Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

Response of Superheated Emulsions to heavy ionsM. Das1, N. Yasuda2, G. Oinuma1, T. Sawamura1
Dept. of Quantum Energy Engineering, Hokkaido University, Sapporo
2National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage, Chiba
1

Abstract
The response of superheated emulsion to heavy ion has
been investigated at the heavy ion medical accelerator
facility (HIMAC), Chiba, Japan. The response has been
observed by counting the nucleation rate by varying theoperating temperature of the emulsion. The present study
is important in order to explain the nucleation insuperheated emulsion by heavy ion.

1. INTRODUCTION
The response of the superheated emulsions to heavyions was investigated at the Heavy Ion MedicalAccelerator Facility (HIMAC), Chiba, Japan. The
superheated emulsions used in the present work consistsof drops of the superheated liquid, R114 [C2Cl2F4, b.p.
3.77oC] suspended in a firm polymer matrix. Theemulsions were irradiated by 40Ar (500MeV/u), 20Ne
(230MeV/u). The response was observed by counting thenucleation rate and by varying the operating temperatureof the emulsions. The present work studies the nature of
the response of the superheated emulsions to heavy ionsof different energies by varying the operating temperatureof the emulsions and measures the threshold temperatureof nucleation. The results obtained from the experiments
were also compared with the estimations.

2. PRESENT WORK
It is known that there is a minimum energy required fornucleation to occur in superheated liquid, the so calledcritical energy. The critical energy decreases withincreases in the operating temperature and by varying theoperating temperature of the emulsions, ions of different
energies can be detected. The critical energy for
nucleation obtained from reversible thermodynamics canbe expressed as

16 π γ 3 (T )W =3 { Pv (T ) − Pl } 2(1)where γ (T ) is the surface tension at temperature T,Pv (T ) is the vapour pressure of the liquid at temperature
T, and Pl is the liquid pressure [1]. The critical energy( W ) is related to the critical LET of ions in the sensitive
liquid by the following equation

 dE W= kR c dx L(2)e f f ,c dE 
 isWhere  d E is the critical LET and  dx dx
 Leff L e f f ,cthe track averaged LET over Leff with Leff = 2Rc and‘k’ is the nucleation parameter [2-5].
The response of the superheated emulsions to auniform beam incident normally to the emulsions is givenby1  dN  = sdI B N o  dt  t = o(3)When the operating temperature (T) is at or above thethreshold temperature, equation (3) holds, and when T is
below the threshold temperature, 1  dN  = 0 .
I B N o  dt  t =oTherefore the ideal case is that the response should bezero below the threshold and at or above threshold it
should be a constant (sd).
The superheated emulsions used in the experiments
were about 8 cm long and 1.5 cm in diameter. The interestof the present work is to observe the response of theemulsions by the higher energy part of the heavy ions oflower LET in the emulsions. The reported work withheavy ions studied the threshold LET by irradiating theemulsions with the axis parallel to the beam and bymeasuring the path length of the ions in the emulsions [6].
In the present experiments, the detector was irradiatedvertically, so that the maximum path length of the ions in
the emulsions is about 1.5 cm, which eliminates the Bragg
peak of the ions and the threshold temperature ofnucleation could be measured by increasing the operating
temperature above the room temperature. The temperature
was changed from room temperature, 25oC, to 80oC by
the temperature controller (SDC21Yamatake Honeywell,Japan) with an accuracy of ± 0.1 oC. The nucleation wasobserved by counting the number of nucleated drops with____________________________________________

E-mail: sawamura@eng.hokudai.ac.jp441

Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

Normalised nucleation rate (cm2)

2.0x10-6

Ar-500 MeV/u
Ne-230 MeV/u

1.0x10-6Testimate=55.1 ℃(Ne)Testimate=46.5 ℃(Ar)0.02530354045505560657075

Operating Temperature (oC)

Figure 1: Observed response of the emulsions for Ar and Ne ions with the theoretically estimated threshold
temperatures (46.5, 55.1℃)

an acoustic transducer, preamplifier, pulse shaping circuit,and the multi-channel analyzer [7].

the estimation are also shown in Figure-1.

4. DISCUSSION

3. RESULTS
The

experimentallyobservedvariation

inthe 1  dN    in N o I B  dt  t =0 normalized initial nucleation rate the presence of Ar and Ne ions of different energies isshown in Figure-1. The response of the emulsions showsa gradual increase with temperature near the threshold.
Figure-1 shows that at low temperatures the response isvery low and almost flat. The response above this value
has been taken as the true response for the ion with aspecific energy. The origin of this flat part at lower
temperature is supposed to be due to scattered ions ofdifferent energies at the experimental site during theirradiation. The threshold temperature has been evaluatedas the midpoint between the temperatures at the flat partof the response and the response at the next immediatehigher experimental temperature.
Before entering into the emulsions, the ions passthrough 6 m of air, 1 mm of Al, and 1 mm of glass. The
ion energies in the emulsions were estimated using TRIM
code by considering the energy loss in air, Al, and glass.
The estimation of the threshold temperature was madeby relating W
to the LET of the ions in R114 with ‘k’kR c(=0.11) as a constant nucleation parameter employing
equation (1). The threshold temperatures obtained from

The response of the emulsions with rising temperatureshows a gradual increase near the threshold. There is areasonable agreement between the estimated threshold
temperatures and the experimental results. According to
the calculation, the response of the emulsions should
increase sharply at the threshold, but the experimentalresults show that the response increases gradually near thethreshold. Though the incident heavy ions are monochromatic, there is scattering of the ions by the material(such as Al, Glass) surrounding the emulsions, and thisproduces the ions of lower energies incident on theemulsions. There is also a distribution of the size of thedrops. These various factors affect the response of theemulsions and prevent the appearance of the expected
sharp cut off at the threshold temperature. The present
study is important in order to explain the nucleation insuperheated emulsions by heavy ion.

ACKNOWLEDGEMENTS
The authors want to acknowledge the help of the staff ofHIMAC, Japan, for the operation of the heavy ion beamfacility. This work is a part of the research project withHeavy Ions at NIRS-HIMAC in Chiba, Japan. The work
is partially funded by Japan Society for the Promotion ofScience (JSPS).

___________________________________________

１E-mail:sawamura@eng.hokudai.ac.jp442

Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

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[5]. M. Das, T. Sawamura, “Nucl. Instrum. Meth.” A,2004 (in press).
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