Radiation Dosimeters for Foods. Ashish Anand Department of Biological and Agricultural Engineering Texas A&M University

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1 Radiation Dosimeters for Foods Ashish Anand Department of Biological and Agricultural Engineering Texas A&M University

2 Introduction Interest in food irradiation technology is increasing world-wide. Some countries are using it as a commercial process for food processing.

3 Advantages of Irradiation It can serve following purposes- Reduce food losses during post harvest. Can control causes of food borne diseases. Can reduce food spoilage.

4 Why do we need dosimeters? 1. To comply statuary regulations. 2. To ensure food quality. 3. To maintain the standards for food safety. 4. To optimize and design process.

5 Theory of Radiation Dosimetry Radiation field can be described by the average number of rays per unit area, per unit time at each Point. The rate of energy deposition in a volume element of the material is given by following expression: de 1 D' = ϕ dx ρ ϕ = Fluence rate; (-de/dx)1/ρ= Mass stopping power of the material.

6 Dosimetry Methods Following methods are used for dosimetry 1. Ionization method 2. Chemical methods 3. Calorimetric methods 4. Gamma dosimetry 5. Beta dosimetry 6. Neutron dosimetry

7 Ionization Method This is most widely used method and it uses ion pairs to estimate dose. Number of ion pairs produced is given by I 1 1 = d 1 ε n ( E) w ( ε ) where B is the lower limit of energy loss and w 1 (ε 1 ) is the energy required for a particle at energy ε to produce an ion pair. 1 1 ε

8 Chemical Methods In some systems the chemical composition is changed by the absorbed radiation. The observed chemical change(y) can be given by Y = ε1 n1 ε1, E) G1( ε1) d 1 ( ε where G 1 (ε 1 ) is the yield per unit absorbed. If G is independent of particle type and ε 1 then Y= G*ε T

9 Calorimetric Methods In some cases the radiation energy absorbed in the dosimeter in the dosimeter changes into thermal energy and raises the dosimeter temperature. The temperature change is given by 1 T = ε1 n1( ε1, E) F1 ( ε1) dε1 c where ε 1 n 1 (ε 1, E) dε 1 is the amount of energy absorbed in a unit mass F 1 (ε 1 ) is the fraction of charged particle energy that is degraded to heat c is the thermal capacity of the substance F (ε 1 ) is approximately constant near unity so that ΣT = εt/c

10 Gamma dosimetry In case of gamma- rays for the point source dose, if dose rate is defined as the energy absorbed per unit volume, then it is given by D ' = µi( E, r) where I (E, r) is the flux density of energy E at a distance r from the point source. If the point source strength is S then I( E, r) = S 4πr When no attenuation in the surrounding material is assumed. With attenuation the flux is S I ( E, r) = exp( µ r) 2 4πr 2

11 Cont., If the source strength C is expressed in curie then S= 3.7*10^10 CE and the dose rate: µ CE MeV D' = exp( µ r) 2 3 r cm s The total dose is obtained by the time integration of the dose rate: D= D'dt or if the dose rate is constant D= D't.

12 Beta dosimetry There are several methods of calculation of beta dose and different applications of these methods according to the different source geometry A point source rate can be given by ρ D' ( r) = 2 4πr µ CE ρ avg exp( µ r) rad h where C is in curies, E avg is in Mev, ρ in g/cm 3, µ/ ρ = 17 E cm 2 /g, and r in g/cm 2.

13 Important dosimetry systems for radiation processing Dosimeter Type Calorimeters Method of Readout Temperature measurement Examples Graphite, Water, Polystyrene Typical Absorbed Dose Range (Gy) Organic crystals EPR spectrometry Alanine, Sucrose, Cellulose Diamonds Electrical measurements, EPR spectrometry Diamonds crystals and films Dose rate measurements Semi conductors Electrical measurement Si diodes, MOSFETs Inorganic crystals Spectrophotometry, EPR spectrometry LiF, SiO 2, Suprasil TM glass

14 Cont., Chemical solutions Spectrophotometry, Spectroflurometry Ceric-cerous, organic acids, Ethanolcholrobenzene Radiochromic films and optical wave guides Spectrophotometry, Micro densitometry Dyed Plastics, Polydiacetylenes Fluorescent systems Spectrofluorimetry, Spectrophotometry Inorganic and organic fluors

15 Criteria for the selection of routine dosimeter Following points must be taken into account while selecting a dosimeter for food application Calibrated response over a specified does range for a given radiation and energy Suitability of a given dosimetry system and its response function over the dose range of interest Measurement reproducibility and stability Energy deposition characteristics Conditions before and after radiation. Conditions during radiation Packaging, handling, geometrical conditions of irradiation Ease and speed of read out Cost and availability versus application.

16 Recent advances in the dosimetry system 1. Diamond Crystals and films Nitrogen doped synthetic diamond microcrystalline films and chips are useful as real dose-rate monitors. They consists of a conglomeration of micro crystalline material attached to electrical leads and a small DC voltage bias (up to 100V) Dose rates up to 10 3 Gy/min can be read. 2. Lithium fluoride optical- quality crystals One of the most useful dosimeters for measuring large doses ( Gy) Upon irradiation alkali halides (LiF) show lattice defects called color centers are induced which are manifested by discrete optical absorption band in the near UV visible spectrum. M- center exhibits the strongest optical luminescence at 530nm and 670nm.

17 Dosimeters used for food application Various dosimeters are used for food processing and sterilization. They are following: 1. Alanine dosimeter 2. Cyanocobalamin dosimeter 3. Sunna dosimeter 4. Aqueous solution of brilliant green

18 Alanine Dosimeter This is a traditional dosimeter used for food application. Principle: When radiation energy is absorbed by D-alanine production and accumulation of stable free radicals starts. These free radicals can be analyzed quantitatively by means of electron spin resonance(esr) spectroscopy. Characteristics: Alanine dosimeter contains dried D-alanine powder which is ground and mixed with EVA polymer powder at high temperature. The size is 15mm*4mm, thickness 0.30mm to 0.40mm and mass mg. Applications: They are mainly used in doses up to 10 4 Gy. The alanine EPR dosimeter is accurate, reliable, small, rugged and low price. Further advantages are the relative insensitivity to ambient conditions and handling non destructive ESR readout and the low fading under ambient conditions. It can be used for several years.

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21 Cyanocobalamin Dosimeter It is used to measure gamma- radiation. Principle: Cyanocobalamin has a typical spectrum with an intense bands at nm. This spectra is very sensitive to any changes in the structure of Cyanocobalamin. In turn all the derivatives produced are highly colored from yellow to brown to red or violet. This wide variation in the spectra is distinguishable by eyes or by reading absorbance. This property is used in designing the dosimeter. When radiolytic or photolytic radicals are present they attack the corrin ring and various colored compounds are produced which changes the absorption spectra and these changes are correlated to the dose.

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23 Cyanocobalamin Dosimeter Characteristics: The dosimeter contains a radiochromic solution of cyanocobalamin which has 0.05M phosphate buffer. The solutions are not sensitive to daylight or fluorescent light and therefore can be stored indefinitely in glass bottles. Applications: Cyanocobalamin solution dosimeter is inexpensive, nontoxic and easy to prepare. It is suitable for measuring gamma rays absorbed doses in the range of kgy. Used for vegetables and fruits.

24 Sunna Dosimeter This dosimeter has been characterized in gamma, electron and bremsstrahlung radiation fields by measuring the optically stimulated luminescence (OSL) at 530nm both below and above 1kGy. Principle: This dosimeter is designed on the basis of following principle- upon irradiation of alkali halides crystals (LiF) lattice defects called color centers are induced, which are manifested by discrete optical absorption bands in the near UV and visible spectrum. Characteristics: Sunna gamma dosimeter consists of a film which is made of polyethylene and LiF fine powder manufactured by injection molding with dimensions of 1cm* 3cm*0.51mm Applications: Sunna dosimeter is used for irradiating red meat, fresh and dried fruits, spice decontamination. Environmental conditions like UV light and humidity does not affect the performance of dosimeter significantly.

25 Brilliant Green Solution Several chemical dosimeters as well as dye solutions have been used for food irradiation dosimetry over a wide range of doses. Similarly brilliant green solution Is used as a dosimeter. Principle: Absorption spectra of unirradiated and irradiated solutions of brilliant green is determined which showed two absorption bands with peaks at 427 and 626nm and a decrease in absorption as the radiation dose in increased Characteristics: Brilliant green solution was prepared by dissolving gm of the compound in 1liter of triply distilled water to make 25µmol/lit solution at natural ph 4.1. The solution was saturated with oxygen by passing oxygen through the solution for about 30 minutes. Applications: Dilute aqueous solutions of brilliant green solution can be used for low dose applications on food irradiation (20-200Gy), such as inhibition of sprouting in fresh vegetables. The solution is stable in light except direct sun light it should be stored at lower temperatures for better results

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27 Conclusions EPR spectrometry of several materials (alanine) is a useful method in dosimetry. Certain aqueous and organic solutions are well established for dose measurements. Optical properties of LiF are useful in designing of dosimeter for gamma rays. More investigation is required for food materials for better understanding.