CN103983715A - TIMS measuring method for oxygen isotope ratio in uranium oxide - Google Patents

Abstract

The invention provides a TIMS measuring method for an oxygen isotope ratio in uranium oxide. The method comprises the following steps: preparing a sample to be measured; filling the prepared sample to be measured and cleaning a sample room and a flight-path tube after filling; selecting a Faraday cup as a data reception apparatus of TIMS; adjusting the evaporation and ionization temperature of the sample to be measured and optimizing optical parameters; running a measuring program to measure the to-be-measured sample so as to obtain a measured value of the oxygen isotope ratio; and carrying out correction and uncertainty calculation on the measured value so as to obtain a final measuring result of the to-be-measured sample. Compared with gas mass spectrometry, the method provided by the invention has the advantages of no need for chemical treatment of the sample and direct measurement and has the characteristics of a fast analysis speed, simple sample preparation, a small usage amount of the sample, etc.

Description

A kind of TIMS measures the measuring method of oxygen isotope ratio in uranium oxide

Technical field

The present invention relates to nuclear material fields of measurement, relate in particular to the measuring method that a kind of TIMS measures oxygen isotope ratio in uranium oxide.

Background technology

Along with international prevention of nuclear proliferation, antinucleus probably and the development of nuclear safeguards situation, core security has become international concern focus.The early 1990s in last century, the relevant black case of nuclear material is seen in report.Be mainly due to after the Soviet Union's dissolution, cause the confusion in nuclear material management, and then occur loss and the illegal transaction of nuclear fuel element, nuclear material or radioactive material, make nuclear non-proliferation situation become very severe.The black appearance of nuclear material, international community is except the nuclear proliferation to traditional is worried, the public also worries that terrorist may utilize core or radioactivity disperser to start to attack.Since first intercepting and capturing smuggling nuclear material case, illegally transport the extremely common people's concern of nuclear material and radioactive material, the sum that illegal nuclear material is transported goods for sale event presents the situation of rising.For black nuclear material is tracked down, review its source, to strengthen management, the core method of having arisen at the historic moment card is learned this emerging comprehensive branch of learning.

Core method card Epidemiological Analysis is from nuclear material is intercepted and captured at scene, by core method card, learn technology, the method confirmed and carry out the analysis of the characteristic attributes such as principal ingredient of sample, and compare with the information of database, review suspicious specimen source, transport pathway, investigate case-involving organization and individual, for detection and the processing of case provides abundant information.Characteristic attribute, generally comprises radioactive type, radioactivity, principal ingredient, isotopic abundance, dopant species and content, macro-size, micromechanism etc.Sometimes by conventional characteristic attribute, can not accurately trace to the source, need to analyze more characteristic attribute, oxygen is exactly a principal character element for geo-location.Because change according to the composition of oxygen isotope in seawater or rainwater, different regions, due to geographic position, have caused natural oxygen isotope to have small difference at occurring in nature content, and difference is approximately 1%～5%.Gas mass spectroscopy is conventional, classical oxygen isotope measuring method, but when gas mass spectroscopy is measured oxygen isotope, amount of samples is large, the technological deficiency that needs chemical treatment and measuring process more complicated.

Summary of the invention

The technical problem to be solved in the present invention is to provide the measuring method that a kind of TIMS measures oxygen isotope ratio in uranium oxide, in order to solve above-mentioned technological deficiency.

In order to reach above-mentioned purpose, one aspect of the present invention has proposed the measuring method that a kind of TIMS measures oxygen isotope ratio in uranium oxide, and the method comprises the following steps:

Step S1, prepares testing sample;

Step S2, the described testing sample dress sample by preparing, cleans sample chamber and dirft tube after dress sample;

Step S3, selects Faraday cup as the data sink of TIMS;

Step S4, regulates evaporation, ionization temperature and the optimizing optical parameter of described testing sample;

Step S5, operating measurement program is measured the measured value of rear acquisition oxygen isotope ratio to described testing sample;

Step S6, to described measured value proofread and correct and uncertainty calculation after obtain the last measurement result of described testing sample.

Further, described step S1 comprises the following steps:

Sample preparation: appropriate uranium oxide sample is ground in little grinding alms bowl, then by the Sample storage after grinding in being full of the exsiccator of Ar;

Sample band pre-treatment: sample band is placed on to 10 of getter ^-7in Pa vacuum bakeout system, carry out degasification, then the sample band after degasification is kept in the exsiccator that is full of Ar;

Sample preparation: get the sample after the grinding of about microgram magnitude with microscale sampler, put it in polyethylene bottle, add the cyclohexane reagent of approximately 20 μ l, make suspending liquid;

Be coated with sample: the sample suspension that vibration prepares, standing 1 minute, with pipettor, get supernatant liquor, drip in the middle part of sample band, after candidate agent evaporation, then get hanging drop in sample band middle part, repeatedly;

Be coated with sample effect observation: be coated with after sample, sample band is placed one day under room temperature environment, after organic reagent volatilization, use SEM to observe the sample on sample band, to check, be coated with sample effect.

Further, the detailed process that fills sample in described step S2 is: after testing sample prepares, testing sample is installed on specimen holder, uses screw to fix, then specimen holder is installed to the sample chamber of TIMS, fix.

The detailed process of further, cleaning sample chamber and dirft tube in described step S2 is: use high-purity N ₂gas rinses 2-3 time sample chamber and dirft tube, and detailed process is, is filled with high-purity N ₂gas, pumping high vacuum, then be filled with high-purity N ₂gas, pumping high vacuum, repeats successively.

Further, described step S3 is specially: select to receive with center Faraday cup AX ²³⁸u ¹⁶o ⁺, H1 Faraday cup receives ²³⁸u ¹⁸o ⁺.

Further, in described step S4, the process of optimizing optical parameter is: wait to occur ²³⁸u ¹⁶o ⁺after signal, manually adjust focusing and deflection voltage, Z direction lens focus and deflection voltage and the slit place voltage parameter of ion gun place voltage, extraction voltage, D direction lens.

Further, described step S5 is specially: adjust ionization temperature and under the condition of maximum signal intensity, center Faraday cup AX receives ²³⁸u ¹⁶o ⁺ion, H1 Faraday cup receives ²³⁸u ¹⁸o ⁺ion, operating measurement program, the acquisition time that each data is set is 1s, and 5 data of each circle collection, gather 10 circulations altogether, and totally 50 circulations, after image data completes, through calculating the measured value that obtains oxygen isotope ratio.

The process of further, in described step S6, described measured value being proofreaied and correct is: first use mass spectrometer measurement standard material ¹⁸o/ ¹⁶o ratio, calculates correction coefficient k, and recycling k value is proofreaied and correct the measurement result of oxygen isotope ratio in sample.

Further, the computing formula of described correction coefficient k is:

$k=\frac{{(O_{18}/O_{16})}_{s\tan \mathrm{dard}}^{\mathrm{true}}}{{(O_{18}/O_{16})}_{s\tan \mathrm{dard}}^{\mathrm{mea}}}$

In formula, oxygen isotope for oxygen isotope standard substance ¹⁸o/ ¹⁶the reference value of O ratio; oxygen isotope for oxygen isotope standard substance ¹⁸o/ ¹⁶the measured value of O;

The computing formula of proofreading and correct is:

${(O_{18}/O_{16})}_{\mathrm{sample}}^{\mathrm{correct}}=k\×{(O_{18}/O_{16})}_{\mathrm{sample}}^{\mathrm{mea}}$

In formula, for sample oxygen isotope ¹⁸o/ ¹⁶the corrected value of O; for sample oxygen isotope ¹⁸o/ ¹⁶the measured value of O.

Further, in described step S6, the computing formula of the uncertainty of measured value is:

$W_{\mathrm{sample}}={(O_{18}/O_{16})}_{\mathrm{sample}}^{\mathrm{correct}}\×\mathrm{\σ}$

In formula, for sample oxygen isotope ¹⁸o/ ¹⁶the corrected value of O, σ is the synthetic standards deviation of sample measurement value;

Wherein, the computing formula of the synthetic standards deviations of sample measurement value is:

$\mathrm{\σ}=\sqrt{{\left({\mathrm{\σ}}_{\mathrm{sample}}^{\mathrm{mea}}\right)}^2+{\left({\mathrm{\σ}}_{s\tan \mathrm{dard}}^{\mathrm{mea}}\right)}^2+\frac{W_{s\tan \mathrm{dard}}}{{(O_{18}/O_{16})}_{s\tan \mathrm{dard}}^{\mathrm{true}}}}$

In formula, for the measurement standard deviation of sample measurement value, for the measurement standard deviation of standard substance, W _standardfor the uncertainty of standard substance, oxygen isotope for oxygen isotope standard substance ¹⁸o/ ¹⁶the reference value of O ratio.

With respect to prior art beneficial effect of the present invention, be: the method is compared with gas mass spectroscopy, does not need to carry out the chemical treatment of sample, can directly measure, and has the features such as analysis speed is fast, sample preparation is simple, amount of samples is little.TIMS is by receiving UO ⁺measure, do not need to transform CO ₂gas is measured again, has reduced intermediate link, has reduced the introducing factor of end product uncertainty, can meet core method card and learn about feature quick, accurate analysis.

Accompanying drawing explanation

Fig. 1 is the process flow diagram that a kind of TIMS provided by the invention measures the measuring method of oxygen isotope ratio in uranium oxide;

Fig. 2 is the process flow diagram of preparing testing sample in measuring method provided by the invention;

Fig. 3 is ²³⁸u ¹⁶o ⁺the graph of a relation of signal intensity and ionic activity.

Embodiment

Below in conjunction with accompanying drawing, technical scheme of the present invention and advantage are described in more detail.

As shown in Figure 1, be the process flow diagram of the measuring method of oxygen isotope ratio in a kind of TIMS measurement uranium oxide provided by the invention, this measuring method comprises the following steps:

Step S1, prepares testing sample.

As shown in Figure 2, for preparing the process flow diagram of testing sample, testing sample preparation process specifically comprises the following steps:

Step S11, sample preparation: appropriate uranium oxide sample is ground in little grinding alms bowl, then by the Sample storage after grinding in being full of the exsiccator of Ar, stand-by.Wherein, the object of grinding both can be removed the oxide layer of sample surfaces, can make again sample evenly, particle diameter diminishes, be conducive to the later stage to be coated with sample.

Step S12, sample band pre-treatment: sample band is placed on to 10 of getter ^-7in Pa vacuum bakeout system, carry out degasification, then the sample band after degasification is kept in the exsiccator that is full of Ar, stand-by.Wherein, degassing procedure both can be removed the moisture on sample band, thereby removed oxygen, also can remove the easily pollution element such as organism and Na, K, Zn simultaneously.After the degasification of sample band, can also improve ionogenic vacuum tightness while measuring, keep good measurement environment.

Step S13, sample preparation: get the sample after the grinding of about microgram magnitude with microscale sampler, put it in polyethylene bottle, add the cyclohexane reagent of approximately 20 μ l, make suspending liquid.Cyclohexane reagent is anaerobic reagent, is mainly that powdered samples is worked to make suspending liquid, transferred particle, simultaneously reagent not can with sample in oxygen exchange, the composition itself containing can not have impact to measurement result yet.

Step S14, is coated with sample: the sample suspension that vibration prepares, and standing 1 minute, with pipettor, get supernatant liquor, drip in the middle part of sample band, after candidate agent evaporation, then get hanging drop in sample band middle part, repeatedly.Sample band adopts rhenium band, is mainly that sample is played to carrying effect, for the observation of painting sample and the TIMS Measurement and analysis of sample

Step S15, is coated with sample effect observation: be coated with after sample, sample band is placed one day under room temperature environment, after organic reagent volatilization, use SEM to observe the sample on sample band, to check, be coated with sample effect.Observe distribution situation and volume containing the sample, painting sample effect, the reagent of powdered sample on sample band and whether be evaporated completely congruence, result determines whether the preparation of testing sample meets the demands according to the observation, if meet testing sample, has prepared, and continues preparation if do not meet.

Step S2, the testing sample dress sample by preparing, cleans sample chamber and dirft tube after dress sample.

After testing sample prepares, testing sample is installed on specimen holder, uses screw to fix, then specimen holder is installed to the sample chamber of TIMS, fix.In order to remove oxygen remaining in sample chamber and dirft tube, before measurement, use high-purity N ₂gas rinses 2-3 time sample chamber and dirft tube, and detailed process is, is filled with high-purity N ₂gas, pumping high vacuum, then be filled with high-purity N ₂gas, pumping high vacuum, repeats successively.

Step S3, selects Faraday cup as the data sink of TIMS.

TIMS is equipped with electron-multiplier and two kinds of data sinks of Faraday cup, does not have cup poor between Faraday cup, and stable, due to ²³⁸u ¹⁸o ⁺with ²³⁸u ¹⁶o ⁺counting can reach the reception requirement of Faraday cup, therefore select to receive with center Faraday cup AX ²³⁸u ¹⁶o ⁺, H1 Faraday cup receives ²³⁸u ¹⁸o ⁺, by measuring ²³⁸u ¹⁸o/ ²³⁸u ¹⁶the ratio of O, obtains ¹⁸o/ ¹⁶the value of O.According to the setting of the conditions such as instrument, mobile H1 Faraday cup, to suitable position, receives ²³⁸u ¹⁸o ⁺ion.

Step S4, evaporation, ionization temperature and the optimizing optical parameter of adjusting testing sample.

The sample band structure of this method adopts single band, and when TIMS measures, the evaporation of sample band, ionization temperature are embodied by the strength of current being carried on sample band.As shown in Figure 3, for ²³⁸u ¹⁶o ⁺the graph of a relation of signal intensity and ionic activity.During lower than 3A, there is not UO in temperature ⁺signal, UO ⁺not ionization, UO ⁺from 3A, start ionization, before ionization temperature reaches 3.75A, along with the rising of ionization temperature, UO ⁺signal intensity increases, when 3.75A, and the highest and held stationary of signal intensity; Along with the rising of ionization temperature, UO ⁺signal declines and is unstable, proves that now temperature has surpassed applicable UO ⁺ionization temperature, therefore by UO ⁺ionization temperature be adjusted between 3.6-3.8A.

The process of optimizing optical parameter is: wait to occur ²³⁸u ¹⁶o ⁺after signal, manually adjust the parameters such as ion gun place voltage, the focusing of extracting voltage, D direction lens and deflection voltage, Z direction lens focus and deflection voltage and slit place voltage, acquisition maximum ²³⁸u ¹⁶o ⁺signal, also obtain ²³⁸u ¹⁸o ⁺signal.

Step S5, operating measurement program is measured the measured value of rear acquisition oxygen isotope ratio to testing sample.

Adjust ionization temperature and under the condition of maximum signal intensity, center Faraday cup AX receives ²³⁸u ¹⁶o ⁺ion, H1 Faraday cup receives ²³⁸u ¹⁸o ⁺ion, operating measurement program, the acquisition time that each data is set is 1s, and 5 data of each circle collection, gather 10 circulations altogether, and totally 50 circulations, after image data completes, through calculating the measured value that obtains oxygen isotope ratio.

Step S6, to measured value proofread and correct and the calculating of uncertainty after obtain the last measurement result of testing sample.

Owing to there being the influence factors such as mass fractionation, space charge effect, make between measured value and actual value there are differences, need to measured value, proofread and correct with standard substance.Trimming process is: first use mass spectrometer measurement standard material ¹⁸o/ ¹⁶o ratio, calculates correction coefficient k, and recycling k value is proofreaied and correct the measurement result of oxygen isotope ratio in sample.

Wherein, the computing formula of correction coefficient k is:

$k=\frac{{(O_{18}/O_{16})}_{s\tan \mathrm{dard}}^{\mathrm{true}}}{{(O_{18}/O_{16})}_{s\tan \mathrm{dard}}^{\mathrm{mea}}}$

In formula, oxygen isotope for oxygen isotope standard substance ¹⁸o/ ¹⁶the reference value of O ratio; oxygen isotope for oxygen isotope standard substance ¹⁸o/ ¹⁶the measured value of O.

The computing formula of proofreading and correct is:

${(O_{18}/O_{16})}_{\mathrm{sample}}^{\mathrm{correct}}=k\×{(O_{18}/O_{16})}_{\mathrm{sample}}^{\mathrm{mea}}$

In formula, for sample oxygen isotope ¹⁸o/ ¹⁶the corrected value of O; for sample oxygen isotope ¹⁸o/ ¹⁶the measured value of O, k is correction coefficient.

The calculating of measured value uncertainty mainly comprises three partial contents, and one is the measurement standard deviation of sample measurement value second is the measurement standard deviation of standard substance the 3rd is the uncertainty (W of standard substance _standard).

Wherein, the computing formula of measurement standard deviation is:

$\mathrm{\σ}=\mathrm{sqrt}[(\mathrm{n\Σ}{x}_i^2-{\left(\mathrm{\Σx}\right)}^2)/\left(n(n-1)\right)]$

In formula, x _irepresent i sampling point value, x represents sample mean, and n represents to sample number.

The synthetic standards deviation computing formula of sample measurement value is:

$\mathrm{\σ}=\sqrt{{\left({\mathrm{\σ}}_{\mathrm{sample}}^{\mathrm{mea}}\right)}^2+{\left({\mathrm{\σ}}_{s\tan \mathrm{dard}}^{\mathrm{mea}}\right)}^2+\frac{W_{s\tan \mathrm{dard}}}{{(O_{18}/O_{16})}_{s\tan \mathrm{dard}}^{\mathrm{true}}}}$

In formula, for the measurement standard deviation of sample measurement value, for the measurement standard deviation of standard substance, W _standardfor the uncertainty of standard substance, oxygen isotope for oxygen isotope standard substance ¹⁸o/ ¹⁶the reference value of O ratio.

The computing formula of the uncertainty of sample measurement value is:

$W_{\mathrm{sample}}={(O_{18}/O_{16})}_{\mathrm{sample}}^{\mathrm{correct}}\×\mathrm{\σ}$

In formula, for sample oxygen isotope ¹⁸o/ ¹⁶the corrected value of O, σ is the synthetic standards deviation of sample measurement value.

After calculating completes, just can provide the last measurement result of sample, last measurement result comprises corrected value and uncertainty.

A kind of TIMS provided by the invention measures the method for oxygen isotope ratio in uranium oxide, by the researchs such as optimization to the solution of oxygen in environment, measuring condition, has set up the method for oxygen isotope in TIMS measurement uranium oxide.The method is compared with gas mass spectroscopy, does not need to carry out the chemical treatment of sample, can directly measure, and has the features such as analysis speed is fast, sample preparation is simple, amount of samples is little.TIMS is by receiving UO ⁺measure, do not need to transform CO ₂gas is measured again, has reduced intermediate link, has reduced the introducing factor of end product uncertainty, can meet core method card and learn about feature quick, accurate analysis.The method can be applicable to core method card learn in for to intercepting and capturing the analysis of sample oxygen isotope, also can be applicable to verify, the surveying work to oxygen isotope in uranium oxide in nuclear safeguards association area simultaneously.

The foregoing is only preferred embodiment of the present invention, is only illustrative for invention, and nonrestrictive.Those skilled in the art is understood, and in the spirit and scope that limit, can carry out many changes to it in claim, revise, and even equivalence, but all will fall within the scope of protection of the present invention.

Claims (10)

1. TIMS measures a measuring method for oxygen isotope ratio in uranium oxide, it is characterized in that, the method comprises the following steps:

Step S1, prepares testing sample;

Step S2, the described testing sample dress sample by preparing, cleans sample chamber and dirft tube after dress sample;

Step S3, selects Faraday cup as the data sink of TIMS;

Step S4, regulates evaporation, ionization temperature and the optimizing optical parameter of described testing sample;

Step S5, operating measurement program is measured the measured value of rear acquisition oxygen isotope ratio to described testing sample;

Step S6, to described measured value proofread and correct and uncertainty calculation after obtain the last measurement result of described testing sample.

2. measuring method according to claim 1, is characterized in that, described step S1 comprises the following steps:

Sample preparation: appropriate uranium oxide sample is ground in little grinding alms bowl, then by the Sample storage after grinding in being full of the exsiccator of Ar;

Sample band pre-treatment: sample band is placed on to 10 of getter ^-7in Pa vacuum bakeout system, carry out degasification, then the sample band after degasification is kept in the exsiccator that is full of Ar;

Sample preparation: get the sample after the grinding of about microgram magnitude with microscale sampler, put it in polyethylene bottle, add the cyclohexane reagent of approximately 20 μ l, make suspending liquid;

Be coated with sample: the sample suspension that vibration prepares, standing 1 minute, with pipettor, get supernatant liquor, drip in the middle part of sample band, after candidate agent evaporation, then get hanging drop in sample band middle part, repeatedly;

Be coated with sample effect observation: be coated with after sample, sample band is placed one day under room temperature environment, after organic reagent volatilization, use SEM to observe the sample on sample band, to check, be coated with sample effect.

3. measuring method according to claim 1, is characterized in that, the detailed process that fills sample in described step S2 is: after testing sample prepares, testing sample is installed on specimen holder, use screw fixes, and then specimen holder is installed to the sample chamber of TIMS, fixes.

4. measuring method according to claim 1, is characterized in that, the detailed process of cleaning sample chamber and dirft tube in described step S2 is: use high-purity N ₂gas rinses 2-3 time sample chamber and dirft tube, and detailed process is, is filled with high-purity N ₂gas, pumping high vacuum, then be filled with high-purity N ₂gas, pumping high vacuum, repeats successively.

5. measuring method according to claim 1, is characterized in that, described step S3 is specially: select to receive with center Faraday cup AX ²³⁸u ¹⁶o ⁺, H1 Faraday cup receives ²³⁸u ¹⁸o ⁺.

6. measuring method according to claim 1, is characterized in that, in described step S4, the process of optimizing optical parameter is: wait to occur ²³⁸u ¹⁶o ⁺after signal, manually adjust focusing and deflection voltage, Z direction lens focus and deflection voltage and the slit place voltage parameter of ion gun place voltage, extraction voltage, D direction lens.

7. measuring method according to claim 1, is characterized in that, described step S5 is specially: adjust ionization temperature and under the condition of maximum signal intensity, center Faraday cup AX receives ²³⁸u ¹⁶o ⁺ion, H1 Faraday cup receives ²³⁸u ¹⁸o ⁺ion, operating measurement program, the acquisition time that each data is set is 1s, and 5 data of each circle collection, gather 10 circulations altogether, and totally 50 circulations, after image data completes, through calculating the measured value that obtains oxygen isotope ratio.

8. measuring method according to claim 1, is characterized in that, the process of in described step S6, described measured value being proofreaied and correct is: first use mass spectrometer measurement standard material ¹⁸o/ ¹⁶o ratio, calculates correction coefficient k, and recycling k value is proofreaied and correct the measurement result of oxygen isotope ratio in sample.

9. measuring method according to claim 8, is characterized in that, the computing formula of described correction coefficient k is:

$k=\frac{{(O_{18}/O_{16})}_{s\tan \mathrm{dard}}^{\mathrm{true}}}{{(O_{18}/O_{16})}_{s\tan \mathrm{dard}}^{\mathrm{mea}}}$

In formula, oxygen isotope for oxygen isotope standard substance ¹⁸o/ ¹⁶the reference value of O ratio; oxygen isotope for oxygen isotope standard substance ¹⁸o/ ¹⁶the measured value of O;

The computing formula of proofreading and correct is:

${(O_{18}/O_{16})}_{\mathrm{sample}}^{\mathrm{correct}}=k\×{(O_{18}/O_{16})}_{\mathrm{sample}}^{\mathrm{mea}}$

In formula, for sample oxygen isotope ¹⁸o/ ¹⁶the corrected value of O; for sample oxygen isotope ¹⁸o/ ¹⁶the measured value of O.

10. measuring method according to claim 9, is characterized in that, in described step S6, the computing formula of the uncertainty of measured value is:

$W_{\mathrm{sample}}={(O_{18}/O_{16})}_{\mathrm{sample}}^{\mathrm{correct}}\×\mathrm{\σ}$

In formula, for sample oxygen isotope ¹⁸o/ ¹⁶the corrected value of O, σ is the synthetic standards deviation of sample measurement value;

Wherein, the computing formula of the synthetic standards deviations of sample measurement value is:

$\mathrm{\σ}=\sqrt{{\left({\mathrm{\σ}}_{\mathrm{sample}}^{\mathrm{mea}}\right)}^2+{\left({\mathrm{\σ}}_{s\tan \mathrm{dard}}^{\mathrm{mea}}\right)}^2+\frac{W_{s\tan \mathrm{dard}}}{{(O_{18}/O_{16})}_{s\tan \mathrm{dard}}^{\mathrm{true}}}}$

In formula, for the measurement standard deviation of sample measurement value, for the measurement standard deviation of standard substance, W _standardfor the uncertainty of standard substance, oxygen isotope for oxygen isotope standard substance ¹⁸o/ ¹⁶the reference value of O ratio.