Patent Publication Number: US-2006000287-A1

Title: Apparatus for producing hydrogen

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2004-194618 filed on Jun. 30, 2004, the entire contents of which are incorporated herein by reference.  
     FIELD OF THE INVENTION  
      This invention relates to an apparatus for producing hydrogen continuously, and in particular, an apparatus for producing hydrogen by using IS (Iodine-Sulfur) process that can precisely measure a position of a boundary surface of multilayer liquids, constituent and concentration of the liquids in a reaction vessel.  
     DESCRIPTION OF THE BACKGROUND  
      Hydrogen is being spotlighted as one option for clean energy in the next generation. As many investigations for utilizing hydrogen as an energy source such as a fuel cell are studied, investigations for producing hydrogen as a fuel are also being made.  
      With regard to a hydrogen producing process, it is known that a process of thermochemical decomposition of water (also referred to as an “IS process”) can produce hydrogen continuously. The IS process can be operated by utilizing heat of, for example, a high-temperature gas-cooled reactor.  
      For the IS process operation by utilizing heat of a high-temperature gas-cooled reactor, a basic concept or a configuration for an apparatus is being investigated. However, sufficient study necessary for operating the apparatus, such as a technology for measurement or an operation control technology, has not been investigated yet.  
      When producing hydrogen by using the IS process, it is necessary to keep the ratio of amount of hydrogen and oxygen, which are produced in the apparatus, at a 2:1 value, and it is also necessary to keep the constituent of processed solutions before and after the process the same. Therefore, it is needed to develop a method for controlling and operating for the IS process which meets the above two necessites during the process. For the IS process, a solution of hydriodic acid (also referred to as HI) and a solution of sulfuric acid (also referred to H 2 SO 4 ) are generated in a reaction vessel. Thus, it is necessary to develop a non-contact liquid level measuring apparatus that can measure the generation ratios of the hydriodic acid solution and the sulfuric acid solution, or that can measure the constituents or concentrations of these solutions in the reaction vessel.  
      Related to these technologies, Japanese patent publication (Kokai) No. 8-14990 discloses a liquid level measuring apparatus that utilizes ultrasonic waves to detect the level of oil in an airtight container of power equipment which high voltage is applied to. This apparatus enables to detect infiltrations of rainwater into the airtight container or leak of oil from the container. Further, Japanese patent publication (Kokai) No. 4-33620 discloses an apparatus that can detect a boundary between two non-mixing liquids in a tank by utilizing ultrasonic waves. These non-contact liquid level measurement techniques are intended to detect a level or a surface boundary of a liquid that is enclosed and is stable in the container or the tank.  
      On the other hand, in the reaction vessel of a hydrogen producing apparatus using the IS process, water (H 2 O), Iodine (I 2 ) and sulfur dioxide (SO 2 ) are reacted and providing hydriodic acid (HI) and sulfuric acid (H 2 SO 4 ). This reaction is referred to as a “Bunsen reaction”. To produce hydrogen continuously, it is necessary to estimate the amount of HI and H 2 SO 4  precisely in the operation for producing hydrogen in the IS process.  
     SUMMARY OF THE INVENTION  
      Accordingly, an advantage of an aspect of the present invention is to provide an apparatus for producing hydrogen that is able to measure a position of a interface or a surface boundary of liquids in a reaction vessel without contact.  
      To achieve the above advantage, one aspect of the present invention is to provide an apparatus for producing hydrogen by IS process that comprises a reaction vessel where reacting liquids are to be introduced, a first ultrasonic probe provided at the bottom of the reaction vessel to detect a position of a boundary surface of the reacting liquids, and a second ultrasonic probe, provided at the side wall of the reaction vessel to compensate the sound velocity.  
      Another aspect of the patent invention is to provide an apparatus for producing hydrogen by IS process that comprises a reaction vessel, in which reacting liquids are to be introduced, an ultrasonic transmission probe provided at a first side wall of the reaction vessel, an ultrasonic receiving probe provided at a second side wall which faces to the first side wall where the first ultrasonic probe is provided, an ultrasonic transmitter connected to the ultrasonic transmission probe, an ultrasonic receiver connected to the ultrasonic receiving probe, and a data processing unit connected to the ultrasonic transmitter and the ultrasonic receiver, which calculates a position of a boundary surface of the reacting liquid inside the reaction vessel.  
      Further features, aspects and advantages of the present invention will become apparent from the detailed description of preferred embodiments that follows, when considered together with the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic drawing showing a principle of an IS process.  
       FIG. 2  is a schematic sectional view of an apparatus for producing hydrogen by IS process in accordance with the first embodiment.  
       FIG. 3  shows a schematic graph of observed ultrasonic pulses transmitted or received from the ultrasonic probe used in the first embodiment.  
       FIG. 4  is a schematic sectional view of a modification of the first embodiment.  
       FIG. 5  is a schematic sectional view of a further modification of the first embodiment.  
       FIG. 6  is a schematic sectional view of another modification of the first embodiment.  
       FIG. 7  is a schematic sectional view of an apparatus for producing hydrogen by IS process in accordance with the second embodiment.  
       FIG. 8  is a schematic sectional view of a modification of the second embodiment.  
       FIGS. 9 and 10  are schematic sectional views of another modification of the second embodiment.  
       FIG. 11  is a schematic sectional view of an apparatus for producing hydrogen by IS process in accordance with the third embodiment.  
       FIG. 12  is a schematic sectional view of an apparatus for producing hydrogen by IS process in accordance with the forth embodiment.  
       FIG. 13  is a schematic sectional view of an apparatus for producing hydrogen by IS process in accordance with the fifth embodiment.  
       FIG. 14  is a schematic sectional view of an apparatus for producing hydrogen by IS process in accordance with the sixth embodiment.  
       FIG. 15  is a graph showing the relations between neutron energy and a neutron reaction cross section or a neutron absorption cross section of sulfur.  
       FIG. 16  is a graph showing the relations between neutron energy and a neutron reaction cross section or a neutron absorption cross section of iodine. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The preferred embodiments in accordance with the present invention are described below with reference to the drawings.  
       FIG. 1  is a schematic drawing showing a principle of an IS process, which is a process of thermochemical decomposition of water used in the embodiments. It is known that the IS process according to  FIG. 1  can produce hydrogen continuously.  
      It is generally known that hydrogen can be produced inexhaustibly from water and that hydrogen is an emission-free “clean” fuel because it becomes water after it is used as a fuel. It is regarded that hydrogen may be an alternative for fuel used for a home, an industrial site, a vehicle, or an aircraft since it is also relatively easy to store.  
      To produce hydrogen by a high-temperature gas-cooled reactor, an IS process is known that does not utilize electric decomposition of high temperature steam, or steam reforming of coal or natural gas.  
      The IS process comprises several stages of reactions which apply substances other than water, wherein heat necessary for those reactions can be given from the high-temperature gas-cooled reactor. Generally, the high-temperature gas-cooled reactor can supply heat at about 1,000 degrees centigrade. Thus, the IS process coupled with the high-temperature gas-cooled reactor can easily utilize heat necessary for the reactions.  
      In the IS process, water (H 2 O), sulfur dioxide (SO 2 ) and iodine (I 2 ) are reacted to generate hydriodic acid (HI) and sulfuric acid (H 2 SO 4 ) for the first step. The reaction formula of this reaction is described as below. 
 
2H 2 O+SO 2 +I 2 =2HI+H 2 SO 4    (1) 
 
      This reaction, which is referred to as a “Bunsen reaction”, is an exothermic reaction that occurs in a condition of temperature that is from room temperature to about 100 degrees centigrade.  
      By the reaction (1), HI and H 2 SO 4  are obtained in a reaction vessel, which will be described later. Because HI and H 2 SO 4  are non-mixing liquids that do not mix with each other, hydriodic acid and sulfuric acid are obtained as they form two layers in the reaction vessel by difference of their respective density.  
      Hydrogen can be produced by a thermal decomposition of hydriodic acid produced by the Bunsen reaction (1). The thermal decomposition of the hydriodic acid occurs at about 400 degrees centigrade. This reaction is described as below. 
 
2HI═H 2 +I 2    (2) 
 
      While hydrogen is obtained by the reaction (2), a solution of sulfuric acid produced by the Bunsen reaction (1) is decomposed to oxygen, water, and sulfur dioxide by a thermal decomposition reaction that occurs at about 800 degrees centigrade or higher. This thermal decomposition reaction of sulfuric acid is endothermic reaction. The reaction formula is described as below.  
                 H   2     ⁢     SO   4       =       SO   2     +       H   2     ⁢   O     +       1   2     ⁢     O   2                 (   3   )             
 
      Water (H 2 O) and sulfur dioxide (SO 2 ) obtained by decomposition reaction of the sulfuric acid (3) are utilized in the Bunsen reaction (1) together with the iodine (I 2 ) obtained by decomposition reaction of hydriodic acid (2).  
      As described above, sulfur dioxide and iodine, which are necessary to produce hydrogen in the IS process, are repeatedly used in the reactions of the IS process. Further, because other substances that are produced in the IS process are water and oxygen, the IS process is regarded as a clean closed-loop process to produce hydrogen continuously.  
       FIG. 2  is a schematic sectional view of an apparatus for producing hydrogen by IS process in accordance with the first embodiment of the present invention.  
      A hydrogen production apparatus  10  can precisely measure a surface boundary Fa or Fb such as a boundary surface between two reacting liquids existing in a reaction vessel  11  without contact.  
      Reaction vessel  11  of the hydrogen producing apparatus  10  encloses hydriodic acid (HI) solution A and sulfuric acid (H 2 SO 4 ) solution B resulted from the Bunsen reaction. Hydriodic acid (HI) solution A and sulfuric acid (H 2 SO 4 ) solution B are both in a liquid state, as reacting liquids.  
      Since HI solution A and H 2 SO 4  solution B (also referred to as reacting liquids) are non-mixing liquids with each other, they are separately held in the reaction vessel  11 , wherein the H 2 SO 4  solution B, which has lower density than HI solution A, is disposed above the HI solution B. In the space above the H 2 SO 4  solution B in the reaction vessel  11 , gas C produced in the reaction vessel  11 , such as oxygen or hydrogen may exist.  
      Reaction vessel  11  has a box-shaped (e.g., rectangular) enclosure. The reaction vessel  11  includes at least one ultrasonic transducer  13  at a bottom  11   a  of the reaction vessel  11 . The reaction vessel  11  shown in  FIG. 2  includes three ultrasonic transducers  13  at the outside surface of the bottom  11   a . Here, the ultrasonic transducer  13  is defined as the ultrasonic prove and ultrasonic transmitter and receiver.  
      The ultrasonic transducer  13  may be in the bottom  11   a  of the reaction vessel  11 . Reaction vessel  11  further includes a plurality of ultrasonic transducers  14 ,  15  at a side wall  11   b  of the reaction vessel  11 . One of the ultrasonic transducers  14 ,  15  is located at a lower side of the side wall lib, while the other is located at an upper side of the side wall  11   b . Each of the ultrasonic transducers  13 ,  14  or  15  includes a ultrasonic probe  16 ,  17   a  or  17   b , and an ultrasonic transmitter/receiver  18 ,  19   a  or  19   b  coupled with these ultrasonic probe  16 ,  17   a  or  17   b , respectively. The ultrasonic probe  16 ,  17   a  or  17   b  is able to transmit or receive an ultrasonic waves of a predetermined frequency coupled with the ultrasonic transmitter/receiver  18 ,  19   a  or  19   b.    
      When the ultrasonic transducer  13 ,  14  or  15  generates an electric pulse to the ultrasonic probe  16 ,  17   a  or  17   b , an ultrasonic wave, which has for example a frequency of 5 MHz, is transmitted from the ultrasonic probe  16 ,  17   a  or  17   b  inside the reaction vessel  11 .  
      The ultrasonic waves transmitted inside the reaction vessel  11  reflect at a liquid-liquid boundary surface Fa, a gas-liquid boundary surface Fb or a solid-liquid boundary surface Fc due to the difference of the density at the locations. These reflected ultrasonic waves are referred to as reflected echoes. The ultrasonic probe  16 ,  17   a  or  17   b  is also able to detect the reflected echoes. The reflected echoes are received and converted to an echo electric signal by the transmitter/receiver  18 ,  19   a  or  19   b . The echo electric signal is sent to a data processing unit  20 . The data processing unit  20  processes the echo electric signal and calculates positions (height) of the liquid-liquid boundary surface Fa and the gas-liquid boundary surface Fb in the reaction vessel  11  without contact. Results of the calculation are outputted on a display unit  21 .  
      The ultrasonic transducer  13 , which is provided at the bottom  11   a  of the reaction vessel, is used to detect the liquid-liquid boundary surface Fa and the gas-liquid boundary surface Fb in the reaction vessel  11 .  FIG. 2  shows a configuration that has three ultrasonic transducers  13  to detect the liquid-liquid boundary surface Fa and the gas-liquid boundary surface Fb. The ultrasonic transducer  14 , which is provided at a lower side of the side wall  11   b , is for compensation of the sound velocity used for the HI solution A existing as a lower layer in the reaction vessel  11 . The ultrasonic transducer  15 , which is provided at a upper side of the side wall  11   b , is also for compensation of the sound velocity. However, the ultrasonic transducer  15  is used to compensate the sound velocity in the H 2 SO 4  solution B, which separately exists as an upper layer in the reaction vessel  11 .  
      An ultrasonic pulse of the ultrasonic waves transmitted from the ultrasonic probe  16 , by the operation of the ultrasonic transducer  13 , penetrates through the bottom  11   a  of the reaction vessel  11  and goes upwardly in the HI solution A in the reaction vessel  11 .  
      A part of the ultrasonic pulse transmitted in the reaction vessel  11  which reaches at the liquid-liquid boundary surface Fa is reflected downwardly as reflected echoes due to the difference of the acoustic impedance between HI solution A and H 2 SO 4  solution B, which is resulted from the difference of densities and velocity of those solutions. The reflected echoes then come back to the ultrasonic probe  16  through the bottom  11   a.    
      The other part of the ultrasonic pulse transmitted in the reaction vessel  11 , which penetrates through the liquid-liquid boundary surface Fa, further goes upwardly in the H 2 SO 4  solution B. However this ultrasonic pulse is also reflected downwardly as the reflected echoes at the gas-liquid boundary surface Fb and comes back to the ultrasonic probe  16 .  
      Because the difference of the acoustic impedance between the H 2 SO 4  solution B and the gas C is greater than that between the solutions A and B, magnitude of the reflected echoes that come from the gas-liquid boundary surface Fb is also greater than the reflected echoes that comes from the liquid-liquid boundary surface Fa.  FIG. 3  shows a schematic graph of observed ultrasonic pulses transmitted or received from the ultrasonic probe  16 , which explains this situation.  
      As shown in  FIG. 3 , when the ultrasonic pulse is transmitted from the ultrasonic probe  16 , two reflected echoes can be observed. First, lesser magnitude reflected echoes, which are reflected at the liquid-liquid boundary surface Fa, are observed at a time t 1  from the transmission. Then, greater magnitude reflected echoes, which is reflected at the gas-liquid boundary surface Fb, are observed at a time t 2  from the transmission (t 2 &gt;t 1 ).  
      The distance d 1  from the outside surface of the bottom  11   a  to the liquid-liquid boundary surface Fa between the HI solution A and H 2 SO 4  solution B in the reaction vessel can be obtained by the formula (4) using time t 1  and the speed v 1  of the ultrasonic waves in the HI solution A.  
               d   1     =         v   1     *     t   1       2             (   4   )             
 
      The velocity v 1  of the ultrasonic waves in the HI solution A can be measured by utilizing the ultrasonic probe  17   a  in the ultrasonic transducer  14 , which is provided on the outer surface of the lower side of the side wall  11   b  of the reacting vessel  11 . The ultrasonic probe  17   a  is provided as it can receive reflected ultrasonic waves at the inner surface of the side wall  11   b  of the opposite side of the ultrasonic transducer  14 . The ultrasonic probe  17  is also provided at the lower side of the side wall  11   b , where the HI solution A should exist in the reaction vessel  11 . The distance of a propagation of the ultrasonic pulse inside the reaction vessel  11  is 2 L, because the ultrasonic pulse transmitted from the ultrasonic probe  17  reflects at an opposite side wall  11   c  and returns to the ultrasonic probe  17 . This is based on an assumption that the width of the sidewall  11   b  is relatively small compared to the reaction vessel width L. When the width of the sidewall lib can not be neglected, it may be considered. Defining time between transmission and receive of the ultrasonic pulse at the ultrasonic probe  17   a  as T 1 , the velocity v 1  of the ultrasonic waves in the HI solution A can be obtained by a formula (5).  
               v   1     =       2   ⁢   L       T   1               (   5   )             
 
      Therefore, the distance d 1  from the outside surface of the bottom  11   a  to the liquid-liquid boundary surface Fa between the HI solution A and the H 2 SO 4  solution B in the reaction vessel can be calculated by the formula (6).  
               d   1     =           v   1     ·     t   1       2     =       L     T   1       ·     t   1                 (   6   )             
 
      The same situation can be applied to the velocity v 2  of the ultrasonic waves in the H 2 SO 4  solution B. In this embodiment, the ultrasonic transducer  15  is provided on the outer surface of the upper side, where the H 2 SO 4  solution B should exist in the reaction vessel  11 , of the side wall  11   b . Thus the ultrasonic probe  17   b  can receive reflected ultrasonic waves at the inner surface of the side wall  11   b  at the opposite side of the ultrasonic transducer  15 . Since the distance of a propagation of the ultrasonic pulse inside the reaction vessel  11  is also 2 L, the velocity v 2  of the ultrasonic waves in the H 2 SO 4  solution B is obtained by a formula (7) when defining time between transmission and receive of the ultrasonic pulse at the ultrasonic probe  17   b  as T 2 .  
               v   2     =       2   ⁢   L       T   2               (   7   )             
 
      Therefore, the distance d 2  from the outside surface of the bottom  11   a  to the gas-liquid boundary surface Fb between the H 2 SO 4  solution B and the gas C in the reaction vessel can be calculated by the formula (8), by using observed time t 2 , which is the time taken to receive the reflected echoes from the liquid-gas boundary surface Fb after the transmission of the ultrasonic pulse from the ultrasonic probe  16 , shown in  FIG. 3 .  
               d   2     =         d   1     +         v   2     ⁡     (       t   2     -     t   1       )       2       =         L     T   1       ⁢     t   1       +       L     T   2       ⁢     (       t   2     -     t   1       )                   (   8   )             
 
      As explained above, the actual velocity v 1 , v 2  of the ultrasonic wave propagating inside the solutions A and B (reacting liquids) in the reaction vessel  11  can be obtained from the observed data of the ultrasonic transducers  14 ,  15 , which are provided at the side wall  11   b  of the reaction vessel  11 . Therefore, the position of the liquid-liquid boundary surface Fa and the gas-liquid boundary surface Fb can be obtained precisely based on the time taken to receive the reflected echoes at the ultrasonic transducer  13 , which is provided at the bottom  11   b , regardless of the change of the propagation speed of the ultrasonic waves inside the solutions A or B due to the change of temperature or concentration.  
      In this embodiment, a plurality of ultrasonic transducers  13  may be provided at the bottom  11   a  of the reaction vessel  11  to increase the accuracy of the calculated position of the boundary surface Fa and Fb, such as in a case where the boundary surface Fa or Fb is in a rippling condition. The average if the values obtained by the plural ultrasonic transducers may be utilized, for example.  
      Further, instead of using ultrasonic probe  17   a  or  17   b  for both of transmission and receive of the ultrasonic waves by the ultrasonic transmitter/receiver  19   a  or  19   b  for compensation of the sound speed in the reacting liquid as described in this embodiment, a pair of ultrasonic probes may be provided at the side wall  11   b  and at the opposite side wall  11   c . In such a case, one ultrasonic probe is used for transmission of the ultrasonic waves coupled with the ultrasonic transmitter, while the other is used for reception coupled with the ultrasonic receiver.  
       FIG. 4  is a schematic sectional view of a modification of the first embodiment of the apparatus for producing hydrogen by IS process. In  FIG. 4 , the same numeric symbols are used for the same elements as shown in  FIG. 2 . Detailed descriptions may be omitted for those elements.  
      As shown in  FIG. 4 , the reaction vessel  11  of the hydrogen production apparatus  10 D according to the modification also comprises at least one ultrasonic transducer  13  provided at the bottom  11   a  of the reaction vessel  11 . The hydrogen production apparatus  10 D also comprises a set of thermometers  40 ,  41  in the reaction vessel  11 . The thermometer  40  measures the temperature of the HI solution A existing in the lower side of the reaction vessel, while the other thermometer  41  measures the temperature of the H 2 SO 4  solution B. The thermometer  40 ,  41  are preferably sealed because harmful substances are in the reaction vessel  11 . The thermometer  40 ,  41  may be a sheathed thermocouple or a resistance temperature detector, for example.  
      The thermometers  40 ,  41  are connected to the data processing unit, which is not shown in  FIG. 4  but is substantially the same as the data processing unit  20  shown in  FIG. 2 . Temperature data detected in the thermometers  40 ,  41  are sent to the data processing unit.  
      The ultrasonic transducer  13 , which is provided at the bottom  11   a  of the reaction vessel  11 , transmits a pulse of ultrasonic waves upwardly into the reaction vessel  11  to detect the position of the boundary surfaces such as the liquid-liquid boundary surface Fa and the gas-liquid boundary surface Fb in the same way as described in the first embodiment. In the other words, the position of the boundary surfaces Fa and Fb are also measured based upon the time difference between the transmission of the ultrasonic pulse and the receive of the reflected echo at the ultrasonic transducer  13  in this modification.  
      As mentioned in the first embodiment, however, it is necessary to compensate the sound velocity in the solutions A and B to measure the position of the boundary surfaces Fa and Fb precisely. In this modification, the temperature data detected by the thermometer  40 ,  41  are used for the compensations of the sound velocity in the solution A or B.  
      The function that correlates the sound velocity in the HI solution A or H 2 SO 4  solution B with the temperature can be predetermined. Therefore these functions, the correlations between the temperature and the sound velocity in the reacting liquids, are stored in the data processing unit in advance.  
      Once the thermometer  40  or  41  detects the temperature data, the data processing unit can refer to the stored function and obtain the estimated sound speed in the solution A or B. Thus the sound speed in the solutions A and B is compensated, the position of the boundary surfaces Fa and Fb can be measured precisely based upon the time difference between the transmission of the ultrasonic pulse and the reception of the reflected echo at the ultrasonic transducer  13 .  
       FIG. 5  is a schematic sectional view of a further modification of the modified embodiment shown in  FIG. 4 . In  FIG. 5 , the same numeric symbols are used for the same elements as shown in  FIG. 2  and  FIG. 4 . Thus, detailed descriptions have been omitted for those elements.  
      As shown in  FIG. 5 , the reaction vessel  11  of a hydrogen production apparatus  10 E according to this modification further comprises sampling lines  44  and  45  to the modification shown in  FIG. 4 . Other elements are substantially the same as the modification shown in  FIG. 4 .  
      The sampling lines  44 ,  45  are provided in the side wall  11   b  of the reaction vessel  11 , separately in the vertical direction to each other. The sampling line  44  is provided at a lower side of the side wall  11   b , where the HI solution A should exist in the reaction vessel  11 . The sampling line  44  samples the HI solution A in the reaction vessel  11  and detects its concentration. On the other hand, the sampling line  45  is provided at an upper side of the side wall  11   b , where the H 2 SO 4  solution B should exist in the reaction vessel  11 . The sampling line  45  samples the H 2 SO 4  solution B and detects its concentration. The concentration data detected by the sampling line  44  and  45  are sent to the data processing unit.  
      In this modification, the concentration data detected by the sampling line  44 ,  45  are further used for the compensations of the sound speeds in the solution A or B, in addition to the compensation of the sound speed based upon the temperature of the solutions A or B.  
      The function that correlates the sound velocity in the HI solution A or the H 2 SO 4  solution B with the concentration can be predetermined. Therefore these functions, the correlations between the concentration and the sound velocity in the reacting liquids, are stored in the data processing unit in advance together with the functions between the temperature and the sound velocity in the HI solution A and H 2 SO 4  solution B.  
      Using the concentration data detected by the sampling line  44 ,  45 , together with the temperature data detected by the thermometer  40  or  41 , the data processing unit obtains the estimated sound velocity in the solution A or B by referring to the functions stored in the data processing unit. Thus, the sound velocity in the solutions A and B is compensated, the position of the boundary surfaces Fa and Fb can be measured more precisely based upon the time difference between the transmission of the ultrasonic pulse and the receive of the reflected echo at the ultrasonic transducer  13 . Other devices, such as a device using ultrasonic waves or a device emitting radiation instead of the sampling line  44 ,  45 , may detect the concentration data.  
       FIG. 6  is a schematic sectional view showing another modification of the first embodiment. In  FIG. 6 , the same numeric symbols are used for the same elements as shown in  FIG. 2 . Accordingly, detailed descriptions have been omitted for those elements.  
      A hydrogen production apparatus  10 F further comprises a float  47 , which reflects the ultrasonic waves, in the reaction vessel  11 . The density of the float  47  is adjusted in between the density of the HI solution A and the H 2 SO 4  solution B. Therefore, the float  47  floats on the liquid-liquid boundary surface Fa. The hydrogen production apparatus  10 F further comprises a guide  48  that restricts the horizontal movement of the float  47 . The guide  48  is preferably of a cylindrical shape, having holes  49  in the side wall of the cylinder that enables the solutions A and B to go inside of the guide  48  through the holes  49 . The guide  48  may alternatively be a cylindrical net. The float  47  and the guide  48  are provided above the ultrasonic probe  16   a  that is provided at the bottom  11   a  of the reaction vessel  11 . At least two ultrasonic probes  16   a  and  16   b  may be provided at the bottom  11   a.    
      All the ultrasonic pulses transmitted from the ultrasonic probe  16   a  are reflected at the float  47  because the float  47  is a reflector of the ultrasonic waves. Therefore, the ultrasonic probe  16   a  can receive the reflected echo, which is reflected at the surface of the float  47 , with a strong intensity. Thus, the ultrasonic probe  16   a  is used to detect the liquid-liquid boundary surface Fa.  
      On the other hand, a part of the ultrasonic pulse transmitted from the ultrasonic probe  16   b  goes through the liquid-liquid boundary surface Fa and is reflected at the surface of the gas-liquid boundary surface Fb. The ultrasonic probe  16   b  is used to detect the gas-liquid surface Fb.  
      This modification may be useful when multiple reflections of the ultrasonic waves occur at the liquid-liquid boundary surface Fa and the reflected echoes from the liquid-liquid boundary surface Fa cannot be received with enough intensity at the ultrasonic probe  16   b.    
      The ultrasonic transducer  14  and  15 , which are provided at the side wall  11   b  of the reaction vessel  11 , compensate the sound velocity in the solution A and B in the same manner described in the first embodiment shown in  FIG. 2 . Devices shown in  FIG. 4  or  5 , such as thermometers  40 ,  41  or sampling lines  44 ,  45 , may be used for compensation of the sound speed in the solution A and B.  
       FIG. 7  is a schematic sectional view of an apparatus for producing hydrogen by IS process in accordance with the second embodiment. In  FIG. 7 , the same numeric symbols are used for the same elements as shown in  FIG. 2 . Accordingly detailed descriptions have been omitted for those elements.  
      A hydrogen production apparatus  10 A can precisely measure a surface boundary Fa or Fb, such as a boundary surface between two reacting liquids existing in a reaction vessel  11 , without contact.  
      Reaction vessel  11  of the hydrogen producing apparatus  10  encloses hydriodic acid (HI) solution A and sulfuric acid (H 2 SO 4 ) solution B resulted from the Bunsen reaction. Hydriodic acid (HI) solution A and sulfuric acid (H 2S O 4 ) solution B are both in a liquid state, as reacting liquids.  
      Since HI solution A and H 2 SO 4  solution B are non-mixing liquids that do not mix with each other, they are separately held in the reaction vessel  11 , wherein the H 2 SO 4  solution B, which has lower density than HI solution A, is disposed above HI solution B. In the space above the H 2 SO 4  solution B in the reaction vessel  11 , gas C produced in the reaction vessel  11 , such as oxygen or hydrogen, may exist.  
      Reaction vessel  11  has a box-shaped (e.g., rectangular) enclosure. In this embodiment, the reaction vessel  11  includes ultrasonic transducer  13  at a bottom  11   a  of the reaction vessel  11 . The reaction vessel  11  shown in  FIG. 7  includes three ultrasonic transducers  13  at the outside surface of the bottom  11   a . The ultrasonic transducer  13  may be in the bottom  11   a  of the reaction vessel  11 . The ultrasonic transducer  13  includes an ultrasonic probe  16  and an ultrasonic transmitter/receiver (not shown) coupled with the ultrasonic probe  16 . The ultrasonic probe  16  is able to transmit or receive ultrasonic waves of a predetermined frequency coupled with the ultrasonic transmitter/receiver.  
      Reaction vessel  11  further includes an ultrasonic transducer  25  at the side wall  11   b , and ultrasonic transducers  26  provided at the opposite side wall  11   c  which faces the side wall  11   b  where the ultrasonic transducer  25  is provided. The ultrasonic transducer  25  includes ultrasonic transmission probe  27  coupled with a ultrasonic transducer (not shown), while the ultrasonic transducer  26 , which is provided at the opposite side wall  11   c  against the side wall  11   b  having the ultrasonic transducer  25 , includes an ultrasonic receiving probe  28  coupled with an ultrasonic receiver (not shown). The ultrasonic transmission probe  27  and the ultrasonic receiving probe  28  are provided such that they tilt upwardly against the side wall  11   b  or  11   c . The ultrasonic transmission probe  27  is provided at an upper side of the side wall  11   b , where the H 2 SO 4  solution B should exist in the reaction vessel. The ultrasonic transducer  28  includes a plurality of ultrasonic receiving probes  28 , for example five ultrasonic receiving probes  28  as shown in  FIG. 7 , aligned in the vertical direction. The ultrasonic transmission probe  27  and ultrasonic receiving probes  28  have directivity.  
      When the ultrasonic transducer  13  or  25  generates an electric pulse to the ultrasonic probe  16  or  27 , ultrasonic waves, which have for example a frequency of 5 MHz, are transmitted from the ultrasonic probe  16  or  27  inside the reaction vessel  11 .  
      The ultrasonic waves transmitted inside the reaction vessel  11  reflect at a liquid-liquid boundary surface Fa, and of a gas-liquid boundary surface Fb due to the difference of the density at these boundaries. These reflected ultrasonic waves are referred to as reflected echoes.  
      The ultrasonic probe  16  and the ultrasonic receiving probe  28  detect the reflected echoes. The reflected echoes are received and converted to an echo electric signal by the ultrasonic transmitter/receiver coupled with the ultrasonic probe  16  or the ultrasonic receiver coupled with the ultrasonic receiving probe  28 . The echo electric signal is sent to a data processing unit (not shown). The data processing unit processes the echo electric signal and calculates positions (height) of the liquid-liquid boundary surface Fa and the gas-liquid boundary surface Fb in the reaction vessel  11  without contact. Results of the calculation are preferably outputted on a display unit (not shown).  
      The ultrasonic transducer  13 , which is provided at the bottom  11   a  of the reaction vessel, is used to detect the liquid-liquid boundary surface Fa in the reaction vessel  11 .  FIG. 7  shows a configuration that has three ultrasonic transducers  13  to detect the liquid-liquid boundary surface Fa. The ultrasonic transducers  25 ,  26 , which is provided at the side wall  11   b ,  11   c , are used to detect the gas-liquid boundary surface Fb.  
      An ultrasonic pulse of the ultrasonic waves transmitted from the ultrasonic probe  16 , by the operation of the ultrasonic transducer  13 , penetrates through the bottom  11   a  of the reaction vessel  11  and goes upwardly in the HI solution A in the reaction vessel  11 .  
      The ultrasonic pulse transmitted in the reaction vessel  11  which reaches at the liquid-liquid boundary surface Fa is reflected downwardly as reflected echoes due to the difference of the acoustic impedance between HI solution A and H 2 SO 4  solution B, which is resulted from the difference of densities of those solutions. The reflected echoes then come back to the ultrasonic probe  16  through the bottom  11   a . The position (height) of the liquid-liquid boundary surface Fa can be calculated based upon time difference between the transmission of the ultrasonic pulse and the receive of the reflected echoes at the ultrasonic probe  16 . Compensation of the sound velocity can be accomplished by such a way shown in the first embodiment.  
      Some part of the ultrasonic pulse transmitted in the reaction vessel  11  penetrates through the liquid-liquid boundary surface Fa, and further goes upwardly in the H 2 SO 4  solution B. This ultrasonic pulse is also reflected downwardly as the reflected echoes at the gas-liquid boundary surface Fb and comes back to the ultrasonic probe  16 . Therefore, as shown in the first embodiment, the position (height) of the gas-liquid boundary surface Fb may be calculated based upon the time difference between the transmission of the ultrasonic pulse and the reception of the reflected echoes from the gas-liquid boundary surface Fb.  
      However, multiple reflection which occurs at the liquid-liquid boundary surface Fa may overlap on the reflected echoes from the gas-liquid boundary surface Fb in some situations. In such a situation, it is difficult to detect the reflected echoes from the gas-liquid boundary surface Fb with the ultrasonic probe  16 .  
      The second embodiment shown in  FIG. 7  utilizes the ultrasonic transducer  25  and  26  to detect the position (height) of the gas-liquid boundary surface Fb instead of the ultrasonic transducer  13  provided at the bottom  11   a.    
      An ultrasonic pulse of the ultrasonic waves transmitted from the ultrasonic transmission probe  27 , by the operation of the ultrasonic transducer  25 , penetrates through the side wall  11   b  of the reaction vessel  11 . As mentioned, the ultrasonic transmission probes  27  and  28  have a tilt (e.g., are angled) against the side wall  11   b  and  11   c . Therefore, the ultrasonic pulse is transmitted aslant towards the gas-liquid boundary surface from the ultrasonic transmission probe  27  at an angle ψ against the side wall  11   b . The transmitted ultrasonic pulse then reflects at the gas-liquid surface Fb and is received by one of the ultrasonic receiving probes  28 . Because an angle of incidence of the ultrasonic pulse received at the ultrasonic receiving probe  28  is also ψ, the position (height) of the ultrasonic receiving probe  28  differs due to the position (height) of the gas-liquid boundary surface Fb. In this embodiment, since a plurality of ultrasonic receiving probes  28  are aligned vertically, the position of the incoming point (height h 3 ) of the reflected echoes, where the reflected echoes reach at the opposite side wall  11   c , can be readily detected. The position of the incoming point (height h 3 ) may be determined precisely by a distribution of intensity of reflected echoes detected at the ultrasonic receiving probes  28 . Data of detected position of the incoming point is sent to a data processing unit (not shown but see  FIG. 2 ).  
      When the height from the height h 2  of the ultrasonic transmission probe  27  to the detected position h 3  of the incoming point of the reflected echoes is dh 2 , the height d 2  of the gas-liquid boundary surface Fb is determined by a formula (9), using the height of the ultrasonic transmission probe  27  as h 2 , the angle of the transmitted ultrasonic pulse from the ultrasonic transmission probe as A, and the width of the reaction vessel  11 , which is a horizontal distance between the ultrasonic transmission probe  27  and the ultrasonic receiving probes  28 , as L.  
               d   2     =         h   2     +         L   ⁢           ⁢   cot   ⁢           ⁢   ψ     +     (       h   3     -     h   2       )       2       =       h   2     +         L   ⁢           ⁢   cot   ⁢           ⁢   ψ     +     d   ⁢           ⁢     h   2         2                 (   9   )             
 
      With the formula (9), the height d 2  of the gas-liquid boundary surface can be determined regardless of the difference of the sound velocity in the H 2 SO 4  solution B because it uses geometric information, such as h 2 , h 3 , L, and predetermined angle ψ of the ultrasonic probes  27 ,  28 .  
      According to this embodiment, the liquid-liquid boundary surface Fa and the gas-liquid boundary surface Fb may be determined precisely even if there is multiple reflection occurring at the liquid-liquid boundary surface Fa which overlaps on the reflected echoes from the gas-liquid boundary surface Fb.  
      Further, it should be noted that this principle for the measurement of the position (height) of the gas-liquid boundary surface Fb can be applied to measure the position (height) of the liquid-liquid boundary surface Fa when using another ultrasonic transmission probe provided at the lower side of the side wall  11   b , where the HI solution A should exist in the reaction vessel  11 . In such a case, it is not necessary to use the ultrasonic probe  16  provided at the bottom  11   a  and the means for compensation of the sound speed to detect the position of the liquid-liquid boundary surface Fa. However it may be utilized for further credibility of the measured position of the boundary surfaces Fa and Fb.  
       FIG. 8  is a schematic sectional view of a modification of the second embodiment shown in  FIG. 7 . In  FIG. 8 , the same numeric symbols are used for the same elements as shown in  FIG. 7 . Accordingly, detailed descriptions may be omitted for those elements.  
      The ultrasonic transmission probe  27  and the ultrasonic receiving probe  28  have directivity, and the ultrasonic pulse is transmitted aslant to the gas-liquid boundary surface Fb in the second embodiment shown in  FIG. 7 . In this modification, the reaction vessel  11  of a hydrogen production apparatus  10 B includes an ultrasonic transmission probe  30  provided at the sidewall  11   b  and an ultrasonic receiving probe  31  provided at the opposite side wall  11   c , instead of the ultrasonic transmission probe  27  and ultrasonic receiving probes  28  shown in  FIG. 7 . The ultrasonic transmission probe  30  and the ultrasonic receiving probe  31  have no directivity. Therefore, the ultrasonic pulse is transmitted from the ultrasonic transmission probe within wide angles. The ultrasonic receiving probe  31  is provided at the upper side of the opposite side wall  11   c , where the H 2 SO 4  solution B should exist in the reaction vessel  11 . It is not necessary to place the ultrasonic receiving probe  31  at the same height as the ultrasonic transmission probe  30 .  
      A part of the ultrasonic pulse transmitted from the ultrasonic transmission probe  30  reaches directly to the ultrasonic receiving probe  31  in the shortest distance. Other part of the ultrasonic pulse reflects at the gas-liquid boundary surface Fb and the reflected echoes reach to the ultrasonic receiving probe  31 . An incidence angle ψ of the reflected echoes can be determined by a propagation distance  1  of the reflected echoes from the ultrasonic transmission probe  30  to the ultrasonic receiving probe  31 . The incidence angle ω is obtained by using the propagation distance  1  and width L of the reaction vessel  11  as formula (10)  
               sin   ⁢           ⁢   ω     =     L   l             (   10   )             
 
      The propagation distance  1  can be calculated based upon the time difference between the transmission of the ultrasonic pulse and receive of the reflected echo. When calculating the propagation distance, it is necessary to determine the velocity of sound inside the H 2 SO 4  solution B. However, it can be obtained by the time taken to receive the ultrasonic pulse that reaches directly from the ultrasonic transmission probe  30  because a distance between the ultrasonic transmission probe  30  and the ultrasonic receiving prove  31  is readily obtained by geometric position of those probes  30  and  31 . Thus, the speed of sound in the H 2 SO 4  solution B is compensated and the incidence angle ω can be obtained precisely.  
      When the incidence angle ω is obtained, the position (height) d 2  of the gas-liquid boundary surface Fb is calculated, by using h 2  as the height of the ultrasonic transmission probe  30 , h 3  as the height of the ultrasonic receiving probe  31 , as formula (11), which is substantially the same as the formula (9).  
               d   2     =         h   2     +         L   ⁢           ⁢   cot   ⁢           ⁢   ω     +     (       h   3     -     h   2       )       2       =       h   2     +         l   ⁢           ⁢   cos   ⁢           ⁢   ω     +     (       h   3     -     h   2       )       2                 (   11   )             
 
      According to this modification, the liquid-liquid boundary surface Fa and the gas-liquid boundary surface Fb may be determined precisely even if there is multiple reflection occurring at the liquid-liquid boundary surface Fa which overlaps on the reflected echoes from the gas-liquid boundary surface Fb, same as the embodiment shown in  FIG. 8 .  
      Further, it should be noted that this principle for the measurement of the position (height) of the gas-liquid boundary surface Fb can be applied to measure the position (height) of the liquid-liquid boundary surface Fa when using another ultrasonic transmission probe provided at the lower side of the side wall  11   b , where the HI solution A should exist in the reaction vessel  11 . In such a case, it is not necessary to use the ultrasonic probe  16  provided at the bottom  11   a  and the means for compensation of the sound velocity to detect the position of the liquid-liquid boundary surface Fa. However, it may be utilized for further credibility of the measured position of the boundary surfaces Fa and Fb.  
       FIGS. 9 and 10  are schematic sectional views of another modification of the second embodiment shown in  FIG. 7 . In  FIGS. 9 and 10 , the same numeric symbols are used for the same elements as shown in  FIG. 7 . Accordingly, detailed descriptions has been omitted for those elements.  
      In this modification, no ultrasonic transducer is provided at the bottom  11   a  of the reaction vessel  11 . The reaction vessel  11  of a hydrogen production apparatus  10 C comprises an ultrasonic transducer  35  provided at the side wall  11   b  and an ultrasonic transducer  36  provided at the opposite side wall  11   c.    
      The ultrasonic transducer  35  includes an ultrasonic transmission probe unit  37 . The ultrasonic transmission probe unit  37  is provided at an upper side of the side wall  11   b , where the H 2 SO 4  solution B should exist in the reaction vessel. The ultrasonic transmission probe unit  37  comprises a downwardly tilted ultrasonic probe  37   a  and an upwardly tilted ultrasonic probe  37   b . The ultrasonic transducer  36  includes a plurality of ultrasonic receiving probes  38  vertically aligned to each other. The ultrasonic pulse from the downwardly tilted ultrasonic probe  37   a  is transmitted aslant toward the liquid-liquid boundary surface Fa at an angle Φ, which is a tilted angle of the downwardly tilted ultrasonic probe  37   a , against the side wall  11   b . The transmitted ultrasonic pulse reflects at the liquid-liquid surface Fa as reflected echoes, and the reflected echoes reach at one of the ultrasonic receiving probes  38  at an incidence angle Φ. The position (height) of an incoming point of the reflected echoes, where the reflected echoes reach at the ultrasonic receiving probes  38 , differs when the position (height) of the liquid-liquid boundary surface changes. Therefore, the position (height) d 1  of the liquid-liquid boundary surface Fa can be calculated according to the position (height) h 3  of the incoming point of the reflected echoes, where the reflected echoes reach at the ultrasonic receiving probes  38 . The position (height) h 3  of the incoming point of the reflected echoes may be obtained by intensity distribution of the reflected echoes detected by the ultrasonic receiving probes  38 . When the vertical distance from the position (height) h 1  of the downwardly tilted ultrasonic probe  37   a  to the position (height) h 3  of the incoming point is defined as dh 1 , which is h 3 -h 1 , the height d 1  of the liquid-liquid boundary surface Fa is calculated by a formula (12), using the width L of the reaction vessel  11 .  
               d   1     =         h   1     -         L   ⁢           ⁢   cot   ⁢           ⁢   ϕ     +     (       h   3     -     h   1       )       2       =       h   1     -         L   ⁢           ⁢   cot   ⁢           ⁢   ϕ     +     d   ⁢           ⁢     h   1         2                 (   12   )             
 
      The gas-liquid boundary surface Fb can be calculated in the same manner by utilizing the upwardly tilted ultrasonic probe  37   b  as shown in  FIG. 10 . When defining a incidence angle of the transmission of the ultrasonic pulse against the side wall  11   b  as Ψ, the position (height) d 2  of the gas-liquid boundary surface Fb can be obtained as formula (13) by using the height dh 2  from the height h 2  of the upwardly tilted ultrasonic probe  37   b  to the height h 4  of the incoming point of the reflected echoes, where the reflected echoes reach at the ultrasonic receiving probe  37   b , and width L of the reaction vessel  11 .  
               d   2     =         h   2     +         L   ⁢           ⁢   cot   ⁢           ⁢   ψ     +     (       h   4     -     h   2       )       2       =       h   2     +         L   ⁢           ⁢   cot   ⁢           ⁢   ψ     +     dh   2       2                 (   13   )             
 
      In this modification, the ultrasonic transducer  36  may include an ultrasonic receiving probe, which is movable in the vertical direction, instead of a plurality of ultrasonic receiving transducers  38  aligned vertically. In this modification, it is important to measure the position (height) of the incoming point of the reflected echoes.  
      According to this modification, the liquid-liquid boundary surface Fa and the gas-liquid boundary surface Fb may be determined precisely regardless of the changing of the speed of sound (sound velocity) in the reacting liquids.  
       FIG. 11  is a schematic sectional view of an apparatus for producing hydrogen by IS process in accordance with the third embodiment.  
      A hydrogen production apparatus  55 , which produces hydrogen continuously by IS process, can detect constituent (component) and concentration (density) of the reacting liquid inside the apparatus.  
      A reaction vessel  11  of the hydrogen producing apparatus  55  encloses hydriodic acid (HI) solution A and sulfuric acid (H 2 SO 4 ) solution B resulted from the Bunsen reaction. Hydriodic acid (HI) solution A and sulfuric acid (H 2 SO 4 ) solution B are both in a liquid state, as reacting liquids.  
      Since HI solution A and H 2 SO 4  solution B (also referred to as reacting liquids) are non-mixing liquid with respect to each other, they are separately held in the reaction vessel  11 , wherein the H 2 SO 4  solution B, which has lower density than HI solution A, is disposed above HI solution B. In the space above the H 2 SO 4  solution B in the reaction vessel  11 , gas C produced in the reaction vessel  11 , such as oxygen or hydrogen may exist.  
      Reaction vessel  11  has a box-shaped (e.g., rectangular) enclosure. The reaction vessel  11  of the hydrogen production apparatus  55  comprises a plurality of gamma-ray sources  56  provided at the side wall  11   b  as radiation source, and radiation detectors  57  provided at the opposite side wall  11   c , which faces toward the side wall  11   b  with the gamma-ray sources  56 . The radiation detectors  57  are connected to a data processing unit  58 . Thus the data detected at the radiation detectors  57  are sent to the data processing unit  58 .  
      Each of the gamma-ray sources  56  is provided separately in the vertical direction, and the each of the radiation detector  57  is provided at the opposite side wall  11   c  on the height corresponding to the gamma-ray source  56  at the side wall  11   b . The gamma-ray source  56  and the corresponding radiation detector  57  are provided such that the gamma ray emitted from the gamma-ray source  56  to the corresponding radiation detector  57  transmits horizontally through the reaction vessel  11 .  FIG. 11  shows an example which includes three gamma-ray sources  56  and three radiation detectors  57 . In this embodiment, one set of the gamma-ray source  56  and the radiation detector  57  is provided at the position where the HI solution A should exist in the reaction vessel  11 , another set is provided at the position where the H 2 SO 4  solution B should exist in the reaction vessel  11 , and the last set is provided at the position where the gas C should exist in the reaction vessel  11 .  
      The gamma-ray source  56  emits gamma ray toward the corresponding radiation detector  57 . The gamma ray emitted from the gamma-ray source  56  transmits through HI solution A, the H 2 SO 4  solution B or gas C, respectively, and reaches to the radiation detector  57 . A correlation between the count and intensity of gamma ray detected at the radiation counter  57  is given by the formula (14).  
                   A   i       4   ⁢   π   ⁢           ⁢     L   2         =     exp   ⁢       {       -     (         σ     1   ⁢   i       ⁢     ρ   1       +       σ     2   ⁢   i       ⁢     ρ   2       +       σ     3   ⁢   i       ⁢     ρ   3       +       σ     4   ⁢   i       ⁢     ρ   4       +       σ     5   ⁢   i       ⁢     ρ   5         )       ·   L     }     ·     1     f   i         ⁢     N   i         ,           (   14   )             
 
 where the numeric symbols used in the formula are as below; 
 
      σ: cross section of absorption of the gamma ray with energy i emitted from the gamma ray source  56  (characteristic value by the energy of the gamma-ray and the substance)  
      ρ: density  
      L: width of the reaction vessel  11   
      f i : sensitivity of the radiation detector  57  for the gamma ray with energy i emitted from the gamma ray source  56  (including the effect of attenuation of the radiation by the reaction vessel)  
      A i : number of the gamma ray with energy i emitted from the gamma ray source  56  per unit time  
      N i : count of the radiation detector  57  against the gamma ray with energy i emitted from the gamma ray source  56   
      ρ 1 : density of gas  
      ρ 2 : density of fluid  
      ρ 3 : density of hydriodic acid  
      ρ 4 : density of water  
      ρ 5 : density of iodine  
      In the formula (14), the number A of the emitted radiation, the width L of the reaction vessel  11 , and the sensitivity f of the radiation detector  57 , are known information. Further, the count N of the radiation detector  57  can be obtained as a measured value. Therefore, substituting those values in the formula ( 14 ), the product σ*ρ of the gamma-ray absorption cross section σ and the density ρ can be obtained.  
      The product σ*ρ of the gamma-ray absorption cross section σ and the density ρ can be expressed as; 
 
σ*ρ=σ 1i *ρ 1 +σ 2i *ρ 2 +σ 3i *ρ 3 +σ 4i *ρ 4 +σ 5i *ρ 5    (15) 
 
      σ 1i *ρ 1 , which is regarding the gas in the right-hand side of the formula (15), is relatively smaller than the value regarding the liquids. Thus, the approximations (16) described below can be satisfied. 
 
σ 1i *ρ 1 &lt;&lt;σ 2i *ρ 2  
 
σ 1i *ρ i &lt;&lt;σ 3i *ρ 3  
 
σ 1i *ρ 1 &lt;&lt;σ 4i *ρ 4  
 
σ 1i *ρ 1 &lt;&lt;σ 5i *ρ 5    (16) 
 
      When the substance, which the radiation transmits through, is gas (which means ρ 2 =ρ 3 =ρ 4 =ρ 5 =0), the product σ*ρ satisfies the formula σ*ρ=σ 1i *ρ 1 . In such a situation, the product σ*ρ is much smaller than the situation when the radiation transmits through liquid. Therefore, one can determine the substance, which the radiation transmits through, is gas. Furthermore, the gamma-ray absorption cross section σ 1i  is a known value. Thus the density of the gas ρ i  can be obtained by dividing the product σ*ρ by σ 1i .  
      On the other hand, when the substance, which the radiation transmits through, is liquid, the product σ*ρ of the gamma-ray absorption cross section σ and the density ρ is much larger than the situation when the radiation transmits through gas. Therefore one can determine the substance, which the radiation transmits through, is liquid. Discrimination of the constituent and calculation of the density can be accomplished as described below. When emitting four energies (i=1 to 4), the formula (14) are as follow.  
                   A   1       4   ⁢   π   ⁢           ⁢     L   2         =     exp   ⁢       {       -     (         σ   21     ⁢     ρ   2       +       σ   31     ⁢     ρ   3       +       σ   41     ⁢     ρ   4       +       σ   51     ⁢     ρ   5         )       ·   L     }     ·     1     f   1         ⁢     N   1         ⁢     
     ⁢         A   2       4   ⁢   π   ⁢           ⁢     L   2         =     exp   ⁢       {       -     (         σ   22     ⁢     ρ   2       +       σ   32     ⁢     ρ   3       +       σ   42     ⁢     ρ   4       +       σ   52     ⁢     ρ   5         )       ·   L     }     ·     1     f   2         ⁢     N   2         ⁢     
     ⁢         A   3       4   ⁢   π   ⁢           ⁢     L   2         =     exp   ⁢       {       -     (         σ   23     ⁢     ρ   2       +       σ   33     ⁢     ρ   3       +       σ   43     ⁢     ρ   4       +       σ   53     ⁢     ρ   5         )       ·   L     }     ·     1     f   3         ⁢     N   3         ⁢     
     ⁢         A   4       4   ⁢   π   ⁢           ⁢     L   2         =     exp   ⁢       {       -     (         σ   24     ⁢     ρ   2       +       σ   34     ⁢     ρ   3       +       σ   44     ⁢     ρ   4       +       σ   54     ⁢     ρ   5         )       ·   L     }     ·     1     f   4         ⁢     N   4                 (   17   )             
 
      In the formula (17), the numbers A 1  to A 4  of the emitted radiation, the width L of the reaction vessel  11 , the sensitivities f 1  to f 4  of the radiation detector  57 , and the gamma-ray absorption cross section σ ji  (i=1 to 4, j=1 to 4) are known information. Further, the count N of the radiation detector  57  can be obtained by measurements.  
      Therefore, the solution for the equations (17) can be solved. Thus, the densities ρ 2 , ρ 3 , ρ 4 , and ρ 5  can be obtained, and the constituents and density of the substance in the reaction vessel  11  are identified. The data processing unit  58  may store data regarding constituents in each layer in the reaction vessel  11  and cross sections of absorption of each gamma ray emitted from the gamma ray sources for the constituents for the calculation described above.  
      The embodiment described above is to identify the constituents and densities for four constituent liquids; however, the gamma-ray sources emitting N species of gamma-ray having different energies can be applied to identify the constituents and densities for N of constituent liquids.  
      The embodiment shown in  FIG. 11  utilizes gamma-ray source  56  as radioactive sources, however neutron sources can be applied to identify the constituents and density (concentration) of the liquid in the same manner described above.  
      According to the embodiment, the constituent and concentration (density) of the liquid enclosed in the reaction vessel  11  can be identified from the outside of the reaction vessel without contacting any of the liquids.  
       FIG. 12  is a schematic sectional view of an apparatus for producing hydrogen by IS process in accordance with the fourth embodiment.  
      A hydrogen production apparatus  60 , which produces hydrogen continuously by IS process, can detect constituent (component) and concentration (density) of the reacting liquid inside the apparatus.  
      A reaction vessel  11  of the hydrogen producing apparatus  60  encloses hydriodic acid (HI) solution A and sulfuric acid (H 2 SO 4 ) solution B resulted from the Bunsen reaction. Hydriodic acid (HI) solution A and sulfuric acid (H 2 SO 4 ) solution B are both in a liquid state, as reacting liquids.  
      Since HI solution A and H 2 SO 4  solution B (also referred to as reacting liquids) are non-mixing liquid with each other, they are separately held in the reaction vessel  11 , wherein the H 2 SO 4  solution B, which has lower density than HI solution A, is provided above HI solution B. In the space above the H 2 SO 4  solution B in the reaction vessel  11 , gas C produced in the reaction vessel  11 , such as oxygen or hydrogen may exist.  
      Reaction vessel  11  has a box-shaped (e.g., rectangular) enclosure. The reaction vessel  11  of the hydrogen production apparatus  60  comprises a plurality of neutron sources  61  provided at the side wall  11   b  as radiation sources, and gamma-ray detectors  62  provided at the opposite side wall  11   c  as radiation detectors, which faces toward the side wall  11   b  with the neutron sources  61 . The gamma-ray detectors  62  are connected to the data processing unit  58 . Thus, the data detected at the radiation detectors  57  are sent to the data processing unit  58 .  
      Each of the neutron sources  56  is provided separately in the vertical direction, and the each of the gamma-ray detector  62  is provided at the opposite side wall  11   c  on the height corresponding to the neutron source  61  at the side wall  11   b . The neutron source  61  and the gamma-ray detector  62  are provided such that the neutron from the neutron source  61  penetrates (transmits) horizontally through the reaction vessel  11  to the corresponding gamma-ray detector  62 .  FIG. 12  shows an example which includes three neutron sources  61  and three gamma-ray detectors  62 . In this embodiment, one set of the neutron source  61  and the radiation detector  62  is provided at the position where the HI solution A should exist in the reaction vessel  11 , another set is provided at the position where the H 2 SO 4  solution B should exist in the reaction vessel  11 , and the last set is provided at the position where the gas C should exist in the reaction vessel  11 .  
      The gamma-ray detectors  62  detect neutron capture gamma-ray, whose intrinsic energy differs by the substance which the neutron penetrates (transmits) through. In this embodiment, utilizing this difference identifies constituents and densities of the substance in the reaction vessel  11 .  
      The neutron source  61  emits neutron toward the corresponding gamma-ray detector  62 . The neutron emitted from the neutron source  61  penetrates (transmits) through HI solution A, the H 2 SO 4  solution B or gas C, respectively, and are captured by constituents in the substance A, B or C in a probability characteristic to those constituents. A molecule of the constituent of the substance A, B or C that captures the neutron radiates gamma-ray, which has an energy characteristic corresponding to the constituent of the substance.  
      For example, this energy of gamma-ray (also referred to as “characteristic gamma-ray energy”), which is a characteristic of the constituent of the substance, is 2.22 MeV for hydrogen, 4.14 MeV for oxygen, 8.64 MeV for sulfur, 6.83 MeV for iodine, and 7.65 MeV for iron, which is the main component of reaction vessel  11 , when the thermal neutron is captured. A count of gamma rays having any particular energy (which means any particular element regarding to the particular energy) can be obtained by, for example, pulse height discrimination based upon the difference of the characteristic gamma-ray energy regarding the constituent of the substances by any neutron energy.  
      The count Nj at the gamma-ray detector  62  of the gamma ray, which has the characteristic gamma-ray energy due to the capture of the neutron in the constituents of the substances (elements), is expressed by the formula (18) by assuming the number of the constituents of the substance is j, wherein j=1 to n.  
                 1     f   j       ⁢     N   j       =           B   j     ⁢     σ   j     ⁢     ρ   j           ∑     i   =   1     n     ⁢       σ   i     ⁢     ρ   i           ⁢       (     1   -     exp   ⁢     {     -       ∑     i   =   1     n     ⁢       σ   i     ⁢       ρ   i     ·   L           }         )     ·   ϕ               (   18   )             
 
 where the numeric symbols used in the formula are as below; 
 
      σ i : (n,gamma) corss section for the element i (characteristic value depends on the energy of emitted neutron and the constituent of the substance (element))  
      B j : gamma-ray branching ratio (ratio of the gamma ray having characteristic energy in all of the neutron capture gamma-ray for the element j)  
      ρ i : density of the element i (i=1 to n)  
      L: width of the reaction vessel  11   
      f i : sensitivity of the radiation detector for the characteristic gamma-ray energy of species j (including such as a compensation of solid angle due to the distance between the position of the neutron reaction and the radiation detector)  
      Φ: neutron flux (1/cm 2 /s)  
      Since formula (18) is satisfied for each of characteristic energies for n species of the constituents of the substance (element). Therefore, the formula (18) constitutes simultaneous equations, expressly,  
                   1     f   1       ⁢     N   1       =           B   1     ⁢     σ   1     ⁢     ρ   1           ∑     i   =   1     n     ⁢       σ   i     ⁢     ρ   i           ⁢       (     1   -     exp   ⁢     {     -       ∑     i   =   1     n     ⁢       σ   i     ⁢       ρ   i     ·   L           }         )     ·   ϕ         ⁢     
     ⁢         1     f   2       ⁢     N   2       =           B   2     ⁢     σ   2     ⁢     ρ   2           ∑     i   =   1     n     ⁢       σ   i     ⁢     ρ   i           ⁢       (     1   -     exp   ⁢     {     -       ∑     i   =   1     n     ⁢       σ   i     ⁢       ρ   i     ·   L           }         )     ·   ϕ         ⁢     
     ⁢   …   ⁢     
     ⁢         1     f   n       ⁢     N   n       =           B   n     ⁢     σ   n     ⁢     ρ   n           ∑     i   =   1     n     ⁢       σ   i     ⁢     ρ   i           ⁢       (     1   -     exp   ⁢     {     -       ∑     i   =   1     n     ⁢       σ   i     ⁢       ρ   i     ·   L           }         )     ·   ϕ                 (   19   )             
 
      In equations (19), reaction cross section σ i , gamma-ray branching ratio B j , width L of the reaction vessel  11 , sensitivity f i  of the radiation detector  62 , and neutron flux Φ are known information. Further, the count Ni can be measured at the radiation detector  62 . Thus, the densities ρ i  (i=1 to n) of the constituents of the substance (element) are the only unknown quantities in equation (19). Since the number of the equations (19) and unknown quantities are both n, one set of solution for the unknown quantities can be obtained by solving the simultaneous equations (19). Therefore, the densities ρ i  (i=1 to n) of the constituents of the substance (element) are obtained.  
      The densities ρ i  of the constituents of the substance (element) and the densities of the components of the solution A and B, which are sulfuric acid molecule (H 2 SO 4 ), water molecule (H 2 O), iodine molecule (I 2 ), and hyriodic acid (HI), satisfy simultaneous equations (20) as below.  
                 ρ   ⁡     (   S   )       =         A   ⁡     (   S   )         A   ⁡     (       H   2     ⁢     SO   4       )         ⁢     ρ   ⁡     (       H   s     ⁢     SO   4       )           ⁢     
     ⁢       ρ   ⁡     (   I   )       =           A   ⁡     (   I   )         A   ⁡     (   HI   )         ⁢     ρ   ⁡     (   HI   )         +     ρ   ⁡     (     I   2     )           ⁢     
     ⁢             ρ   ⁡     (   H   )       =       ⁢           A   ⁡     (     H   2     )         A   ⁡     (       H   2     ⁢     SO   4       )         ⁢     ρ   ⁡     (       H   s     ⁢     SO   4       )         +                     ⁢           A   ⁡     (     H   2     )         A   ⁡     (       H   2     ⁢   O     )         ⁢     ρ   ⁡     (       H   s     ⁢   O     )         +         A   ⁡     (   I   )         A   ⁡     (   HI   )         ⁢     ρ   ⁡     (   HI   )                   ⁢     
     ⁢       ρ   ⁡     (   O   )       =           A   ⁡     (     O   4     )         A   ⁡     (       H   2     ⁢     SO   4       )         ⁢     ρ   ⁡     (       H   s     ⁢     SO   4       )         +         A   ⁡     (   O   )         A   ⁡     (       H   2     ⁢   O     )         ⁢     ρ   ⁡     (       H   s     ⁢   O     )                     (   20   )             
 
 Where; 
 
      ρ(x) means the density of the constituent of the substance x, and  
      A(x) means the molecular weight of the element x  
      In the simultaneous equations (20), which comprises four equations, there are four unknown quantities, ρ(H 2 SO 4 ), ρ(HI), ρ(I 2 ), and ρ(H 2 O). Therefore, the simultaneous equations (20) can be solved and the densities (concentrations) of the solution A, B and gas C in the reaction vessel  11  can be identified.  
      The data processing unit  63  may store data regarding the constituents of the substances in the reaction vessel  11 , and reaction cross section for the constituents of the substances (elements) for the calculation described above in the data processing unit  63 .  
       FIG. 13  is a schematic sectional view of an apparatus for producing hydrogen by IS process in accordance with the fifth embodiment.  
      A hydrogen production apparatus  64 , which produces hydrogen continuously by the IS process, can detect positions of boundary surfaces inside the apparatus.  
      A reaction vessel  11  of the hydrogen producing apparatus  64  encloses hydriodic acid (HI) solution A and sulfuric acid (H 2 SO 4 ) solution B resulted from the Bunsen reaction. Hydriodic acid (HI) solution A and sulfuric acid (H 2 SO 4 ) solution B are both in a liquid state, as reacting liquids.  
      Since HI solution A and H 2 SO 4  solution B (also referred to as reacting liquids) are non-mixing liquids that do not mix with each other, they are separately held in the reaction vessel  11 , wherein the H 2 SO 4  solution B, which has lower density than HI solution A, is laid above HI solution B. In the space above the H 2 SO 4  solution B in the reaction vessel  11 , gas C produced in the reaction vessel  11 , such as oxygen or hydrogen may exist.  
      Reaction vessel  11  has a box-shaped (e.g., rectangular) enclosure. The reaction vessel  11  of the hydrogen production apparatus  64  comprises a gamma-ray source  56  provided at a lower side of the side wall  11   b  as a radiation source, and a radiation detector  57  provided at an upper side of the opposite side wall  11   c , which faces toward the side wall  11   b  with the gamma-ray sources  56 . The radiation detectors  57  are connected to a data processing unit  58 . Thus, the data detected at the radiation detectors  57  are sent to the data processing unit  58 . The data processing unit  58  calculates the position of the boundary surfaces inside the reaction vessel  11 . The calculated result from the data processing unit  58  may be displayed on a display (not shown).  
      The gamma-ray source  56  emits gamma ray toward the radiation detector  57 . The gamma ray emitted from the gamma-ray source  56  transmits the reaction vessel  11  through the radiation detector  57  goes horizontally through the reaction vessel  11 . As shown in  FIG. 13 , the gamma ray passes aslant through two reacting liquids, which are the solution A and B, in the reaction vessel  11  from the lower side to the upper side. The gamma ray which transmits through the reaction vessel  11  is detected at the radiation detector  57 , which is provided at the upper side of the opposite side wall  11   c.    
      A relation between the intensity of the gamma ray emitted from the gamma-ray source  56  and a count of the gamma ray detected at the radiation detector  57  is expressed in the formula (21) shown below.  
               A     4   ⁢       π   ⁡     (       l   1     +     l   2       )       2         =     exp   ⁢       {     -     (       σρ   ⁢           ⁢     l   1       +       σ   ′     ⁢     ρ   ′     ⁢     l   2         )       }     ·     1   f       ⁢   N             (   21   )             
 
 where the numeric symbols used in the formula are as below; 
 
      l 1 : distance that the gamma ray passes through in the solution A,  
      l 2 : distance that the gamma ray passes through in the solution B.  
      σ: gamma-ray absorption cross section of the solution A  
      σ′: gamma-ray absorption cross section of the solution B  
      ρ: density of the solution A,  
      ρ′: density of the solution B.  
      f: sensitivity of the radiation detector  57  for the gamma-ray source  56   
      A: number of the radiation emitted to the direction 4π by the gamma-ray source  56  per unit time  
      N: count of the radiation detector  57  against the gamma-ray source  56   
      When defining the height that the gamma ray passes through inside the reaction vessel as Y and the height that the gamma ray passes through in the H 2 SO 4  solution B as D, the formula (22) is satisfied.  
                 l   1     D     =       l   2       Y   -   D               (   22   )             
 
      Further, there is a relationship between the l 1 , l 2 , Y and the width L of the reaction vessel  11  as: 
 
( l   1   +l   2 ) 2   =Y   2   +L   2    (23) 
 
      Therefore, formula (21) can be rewritten as:  
               A     4   ⁢     π   ⁡     (       Y   2     +     L   2       )           =     exp   ⁢       {     -     (       σρ   ⁢           ⁢       Y   -   D     Y     ⁢         Y   2     +     L   2           +       σ   ′     ⁢     ρ   ′     ⁢     D   Y     ⁢         Y   2     +     L   2             )       }     ·     1   f       ⁢   N             (   24   )             
 
      In the formula (24), height Y that the gamma ray passes through in the solutions, width L of the reaction vessel  11 , number A of the radiation, and sensitivity f of the radiation detector  57  are known information. Therefore, the radiation detector  57  can measure count N.  
      Further, the product of the gamma-ray absorption and the density, such as σ*ρ and σ′*ρ′, can be obtained according to the third embodiment shown in  FIG. 11 . Therefore, the height D that the gamma ray passes through in the H 2 SO 4  solution B can be obtained according to the formula (24). Thus, the height of the liquid-liquid boundary surface Fa is obtained in accordance with this embodiment.  
      The gamma-ray source  56  might be a neutron source instead of the gamma-ray source  56  shown in  FIG. 13 . The height of the liquid-liquid boundary surface Fa can be calculated in the same manner as explained above in that situation.  
      According to this embodiment, the liquid-liquid boundary surface Fa inside the reaction vessel  11  can be obtained from outside of the reaction vessel without physically contacting the reaction vessel. When the principle explained in the third or the forth embodiment is applied to this embodiment, the constituent (component) and the concentration (density) may also be obtained according to this embodiment.  
       FIG. 14  is a schematic sectional view of an apparatus for producing hydrogen by the IS process in accordance with the sixth embodiment.  
      A hydrogen production apparatus  65 , which produces hydrogen continuously by the IS process, can detect positions of boundary surfaces inside the apparatus. The hydrogen production apparatus  65  can also obtain density (concentration) of a substance of the reacting liquids inside the apparatus. The density is calculated based upon absorption property of sulfur and iodine, which is included in the reacting liquids, against a neutron.  
      A reaction vessel  11  of the hydrogen producing apparatus  65  encloses hydriodic acid (HI) solution A and sulfuric acid (H 2 SO 4 ) solution B resulted from the Bunsen reaction. Hydriodic acid (HI) solution A and sulfuric acid (H 2 SO 4 ) solution B are both in a liquid state, as reacting liquids.  
      Since HI solution A and H 2 SO 4  solution B (also referred to as reacting liquids) are non-mixing liquids that do not mix with each other, they are separately held in the reaction vessel  11 , wherein the H 2 SO 4  solution B, which has lower density than HI solution A, is laid above HI solution B. In the space above the H 2 SO 4  solution B in the reaction vessel  11 , gas C produced in the reaction vessel  11 , such as oxygen or hydrogen may exist.  
      Reaction vessel  11  has a box-shaped (e.g., rectangular) enclosure. The reaction vessel  11  of the hydrogen production apparatus  65  comprises a DT neutron source  66 , energy sensitive neutron detectors  67 , a data processing unit  68 , and neutron collimators  69 . The DT neutron source  66  is provided at the side wall  11   b  as a radiation source. The energy sensitive neutron detectors  67  is provided at the opposite side wall  11   c , which faces toward the side wall  11   b  with the gamma-ray sources  56 . The data processing unit  68  processes and analyzes data detected at the neutron detector  67  to calculate the height of a liquid-liquid boundary surfaces Fa and a gas-liquid boundary surface Fb. The neutron collimators  69  detect the effect of scattered radiation of neutron to the neutron detector  67 . The energy sensitive neutron detectors  67  are connected to the data processing unit  68 . Thus, the data detected at the detectors  68  are sent to the data processing unit  68 . The calculated result of the boundary surfaces Fa and Fb from the data processing unit  68  may be displayed on a display (not shown).  
      Each of the energy sensitive neutron detectors  67  is provided separately in the vertical direction. Each of the neutron collimators  69  is coupled to each of the energy sensitive neutron detector  67  and provided adjacent to the energy sensitive neutron detectors  67  to filter the scattered radiation of a neutron.  FIG. 14  shows an example which includes three energy sensitive neutron detectors  67  and three neutron collimators  69 .  
      The DT neutron source  66  emits neutrons. The neutron emitted from the DT neutron source  66  has various energies including scattering component. For example, the neutron emitted from the DT neutron source  66  has a maximum intensity of 14 MeV. The neutron, which has having various energies, is emitted within a certain angle in the reaction vessel  11 .  
      In  FIG. 14 , the neutron is emitted directly to the reaction vessel  11  from the DT neutron source  66 . However, a neutron moderator or an energy discriminator may be provided between the DT neutron source  66  and the reaction vessel  11 . The neutron moderator may help flattening energy distribution of the neutron to be emitted in the reaction vessel  11 . The energy discriminator may help emitting only neutrons having particular energy to the reaction vessel  11 .  
      A neutron source other than DT neutron source  66  may be used in this embodiment. It is preferable to use a neutron source that radiates neutrons having particular energy corresponding to the neutron resonance of the substance inside the reaction vessel  11 .  
      The neutron emitted from the DT neutron source  66  penetrates (transmits) through the reaction vessel  11  and reaches the energy sensitive neutron detectors  67  via the neutron collimators  69 . The densities of the reacting liquids in the reaction vessel  11  can be calculated in the data processing unit  68  in the same manner as explained in the forth embodiment shown in  FIG. 12 . Also, the heights of the boundary surfaces Fa and Fb can be calculated in the data processing unit  68  in the same manner as explained in the fifth embodiment shown in  FIG. 13 .  
      In this embodiment, the neutron collimators  69 , provided between the reaction vessel  11  and the energy sensitive neutron detectors  67 , reduce the effect of the scattered neutron to the neutron detector  67 . The neutron collimators  69  may be provided between the DT neutron source  66  and the reaction vessel  11  to emit neutrons only in particular directions. In such a case, signal-noise ratio of the energy sensitive neutron detectors  67  may be improved.  
      The accuracy of the calculated density and height of the boundary surfaces Fa, Fb at the data processing unit  68  may be improved in a manner as described below.  
       FIGS. 15 and 16  are graph showing the relations between neutron energy and a neutron reaction cross section or a neutron absorption cross section of sulfur and iodine, respectively (Citation from Nuclear Data Center, Japan Atomic Energy Research Institute: “Chart of the Nuclides 2000,” http://wwwndc.tokai.jaeri.go.jp/CN00/index.html (2001.12.16)). In  FIGS. 15 and 16 , the neutron reaction cross section is shown as solid lines a 1  and a 2 , while the neutron absorption cross section is shown as solid lines b 1  and b 2 .  
      The neutron reaction cross section is defined as a ratio that a reaction between the substance and the neutron having certain energy occurs. In other words, it is defined as a ratio that the neutron having certain energy is eliminated due to the reaction with the substance. On the other hand, the neutron absorption cross section is defined as a ratio that the neutron having certain energy is absorbed in the substance. The neutron absorption cross section is included in the neutron reaction cross section as one of the reaction. Most of the difference between the neutron reaction cross section and the neutron absorption cross section is due to a reaction that the neutron loses its energy when the neutron and the substance scatter.  
      In  FIGS. 15 and 16 , the neutron reaction cross section drastically changes around the energy within the neutron resonance absorption. On the other hand, the neutron absorption cross section drastically increases around the energy within the neutron resonance absorption. According to  FIGS. 15 and 16 , it can be found that sulfur and iodine have an energy range of the neutron resonance absorption that the neutron absorption cross section drastically swings with particular neutron energy. It is generally known that hydrogen and oxygen do not have neutron resonance absorption below 14 MeV.  
      In the other words, because iodine has an energy range of the neutron resonance absorption around 1 MeV, the neutron having energy of 1 MeV is greatly absorbed in iodine. The distance that the neutron penetrates (transmits) through the constituent including iodine can be estimated according to attenuation based upon this absorption. Especially, because changes are more drastic for the neutron absorption cross section than for the neutron reaction cross section, it is effective to include neutrons whose energy is reduced due to scattering for count at the energy sensitive neutron detector  67 . The distance which a neutron penetrates (transmits) through the liquid containing iodine, such as the HI solution A, may be corrected according to a distribution of a section against energy, such as a ratio of attenuation of a neutron in a range out of the neutron resonance absorption around 1 MeV and attenuation of a neutron in a range of the neutron resonance absorption.  
      For example, because the neutron reaction cross section gets smaller when the energy is lower than the energy which the neutron resonance absorption will occur, the distance which the neutron penetrates (transmits) through may be corrected according to a characteristic against the energy and a ratio of the neutron.  
      In the same manner, since sulfur has an energy range of the neutron resonance absorption around 1 keV, attenuation of a neutron having 1 keV is greater in this energy range. Therefore, the distance that the neutron penetrates (transmits) through the liquid containing sulfur may be estimated in the same manner as explained above. The distance that the neutron penetrates (transmits) through may also be corrected according to the ratio of attenuation of the neutron in a range out of the neutron resonance absorption around the range of the neutron resonance absorption and attenuation of the neutron in the range of the neutron resonance absorption.  
      The distance, which the neutron penetrates (transmits) through substance containing certain constituent, can be calculated according to attenuation of the neutron in three or more energy ranges, which include the energy range of the neutron resonance absorption of sulfur, the energy range of the neutron resonance absorption of iodine, and an energy range, in which neutron resonance absorption of sulfur or iodine does not occur, mainly comprising hydrogen scattering. Densities can be calculated according these values.  
      According to the embodiment shown in  FIG. 14 , the densities and the height of boundary surfaces of the reacting liquids can be calculated by measuring attenuation of neutron due to radiation. The accuracy of the calculated densities and height of the boundary surface can be improved by using the neutron of a certain energy range in which neutron resonance absorption occurs.  
      Other embodiments of the present invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and example embodiments be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following.