (cm)
Sweet Spot Determination
Rys. 3
cd.
Źródło: Opracowanie własne.
