1. Introduction :

The ‘Superheated Drop Detector’ or SDD invented by Apfel in 1979 [1] is one of the most useful devices in neutron detection. The basic principle of operation of this detector is the same as a bubble chamber. Here the superheated drops are suspended in a dust free visco-elastic gel medium. Upon nucleation by energetic radiations the drops form bubbles and the drops nucleate independent of each other. So one nucleation does not consume the whole liquid and the repressurisation process that is needed in bubble chambers, is not required here. This is an advantage of SDD over the bubble chamber. Each drop stores mechanical energy that is released when triggered by radiation. The superheated liquid can be prepared by increasing the temperature of the liquid at a given pressure or alternatively it can be prepared by lowering the pressure of the liquid at a given temperature. This detector can be made on a polymer matrix where the bubbles formed after the nucleation of drops are tightly bound as was carried out by Ing and his group, called the ‘Bubble Detector’ (BD). Here the nucleation is observed by counting visually the number of bubbles trapped in the gel [2]. The test liquid remains in a glass tube under pressure created by another liquid and just before the experiment, the liquid is sensitized by unscrewing the cap of the tube and allows the liquid to become superheated. The superheated drops serve as an excellent detector for neutrons.

There are different detecting systems by which the nucleation in superheated drops can be measured. One way is to count acoustically the pulses produced by drop vaporization with the help of a piezo electric transducer and a drop counter [3]. Another way is to measure the volume of the vapor formed upon nucleation by a passive method. This system consists of a vertical graduated pipette [4] or horizontal glass tubes placed on a graduated platform [5,6] with an indicator (gel piston or coloured water column) indicating the volume of the vapor formed upon nucleation. Changing the diameter of the glass tube can vary the sensitivity of this type of detector. This system does not require any power source and can be used as an alarm dosimeter, in area monitoring etc. The third way is to count visually the bubbles trapped in hard polymer matrix [2]. The suitability of using superheated drops as a neutron dosimeter [7,8,9,10] has already been established. It is a very sensitive neutron dosimeter and can measure the neutron dose as low as 0.1m Sv. The SDD and BD neutron dosimeters are now commercially available from Apfel Enterprises Inc, USA and from Bubble Technology Industries Ltd., Canada respectively.

2. Principle of neutron spectrometry :

Since its discovery, attempts have been made on the application of this detector in neutron spectrometry. There is a minimum energy required for nucleation at a given temperature below which no nucleation occurs. This minimum energy is called the threshold energy (W) for nucleation that can be obtained from reversible thermodynamics [11]. The threshold energy decreases as the degree of superheat of the liquid increases. The degree of superheat of a liquid is the difference between the vapor pressure of the liquid at a given temperature and the ambient pressure or the difference between the boiling point of the liquid and the ambient temperature. Therefore liquid with lower boiling point possesses a higher degree of superheat at a given temperature and as the ambient temperature increases the liquid becomes more and more superheated. This property of the superheated liquid is being utilized to develop the neutron spectrometry. There are different ways by which superheated drops can be used in neutron spectrometry. One of the ways is to use the different superheated liquids of different degree of superheat and the threshold neutron energies can be obtained by irradiating the detectors with different monochromatic neutron sources [12]. Another way is to use the same detector operating at a different temperature. The threshold energy depends on the operating temperature of the detector, hence by suitably varying the temperature of the detector, neutrons of different energies can be detected, as was achieved by d’Errico et al. [13]. Two superheated liquids operating at four different temperatures were used to obtain eight different threshold neutron energies. It is to be noted that in order to get good resolution of the spectrum, temperature variation at a close grid is necessary.

3. Present work :

There is a different approach by which the neutron energy spectrum can be obtained from the temperature dependence of threshold energy of a superheated liquid. After the interactions of the neutrons with the nuclei of the constituting atoms of a superheated liquid, ions of different energies are formed. The ion having the highest value of LET (dE/dx) in the liquid, will play the major role in nucleation. Another important point is that there is a specific length L, along the ion track, and the energy (E_c) deposited over that length will contribute a significant role in nucleation. Actually, a very small fraction of the deposited energy is normally used in nucleation i.e. W/E_c is very small and this ratio is called the thermodynamic efficiency of nucleation (h _T). After the deposition of energy by the ions, nucleation occurs with the formation of a critical size vapor bubble of radius r_c inside the liquid drop. It is suggested that L = 2r_c [14,15] and E_c can be expressed as E_c = 2 r_c dE/dx. Therefore,

W and r_c are both functions of temperature and dE/dx is a function of the energy of the projectile ions in the superheated liquid, which can be converted to the energy of the incident neutrons. So this equation relates the threshold neutron energy for nucleation to the ambient temperature. This enables one to convert the temperature of the detector to the energy of the incident neutrons. Therefore using the above equation as a working equation, a neutron energy spectrum can be obtained by observing the detector response at different temperatures. This gives an important application of the superheated drop detector in neutron spectrometry. The nucleation rate in superheated drops is proportional to the total volume of the drops (V), incident neutron flux (y ), neutron-nucleus interaction cross section (s ) and to the neutron detection efficiency (h ) of the detector. Neutron detection efficiency h , is defined as the ratio of the observed nucleation to the incident neutrons. By observing the nucleation rate in superheated drops, h can be obtained from the known values of V, y and s . If one measures h at different temperatures, the derivative of h against temperature resembles the neutron energy spectrum of the source. The temperature axis can be converted to the neutron energy following the method discussed here. For a given neutron energy spectrum, at low temperature only the high energy neutrons take part in nucleation. As temperature increases, threshold energy decreases and so in addition to the high energy neutrons, low energy neutrons are also detected. So for a polychromatic source, h should increase with temperature. When all the neutrons in the spectrum contribute in nucleation, h should be constant with temperature because no more neutrons are left to be detected. For a monochromatic source, there is only one sharp increase of h at a particular temperature corresponding to the energy and it should be constant for the other temperatures. This detector can be made sensitive to different ranges of neutron energies as the user’s choice by varying the temperature of the liquid. This detector can detect neutrons with energies ranging from thermal to fast energy. We have tested the present principle of spectrometry with a 3 Ci Am-Be neutron source using superheated drop detector made of R12. The temperature was varied from –17^oC to about 60^oC with the help of an indigenously made temperature controller. There is a fair agreement between the neutron energy spectrum of Am-Be obtained from our experiment and the available spectrum of the source, the details of which will be published elsewhere [16].

The main advantage of this type of spectrometer is that it is easy to prepare, is low cost and does not require any power supply. Nowadays, superheated drops are widely used in the determination of neutron spectra in space, at high altitude studies, gamma detection, detection of radon, for cold dark matter search, charged particle detection etc. Besides these applications, the radiation induced nucleation in superheated drop detector is itself a very interesting field of research.

References :-

  1.   R. E. Apfel (1979) U. S. Patent 4,143,274.
  2.   H. Ing and H. C. Birnboim (1984) Nucl. Tracks and Rad. Meas. 8, 285.
  3.   R. E. Apfel and S. C. Roy (1983) Rev. Sci. Instrum. 54, 1397.
  4.   R. E. Apfel (1992) Rad. Prot. Dos. 44, 343.
  5.   B. Roy, B. K. Chatterjee, Mala Das and S. C. Roy (1998) Rad. Phys. Chem. 51, 473.
  6.   Mala Das, B. Roy, B. K. Chatterjee and S. C. Roy (1999) Rad. Meas. 30, 35.
  7.   R. E. Apfel and S. C. Roy (1984) Nucl. Inst. Meth. 219, 582.
  8.   R. E. Apfel and Y. C. Lo (1989) Health Phys. 56, 79.
  9.   S. C. Roy, R. E. Apfel and Y. C. Lo (1987) Nucl. Inst. Meth. A255, 199.
10.   H. Ing (1986) Nuclear Tracks 12, 49.
11.   J. W. Gibbs (1875) Translations of the Connecticut Academy III, p.108.
12.   H. Ing, R.A. Noulty and T.D. McLean (1995) Rad.Meas.27, 1.
13.   F. d'Errico (1995) Rad.Prot.Dos, 61, 159.
14.   R. E. Apfel, S. C. Roy and Y. C. Lo (1985) Phys. Rev. A 31, 3194.
15.   M. J. Harper and M. E. Nelson (1990) Rad. Prot. Dos. 47, 535.
16.   Mala Das, B. K. Chatterjee, B. Roy and S. C. Roy (1999) Nucl. Inst. Meth A (submitted).