Characteristics and dead-time of GM-tube

Transcription

1 Caracteristics ad dead-time of GM-tube GM-tubes are te most popular gas ioizatio detectors for te measuremet of - ad -radiatio. Gas ioizatio detectors ave tree types: ioizatio cambers, proportioal couters ad Geiger-Müller tubes. Te radioactive radiatio ioizes te o-coductive gas ad trasforms it coductive. Te formed electric impulses are greatly ifluece by te electric field i te detector. We te electric filed is too low, te ios ad electros move too slowly towards te electrode ad a part of tem recombie. We icreasig te electric field, all ios reac te opposite electrode, te curret does ot depeds o te electric field. t is te rage of ioizatio cambers. Te curret depeds o te itesity, type ad eergy of te radiatio. We te electric field icreases o, ios ad electros are accelerated ad cause secodary ioizatio; te tube acts as a amplifier. Te curret is proportioal to te itesity ad eergy of te radioactive particles. Tis rage is called as te rage of proportioal couters. Te ioizig particles ca be couted ere oe by oe, te amplitude of te electric impulses gives iformatio o te eergy ad type of te particle. Te gas ioizatio detectors are i te so-called GM-rage we te electric field becomes so great tat te ios ad molecules gais so more kietic eergy ta te ioizatio eergy of te gas molecules i te tube. Te free patway of te molecules: RT k () d N p Were d is te diameter of te molecules, p is te pressure, N is te vogadro umber. t pressure 0. MPa of argo 0 4 V/cm electric field is eeded for te formatio of secodary ioizatio. practice, te gas pressure is lower so cca. 000 V/cm is eeded for te operatio of a GM detector. Because of te secodary ioizatio processes, ioizatio takes place i te wole volume of te gas, te electric impulse is great. Te electric impulse does ot deped o te type or te eergy of te radioactive particle. s a result of te great impulses, te electric field decreases to a value below te GM rage, so te ioizatio stops. GM-tubes are supplied troug a M resistace, wic delays te retur of te ormal operatio electric field. Te time of tese processes is te regeeratio time (450 s). Before tis time, te tube produces electric impulses, but tey are smaller. Te istrumet observes tese smaller impulses, too. Te time eeded for te formatio of electric impulses after te previous impulse is called dead-time. t is about 00 s, ad caracteristic of te GM-tube. t is a very

2 importat parameter because it determies te maximal coutig rate. t ig coutig rates ad precise measuremets, te results ave to be corrected wit te dead time. Te operatig voltage of GM-tubes is determied by te caracteristics of te tube. t meas te measuremet of te coutig rate of a give radioactive sample at differet voltage values. Te coutig rate (te umber of te registered impulses durig te measurig time) as a fuctio of voltage gives te caracteristics of te tube. ( cpm ) Voltage (V) Caracteristics of GM-tube s see i te figure, tere is a liear portio of te curve betwee 800 ad 40 V, it is te so-called plateau. ts legt ere is 30 V. Te operatig voltage is selected at te first /3 of te plateau, tat is 670+ (00/3) =740 V. Te slope of te plateau is also importat. t is give i relative uits, i %/00V. our case it is: slope *00*00= 8.5 %/00 V Te slope is usually about 5 5 %/00 V, it is iger i te tube wit aloge gas (operatig voltage is cca V) ta i te tubes cotaiig oble gases ad orgaic gases as quecig gas. Te legt of te plateau cages iversely, it is sorter i te tubes wit aloges (00-00 V). Te legt of te plateau becomes sorter durig log operatio times, wile te slope icreases. Te life-time of a tube is cca impulses. Note tat te tube is workig we it does ot cout impulses! fter measurig te caracteristics, decrease te voltage ad take off te radioactive sample!

3 3 Measuremet of te caracteristics Look for te potetiometer adjustig te voltage o te couter! t is a potetiometer wit 0 turs, sowig te uit turs i a small widow ad /00 turs o te kob. Te te turs ca usually set te voltage i te rage of V. E.g. 5,00 meas 000 V. Look for te switces adjustig te measurig time! Set te measurig time (-0 mi)! Set te discrimiator (aalyzig te amplitude of te electric impulses) to itegral mode, set te cuttig level to miimum 0.5 V to elimiate te electric oise! Put te radioactive sample uder te GM-tube ito te sieldig box. Coose te positio so tat te coutig rate is betwee 0000 ad 0000 cpm at te operatig voltage of te GM-tube! our laboratory, we keep te istrumets adjusted to te optimal parameters, so you ca cotrol te positio of te sample at te begiig of te practice. Decrease te voltage so low tat o impulses ca be observed, ad icrease it from ere to te voltage were te coutig starts (witi 0 V accuracy). Measure te coutig rate tree times at te adjusted voltage, te icrease it by 0 40 V, measure te coutig rate tree times agai, ad so o. crease te voltage so tat 0-5 poits give te total caracteristics. t too ig steps, te curve will ot be ice, at too small steps te measuremets require too log time. crease te voltage util te uiform icrease of te plateau begis to icrease sarply. Do ot measure too log i tis voltage rage because te tube destroys. We te caracteristics are ready, decrease te voltage ad take te sample out of te box! Plot te averages of te coutig rate as a fuctio of voltage! Determie te legt ad te slope of te plateau as well as te operatig voltage! Evaluate te quality ad statemet of te tube! Determiatio of dead-time We is te coutig rate ad te GM-tube caot observed impulses after eac impulses at time, te it does ot work at time, so te real measurig time is -. Te real coutig rate * : *. () Te dead-time is usually determied by te so-called two-sample metod. Te sample older ad te samples are positioed i a well-reproducible maer i te measurig

4 4 istrumets. Te coutig rate of te backgroud, ad te samples, separately ad togeter, are determied (,,,, ). Te coutig rate of te samples is defied ere as te total coutig rate of te sample+backgroud. (Usually, te coutig rate of a sample is defied as a value corrected wit te backgroud.) Because of te dead time +, +. However, for te coutig rates corrected wit te dead-time:. (3) * * * *, Substitute te correctio formula () for eac members of Eq. (3):,. (4), From Eq. (4) ca be expressed, but it gives a rater complicated formula. t small coutig rates, tat is te correctio is oly -%, te members o te great powers are eglected ad a approximate formula is obtaied:. (5),, (Te exact solutio is as follows:,, 3 4., (6) scietific works, te deviatio of a experimetal parameter as to be give. Te deviatio of te coutig rate from parallel measuremets is: j j. (7) Te deviatio of te average is i. ca be calculated by te statistical program of calculators. Two types of deviatios are obtaied: s ad. Tey are very similar, oly -, or is i te deomiator.

5 5 Te deviatio of te dead-time ca be calculated from te deviatio of te coutig rates:,,,,,,,,,,,, Do te followig trasformatio. Let,, B,. Te deviatio of te dead time is: B B B,, (8) (Were i is te average coutig rate of te i t sample, i is te deviatio of te average.) Set te operatig voltage! Put te sample older ito te sieldig box! Put oe of te samples ito te oe place of te sample older ad a iactive pot ito te oter place! tis way, te back-scatterig coditios are te same. Measure te coutig rate tree times! Cage te iactive pot to te oter sample (te first sample remais i place) ad measure te coutig rate of te two samples togeter! Take te first sample off ad put te iactive pot oto its place! Measure te coutig rate of te secod sample tree times! Fially, measure te backgroud, puttig a iactive pot oto te sample older! Calculate te averages, te deviatios, ad te deviatio of te averages! Determie te dead-time by Eq. 6 ad its deviatio by Eq. 8. Ceck te validity of Eq. 4. Write te operatig voltage, te slope of te plateau, te dead-time ad its deviatio o te data seet of te istrumet! Try to evaluate te type ad quality of te GM tube! Wat is te quecig gas?