Theory of microwave effects on bubble dosimeters

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Abstract
Bubble dosimeters measure a neutron flux by its effect upon microscopic droplets of superheated liquid encased in a polymer gel. It has been observed that a microwave field can also induce bubble formation in some of the droplets. This article considers the theory of this phenomenon as an effect of a microwave‐induced temperature increase. Although the droplets are superheated, their confinement by a smooth gel surface and lack of impurities such as dust particles allow only homogeneous nucleation to occur. At room temperature the thermal fluctuations that give rise to critical size bubbles are very rare; the dosimeter thus has a long shelf life and a low spontaneous noise level. In the presence of a microwave field, the gel and droplets absorb energy and can be heated by 1–2 K for moderate powers; a high‐power microwave field is needed to produce an observable nucleation rate. The electromagnetic properties of the dosimeter determine the internal field and the microwave absorption. Then the microwave heating and thermal properties of the dosimeter lead to an elevated steady‐state temperature for the droplets. Finally, the nucleation rate is obtained from classical homogeneous nucleation theory, while the number of bubbles formed in an ensemble of droplets is found by a simple statistical argument. Although a special case is considered and several approximations are invoked, the qualitative results show this effect could lead to spurious neutron readings only for intense microwave fields or for an ambient temperature close to the nucleation temperature. For microwave bubble dosimetry, some nonthermal mechanism would be much more useful.

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