Can you explain radiation, attenuation of X rays, Gamma rays, and Beta particles, and dosage and how it applies to your ACRYLITE® acrylic sheet?

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Can you explain radiation, attenuation of X rays, Gamma rays, and Beta particles, and dosage and how it applies to your ACRYLITE® acrylic sheet?

Radiation Attenuation of ACRYLITE® cast (GP) & ACRYLITE® extruded (FF) Acrylic Sheet

Diagnostic and therapeutic radiation is used extensively today in the medical community. Because of the broad range of uses for radiation, it is important to understand how it affects materials. What follows is a compilation of data characterizing the effects of electromagnetic (X, gamma) and particle (alpha, beta) radiation on ACRYLITE® sheet.

What is Radiation?

In medical and other scientific fields, radiation usually refers to X-rays generated by X-Ray tubes, gamma rays generated by radioactive isotopes of elements such as Cobalt 60 or Cesium 137, and alpha and beta rays generated by particle accelerators. The intensity of these rays is measured in electron volts (eV). An electron volt is a unit of energy equal to that of an electron falling unimpeded through a potential difference of one volt.

Radiation can be broken down into two basic types: particle and electromagnetic. Two types of particle radiation are alpha and beta particles. Alpha particles are identical to the nucleus of a helium atom, consisting of two protons and two neutrons; beta particles are electrons ejected from the nucleus of an atom. Electromagnetic radiation is any wavelength along the electromagnetic spectrum as shown in Figure 1. The radiation discussed here is of very short wavelengths, 0.001 nm to 100 nm, and include X-rays and gamma rays.

For medical and industrial purposes most radiation used is either beta particles, X-rays, or gamma rays. Alpha radiation is not used extensively in these applications.

Attenuation: X-Rays and Gamma Rays

In Table 1, the appropriate minimum thicknesses required to attenuate or reduce the energy intensity of X-rays and gamma rays from 10 keV to 20 MeV by 50%, 90% and 99% are given for unmodified polymethyl methacrylate (PMMA) which is the main ingredient in ACRYLITE® sheet. As can be seen from the tables, ACRYLITE® sheet is almost completely transparent to high intensity X and gamma radiation. To achieve the desired attenuation, several pieces of ACRYLITE® sheet may be combined to obtain the required thickness. Mass attenuation coefficients in the X and Gamma Ray Tables are taken from Dosimetry for Radiation Processing, Taylor & Francis, 1989, pp. 208-209.

Figure 1. Mass Attenuation Coefficients (µ/ρ) and Thicknesses

For Photon Energies 10 keV to 20 MeV

X Rays and Gamma Rays

MEV M/P
(CM2/G)
THICKNESS REQUIRED TO REDUCE RADIATION INTENSITY BY 50%   THICKNESS REQUIRED TO REDUCE RADIATION INTENSITY BY 90%   THICKNESS REQUIRED TO REDUCE RADIATION INTENSITY BY 99%  
    MM IN MM IN MM IN
0.01 3.2700 1.78 0.070 5.92 0.233 11.83 0.466
0.015 1.0800 5.39 0.212 17.92 0.705 35.83 1.411
0.02 0.5620 10.36 0.408 34.43 1.356 68.86 2.711
0.03 0.3010 19.35 0.762 64.28 2.530 128.57 5.062
0.04 0.2340 24.89 0.980 82.69 3.256 165.38 6.511
0.05 0.2070 28.14 1.108 93.48 3.680 186.95 7.360
0.06 0.1920 30.33 1.194 100.78 3.968 201.56 7.935
0.08 0.1750 33.28 1.310 110.57 4.353 221.14 8.706
0.10 0.1640 35.52 1.598 117.98 4.645 235.97 9.290
0.15 0.1460 39.90 1.571 132.50 5.218 265.06 10.435
0.20 0.1330 43.80 1.724 145.48 5.728 290.93 11.455
0.30 0.1150 50.65 1.990 168.26 6.624 336.51 13.248
0.40 0.1030 56.55 2.230 187.86 7.400 375.72 14.790
0.50 0.0941 61.90 2.440 205.63 8.100 411.25 16.190
0.60 0.0870 69.95 2.640 222.41 8.376 444.82 17.510
0.80 0.0764 76.24 3.000 253.27 9.970 506.53 19.940
1.00 0.0687 84.79 3.340 281.65 11.090 563.30 22.180

Dosage:X and Gamma Rays

An amount of radiation is usually referred to as a dose. There is a distinction between radiation exposure and a delivered, or absorbed, dose. For personal protection, it is the exposure that is of interest, whereas for biological or structural damage, the absorbed dose would be of concern.

In most cases, the roentgen is the unit used to measure exposure and the rad is the unit used to measure absorbed dose. These units are most prevalent today although a more general unit of radiation has been established, the Gray (Gy), which is independent of both the type of radiation and the irradiated material.

The following chart gives the relative radiation stability of some polymers. The radiation is measured in kiloGray (kGy).

Which to use, ACRYLITE® extruded (FF) or ACRYLITE® cast (GP)?

Although both ACRYLITE® cast (GP) and axtruded (FF) acrylic sheet can be used in applications where radiation is present, ACRYLITE® cast (GP) sheet is recommended because of its higher molecular weight and its availability in larger thicknesses. X-rays and gamma rays, as stated previously, break the molecular chains and cause degradation. The long molecular chains of ACRYLITE® cast (GP) sheet can withstand the effects of this degradation process longer than the shorter molecular chains of ACRYLITE® extruded (FF) sheet, under the same circumstances.

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