Alanine dosimeters

Alanine dosimeters are based on the ability of 1-a alanine (a crystalline amino acid) to form a very stable free radical when subjected to ionizing radiation. The alanine free radical yields an electron paramagnetic resonance (EPR) signal that is dose dependent, yet independent of the dose rate, energy type, and relatively insensitive to temperature and humidity. Alanine dosimeters are available in the form of pellets or films and can be used for doses ranging from 10 Gy to 200 kGy. A reference calibration service using the alanine EPR system was developed and the scans were sent to the service center by mail. Currently the available system allows transferring the EPR scan to a NIST server for a calibration certificate. This way the procedure has been shortened from days to hours. ... 

PRZYBYTNIAK, G.K., ZAGORSKI Z.P., Orientation of crystals in alanine dosimeter assessed by DRS, as seen in EPR spectra evaluation , J.Radioanal.Nucl.Chem., Letters 212(1996)373-382. 

Alanine dosimeters are commercially produced in the form of pellets, rods, films and cables. For calibration a set of standards irradiated in advance with known doses and prepared from the same alanine dosimeter batches as those under study are employed. The radiation dose of the unknown sample is obtained by comparison of the heights of the central line of the sample and the standards. Usually a reference ESR standard is recorded simultaneously with the alanine spectra to correct for variations in spectrometer sensitivity. 

Fig. 9.6 ESR spectrum of irradiated self-calibrated alanine dosimeter. The diagram is reproduced from [4] with permission from Springer. An editing error in [4] was corrected...

The applied measuring protocols required measuring times up to several hours with the common alanine dosimeter. New materials have therefore been suggested as more practical dosimeters for clinical applications. 

Lithium formate is the most extensively studied new material for ESR-dosimetry [21], It has also been tested for use in clinical applications [53, 54]. Thus, doses in the range 0.2-3.5 Gy due to 6 MeV photons were determined in blind tests shown in Fig. 9.7. A deviation of less than 1.2% from those by ionization chamber measurements was obtained, which is well within the uncertainty of the measurements. It was concluded that no trend could be seen in the ESR dosimeter response, regarding either the dose rate or the beam quality. Each measurement took 15 min, while several hours would be required with the common alanine dosimeter. 

Sensitizing agents The sensitizing effect of metal ion dopants observed in initial studies by Ikeya et al. of certain inorganic substances does not seem to have been further explored. Attempts to increase the dose response of the alanine dosimeter by similar treatments have also not been entirely successful. An increase in the sensitivity of some of the new materials by transition metal ion dopants has, however, been reported [57]. An increase in the free radical yield by up to a factor two and a shortening of the relaxation times contributed to the increase in sensitivity. Those studies are still on an experimental level. 

Typical dose response of the a-L-alanine dosimeter with an insert showing the electron paramagnetic resonance (EPR) signal (Regulla and Deffner 1982]... 

Dosimeters that are sufficiently small, such as thin radiochromic films and alanine pellets, can readily be calibrated against the calorimeter, by irradiating in tandem (with a suitable radiation monitor) while encased in a phantom material that is identical in size, shape and substance to the calorimetric absorber. The main appreciable correction that is required is the ratio of mass energy-absorption coefficients of the two materials (in the case of photon irradiations) or the ratio of mass collision stopping powers of the two materials (in the case of electron beam irradiations) (McLaughlin et al., 1989). 

One of the most promising dosimeters, which may under careful preparation and calibration, qualify as a reference dosimetry system, is L-a alanine as measured by EPR spectrometry (Regulla et al., 1993). It is also proving to be useful as a transfer dosimeters, as shown by its application in the IAEA International Dose Assurance Service (IDAS) (Nam and Regulla, 1989) and in the reference dosimetry service to industry by the National Physical Laboratory (NPL). 

The next example of comparatively simplicity, this time nonaqueous, is the crystalline alanine. There are several products of irradiation of that solid crystalline amino acid. In this state it occurs as zwitterion as NMR shows, i.e. the amine group is protonated -N+H3. Single ionization spurs, of a low energy, cause deamination which leads to detachment of ammonia and formation of a free radical. Pulse radiolysis of single crystals of L-alanine shows, that the alanine derived radical CH3-C H-C02-, which shows the spectrum with maximum at 348 nm [9], stabilizes during 5 milliseconds [10], It is usually observed not spectroscopically but by the EPR method [11] it shows extreme stability, being applied as reference dosimeter. 

Line-width The amplitude of an ESR 1st derivative line is inversely proportional to the line-width squared at a fixed concentration of free radicals. A narrow line-width is therefore desirable. The anisotropy of the g-factor and/or the hyperfine coupling causes line broadenings in several of the commonly used dosimeter materials, e.g. alanine and Li-formate. The line-width also tends to increase at microwave saturation, which is an additional reason for not increasing the power excessively. In practice the signal of an ESR-dosimeter may be distributed over several hyperfine lines, causing loss of sensitivity. For alanine five lines are present (Fig. 9.3). 

L-a-alanine, H2NCH(CH3)COOH, is the most commonly used material for ESR dosimetry. Alanine has excellent characteristics for dosimetric purposes in several respects [4] (1) high free radical yield (G-value), (2) short relaxation times so that high microwave power can be applied, (3) linear ESR response with radiation dose up to 5-10" Gy, (4) high stability of the radiation induced free radicals so that the dosimeter can be kept as a document of the radiation dose, (5) tissue equivalence for medical and biological applications, (6) non-destructive read-out of dosimeter so that the dose accumulation at repeated exposures can be monitored. 

L-a-alanine has been employed as an ESR dosimeter since 1962 [26], Regulla et al. [7, 27] have reported detailed studies on its dosimetric properties. It is the most commonly used material in the field of ESR dosimetry being accepted by the International Atomic Energy Agency (IAEA) [28] and other agencies as a secondary reference and transfer dosimeter for high (industrial) dose irradiation. 

Sophisticated measurement protocols have made it possible to measure low doses with an impressive precision using the alanine ESR dosimeter. Nagy et al. [34] have reported measurements within 1.5 % for doses between 1 and 5 Gy, and Hayes et al. [35] have demonstrated measurement of doses in the range 0.01-1,000 Gy with an uncertainty of 1% for high doses and 10 mGy for low doses. An uncertainty of less than 0.5% was achieved for doses between 5 and 25 Gy [36]. 

The alanine ESR dosimeter is less convenient for low dose dosimetry due to the long measurement times. New materials for the dose determinations in the interval 0.2-10 Gy may therefore be needed if the ESR method shall become a realistic alternative to existing dosimetry systems, e.g. ionisation chambers. Si-diodes, ther-moluminiscence or chemical dosimetry like Ericke solution. Strategies for finding... 

ESR dosimetry in neutron beams has been reported recently using alanine, ammonium tartate, and lithium formate ESR dosimeters [58-60]. A sensitivity improvement by gadolinium addition was observed experimentally and examined theoretically by Monte Carlo simulations. Those materials have also been considered for dosimetry in a mixed radiation field of photons and neutrons. 

In radiation processing practice the dose is sometimes measured by means of the alanine-EPR (CH3CH(NH2)C00H) dosimetry system (ISO/ASTM 51607 Alanine-EPR Dosimetry). The operation of the dosimeter is based on the generation of CHaCH COOH radicals by ionizing radiation. The read-out is made with electron-paramagnetic-resonance analysis. The nominal absorbed dose range is 1-10 Gy, with linear responses up to 10 Gy. 

The build-up of radicals in irradiated solids forms a method of estimating the absorbed dose of ionizing radiation. A standard used in EPR dosimeters is the amino acid alanine, in pellet form, which gives a linear relationship between radical signal amplitude and absorbed dose over several orders of magnitude. Where there has been accidental radiation exposure, the EPR of radicals in hydroxyapatite in tooth enamel has been used to estimate the absorbed dose. Irradiation of foods such as meat may be detected by radicals formed in bone. 

---