Radiation Dosimeters for FoodsRadiation Dosimeters for FoodsAshish Anand

Department of Biological and Agricultural Engineering

Texas A&M University

IntroductionIntroduction• Interest in food irradiation technology is

increasing world-wide. • Some countries are using it as a commercial

process for food processing.

Advantages of IrradiationAdvantages of Irradiation

• It can serve following purposes-• Reduce food losses during post harvest.• Can control causes of food borne

diseases.• Can reduce food spoilage.

Why do we need dosimeters ?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.

Theory of Radiation DosimetryTheory of Radiation Dosimetry

ρϕ 1'

−=dxdED

• 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:

ϕ = Fluence rate; (-dE/dx)1/ρ= Mass stopping power of the material.

Dosimetry MethodsDosimetry Methods

• Following methods are used fordosimetry—

1. Ionization method2. Chemical methods3. Calorimetric methods4. Gamma dosimetry5. Beta dosimetry6. Neutron dosimetry

Ionization MethodIonization Method

∫= 111

11

)()( ε

εε dw

EnI

• This is most widely used method and it uses ion pairs to estimate dose.

• Number of ion pairs produced is given by

where B is the lower limit of energy loss and w1 (ε1) is the energy required for a particleat energy ε to produce an ion pair.

Chemical MethodsChemical Methods

∫= 111111 )(),( εεεε dGEnY

• In some systems the chemical composition is changed by the absorbed radiation.

• The observed chemical change(Y) can be given by

where G1 (ε1) is the yield per unit absorbed.If G is independent of particle type and ε1 then Y= G*εT

Calorimetric MethodsCalorimetric 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

where ε1n1 (ε1, E) dε1 is the amount of energy absorbed in a unit mass F1 (ε1) is the fraction of charged particle energy that is degraded to heat c is the thermal capacity of the substance F (ε1) is approximately constant near unity so that ΣT = εT/c

∫=∆ 111111 )(),(1 εεεε dFEnc

T

Gamma dosimetryGamma 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

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

• When no attenuation in the surrounding material is assumed. Withattenuation the flux is

),(' rEID µ=

24),(

rSrEIπ

=

)exp(4

),( 2 rrSrEI µπ

−=

Cont.,Cont.,• If the source strength C is expressed in curie thenS= 3.7*10^10 CE and the dose rate:

• 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.

−

×=

scmMeVr

rCED 32

9

)exp(1096.2' µµ

Beta dosimetryBeta dosimetry

−

×=

hradr

r

CErD

avg

)exp(4

1014.2)(' 2

26

µπ

ρµρ

• 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

where C is in curies, Eavg is in Mev, ρ in g/cm3, µ/ ρ = 17 E-1.14 cm2 /g, and r in g/cm2.

Important dosimetry systems for radiation processingImportant dosimetry systems for radiation processing

Dosimeter Type Method of Readout

Examples Typical Absorbed Dose

Range (Gy)Calorimeters Temperature

measurementGraphite, Water,

Polystyrene101-104

Organic crystals EPR spectrometry Alanine, Sucrose, Cellulose

100-105

Diamonds Electrical measurements, EPR

spectrometry

Diamonds crystals and films

Dose rate measurements

Semi conductors Electrical measurement

Si diodes, MOSFETs 100-104

Inorganic crystals Spectrophotometry, EPR spectrometry

LiF, SiO2, SuprasilTM

glass103-107

Cont.,Cont.,Chemical solutions Spectrophotometry,

SpectroflurometryCeric-cerous, organic acids,

Ethanol-cholrobenzene

102-105

Radiochromic films and optical wave

guides

Spectrophotometry, Micro densitometry

Dyed Plastics, Polydiacetylenes

100-106

Fluorescent systems

Spectrofluorimetry, Spectrophotometry

Inorganic and organic fluors

100-104

Criteria for the selection of routine dosimeterCriteria 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.

Recent advances in the dosimetry systemRecent advances in the dosimetry system1. 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 103 Gy/min can be read.2. Lithium fluoride optical- quality crystals

• One of the most useful dosimeters for measuring large doses (104-107Gy)

• 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 and670nm.

Dosimeters used for food applicationDosimeters used for food applicationVarious dosimeters are used for food

processing and sterilization. They are following:

1. Alanine dosimeter2. Cyanocobalamin dosimeter3. Sunna dosimeter4. Aqueous solution of brilliant green

Alanine DosimeterAlanine 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

18.0-19.0mg.• Applications:

• They are mainly used in doses up to 104 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.

Cyanocobalamin DosimeterCyanocobalamin Dosimeter• It is used to measure gamma- radiation.• Principle:

• Cyanocobalamin has a typical spectrum with an intense bands at 360.5-361nm.

• 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.

Cyanocobalamin DosimeterCyanocobalamin 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 lightand therefore can be stored indefinitely in glass bottles.

• Applications: • Cyanocobalamin solution dosimeter is inexpensive, non-

toxic and easy to prepare. • It is suitable for measuring gamma rays absorbed doses in

the range of 0.1-2.0 kGy. • Used for vegetables and fruits.

Sunna DosimeterSunna 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 absorptionbands 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.

Brilliant Green SolutionBrilliant 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 0.0121gm 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

ConclusionsConclusions• 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.