Energy determination
 

When main menu is displayed and "Strip dosimeter" selected, choose "Energy Determination" to access the sheet.

I. Introduction
 

The method for the evaluation of the energy of an electron beam is based on the ASTM Standard E 1649 - 94 appendixe X3.

For this specific procedure, a wedge of a given material should be irradiated with a strip of dosimeter inside. The determination of the characteristics of the depth dose curve i.e. Rp and R50 respectively, the practical range of the electrons and the half value depth, permits the calculation of the monoenergetic energy (E), the average energy (Ea) and the most probable energy (Ep) of the electron beam close to 10 MeV.

II. Energy calculation in broad energy spectrum condition in water

 

Ea (MeV) = 2,33 . R50 (cm) and (X3.10)
Ep (MeV) = 0,22 + 1,98 . Rp (cm) + 0,0025 . Rp 2 (cm) (X3.9)

As, in these empirically determined formula, the Rp and R50 parameters are given for water, and the dosimeter irradiated in a different material (m), another simple formula gives the relation between the absorption characteristics for water (w) and the material (m).

(X3.11)

where :
Table X3.1

Material

Density r (g/cm3)

ro for 10 MeV (CSDA range)

Aluminum

2,699

5,859

Polystyrene

1,060

5,155

Polyethylene

0,940

4,833

Water

1,000

4,963

III. Energy calculation in Aluminum

 

1. Experimental "Monoenergetic" condition :


The beam energy may be calculated for "monoenergetic" beams with the following empirical equation :
E(MeV) = 0.256 + 4.91 . Rp(cm) - 0.0248. Rp2 (cm) (X3.14)
Where Rp is the practical range of the electron in aluminum (given cm)


2. Broad energy spectrum condition :


If the Rp/R50 ratio is greater than 1.26 then the beam may not be monoenergetic. Empirical formula give the average and most probable energy of the electrons.
Ea (MeV) = 6.2 . R50 (cm) and (X3.19)
Ep (MeV) = 0.20 + 5.09 . Rp (cm) (X3.18)
Where : R50 is the half value depth in aluminum (given cm)
Rp is the practical range of the electron in aluminum (given cm)

VI. Energy calculation in Aluminum

 

Complete the heading as in the routine strip measurement. Some specific fields are explained here :

Wedge : Four wedge materials are available :

  • Water
  • Aluminum
  • Polystyrene
  • Polyethylene
Choose the wedge materiel used for the energy evaluation.

The Density (g/cm3), CSDA r0 g/cm2)and Angle (degrees) fields are automatically completed. Density and CSDA r0 values are fixed but angle value can be changed.

When the heading is complete, the software checks the communication between the PC, the spectrophotometer, and the motor.

Make sure that the motor is installed in the spectrometer. If not, install it. Make sure that for absorbance, the spectrometer is correctly zeroed without dosimeter but with the motor.

If, while the measurements the communication should be interrupted (RS232, Power supply ...) the Aer'ODE Software will detect the problem, save the data and advise the user.

The current wavelength and absorbance appear on the spectrophotometer’s screen.
A new sheet is displayed on the screen :



  • Input the strip dosimeter in the motor. (As written on the screen)
  • Click the measure button to start measurement.

The dosimeter strip should not be shorter than 15 cm, for this readout equipment. If the dosimeter strip stacks in the drive motor, then stop the measurement (Stop button), take out the strip (Motor Rewind button), cut properly the end of it and try again by a click on the measure button.

During measurement, the absorbance values are displayed on this window.



The end of the measurement is detected automatically when the absorbance value is near zero or stable for a period of time. It can be also decided by the user with a click on the stop button.
  • At the end of measurement, close the window (as written on the screen) by a click on the close button.
The specific absorbance, absorbed dose and uncertainty is automatically calculated and displayed in the table. (See annexe about uncertainty calculation).
The Min, Max and mean dose are displayed in the corresponding fields.
  • Click the Graphic button to display the graphic.

  • Click the energy button to calculate the energy of the beam. The corresponding fields are complete.
In order to make a precise energy measurement, the depth zero position must be set correctly. If a mark is made on the strip before irradiation, a dose peak will appear on the screen. Position this mark at depth zero then it can be used as the position of the entrance of the strip inside the wedge. Care should be taken also in comparing the real length of the dosimeter and the measured length. This can be easily done by making a second mark on the strip before irradiation and measuring by difference the length between the two marks. Should this comparison be out of a specific range (±2%) then the step length has to be updated in the parameter setting routine (See section Parameter Setting for more informations).

  • Click the Close button.

Ep (MeV) and Ea (MeV) fields are completed in the energy determination sheet.