Med Phys 4R06/6R03 Laboratory Experiment #6 MULTICHANNEL PULSE SPECTROMETRY

Transcription

1 Med Phys 4R06/6R0 Laboratory Experment #6 MULICHANNEL PULSE SPECROMERY INRODUCION: In ths experment you wll use the technque o multchannel spectrometry to perorm quanttatve analyss o radoactve partculate concentraton n the ar. Spectral analyss wll be made wth a standard NaI scntllaton detector. Ar samples are obtaned by lowng ar at a known rate through a lter upon whch partculates are deposted. I radoactve materal s present n the ar, ether as a gas or as an aerosol, two potental hazards may exst. I the dmensons o the source are comparable to the mean ree path o the -radaton, then a sgncant external whole body dose may be receved. An example o ths would be the release o sson product rare gases or 4 Ar rom a nuclear generatng staton rradatng the general publc. he other potental hazard would be the nternal dose receved rom the nhalaton o the radoactve materal. Partculate radoactvty n nature arses rom the transport o Rn, the daughter o 6 Ra and ts subsequent decay. he seres produced s 8 Po m 4 Pb 8.8m 4 B 9.7m 4 Po. Inhalaton o these radonucldes leads to lung exposure and s o maor concern both n the mnng ndustry and as a maor component o envronmental radaton dose to the publc. In ths nvestgaton, the concentraton o radon progeny wthn the reactor buldng wll be studed. he decay products o radon are not gaseous under normal crcumstances, and become axed to arborne partculate matter, reerred to as an aerosol. By drawng ar at a known rate through an absolute lter, a sample o the aerosol s obtaned. the actvty o 4 Pb and 4 B on the lter at the end o the samplng perod may be determned by -ray spectroscopy. he -rays assocated wth the decay o 4 Pb have energes (and branchng ratos) o 4 (4%), 95 (9%) and 5 (6%) kev. he most dstnct -ray rom 4 B decay has an energy o 609 kev and a branchng rato o 47%. he derental equaton descrbng the radon n the reactor buldng may be wrtten: 0 = R - N - v V N = R - N o () In the above equaton N 0 s the number o radon atoms n the buldng, R s the rate at whch radon s produced by duson rom the buldng materals, 0 s the radon decay constant, and v/v s the rato o buldng ventlaton rate to buldng volume (»/60 hr - ). he equatons or the radon progeny may be wrtten

2 = - N - - N - k v V N - PN = - N - - N =,, () where k s the ventlaton mxng rato, and P the plate out rate constant or speces. In the steady state, whch wll be establshed over tme perods long compared to the radon eectve perod 0 -, all dervatves vansh. hs leads to the solutons N 0 = R/ 0 () - N = N - he set o equatons (4) can be re-wrtten n terms o actvty relatons as A = ( ) A- =,, (4) Note that the eect o ventlaton and plate-out s to dsturb the smple relatonshp o secular equlbrum, n whch all actvtes are equal. Provded the physcal decay constant greatly exceeds the sum o the ventlaton and plate-out rate, = and secular equlbrum s approxmated. hs may be assumed to hold or =,, n ths laboratory, although t should be stressed that sgncant departures rom equlbrum are oten observed n both mnes and dwellngs. In any event a steady state concentraton C = N /V o speces s establshed. he equatons descrbng the condtons on the lter may be wrtten (5) = FC - N (6) = FC + N - N (7) = FC + N - N where F s the samplng low rate. For collecton tme, such that >>, (8) 0) FC( e ) A ( FC - A (0) = F( C + C ) ( - e )-FC e - - (9) (0)

3 where A = N s taken as the ntal actvty, wth t = 0 dened as the tme at whch the ar sampler s swtched o. he relevant derental equatons or t>0 can be obtaned rom equatons (6) and (7) wth F = 0, and ntal actvtes gven by (9) and (0). he actvty o 4 Pb can be wrtten A (t)= - A (0)(e - t - e - )+ A (0)e he total number o dsntegratons rom 4 Pb occurrng durng a countng nterval begnnng at t = and endng at t = s N D = A (t) - t () = ( e - e ) - e ( - ) A (0) ( e - e ) A (0) he observed area o the ull energy peak correspondng to gamma ray accumulated durng the countng nterval may be wrtten D () P p N () where P s the peak area, s the absolute peak ecency, and p s the emsson probablty per decay. hus the data may be combned to gve the estmate N D P p (4) he ntal actvtes appearng n () are related to the concentratons o 8 Po and 4 Pb and samplng low rate F through the relatons (9) and (0). Assumng equlbrum holds or the radon daughters, then the concentratons are related through C = o (R/V) o =,, (5)

4 EXPERIMENAL PROCEDURE. he pulse processng chan consstng o a charge senstve preampler coupled to a shapng ampler and a peak sensng ADC. Fnd the peak postons o the calbraton sources: 4m In (0.9 MeV), 98 Au (0.4 MeV), 7 Cs (0.66 MeV) and 60 Co (.7 and. MeV).. urn on the ar samplng system located n the samplng area whenever ready. Note the tme and low rate; the low meter s located on the back o the lter. Go on to step whle the sample s beng collected.. Collect the room background spectrum or 0 mn. and save to a le. 4. A source o 4 Na has been prepared usng the reactor pneumatc rradaton aclty. Place the source at 0 cm rom the detector ace, and collect a spectrum or a reasonable lve tme. Determne net photopeak areas and peak postons o the background spectrum usng the analyss program whle the 4 Na spectrum s beng analysed. Save the 4 Na spectrum once spectrum collecton s done. 5. Stop the ar samplng, notng the tme. he lter should be olded n quarters and transerred to the ace o the detector as quckly as possble. Cover the detector ace wth the lead sheld and begn a 000 sec countng. ANALYSIS AND QUESIONS. Usng the results rom the calbraton sources, construct a calbraton lne o peak energy versus poston and use t to denty the radonucldes responsble or the maor room background components.. Extract the absolute total ecences at 4 Na 68.6 and kev lnes rom the attached ntrnsc total ecency curves (Fg. ). Calculate the peak ecences usng the attached peak-to-total rato curve (Fg. ). Estmate the concdence summng correcton reerrng to the lecture note. 4

5 From the areas o the 4 Na peaks measured, calculate the radoactvty o 4 Na at the moment o the countng start. he emsson probabltes o the 68.6 and kev lnes are.00 and , respectvely. From decay tme normaton, calculate the radoactvty at the moment o the rradaton end. he 4 Na sample was produced by rradatng 40 mg NaNO powder. From the rradaton parameters (rradaton start and end tmes), calculate the neutron luence rate at the rradaton ste (Reer to the chapter lecture note. Suppose mono-energy neutrons.). he Na thermal neutron capture cross-secton s avalable at the course webste.. Compare the energes o the peaks observed n the spectrum obtaned wth the lter wth the publshed values. 4. From the relevant peak areas, calculate N D usng eq. (4) 5. Assumng equlbrum, determne the actvty concentraton or radon and ts progeny n the reactor buldng, usng equatons (0), and (5). 6. For mmerson n a rare gas plus daughters, the maxmum permssble concentraton n ar (MPC) a s gven by the ICRP as (MPC) a =.6x0-6 C / <E > cm. where <E > s the sum o the average -ray energy released (MeV) per decay o parent and daughters. How does the observed concentraton compare to ths value? Comment on ths. 7. he man concern nvolved wth exposure to radon s the dose to the lung tssue resultng rom - partcles emtted by the radon daughters whch accumulate there. A derved ar concentraton o 500 Bq/m s gven n ICRP. At ths concentraton, or an annual occupatonal exposure o 000 hrs., a worker wll accumulate the recommended annual lmt o exposure. Compare your actvty concentraton wth the DAC. Comment. 5

6 Fg.. he ntrnsc total ecency o varous thcknesses o NaI(l) or gamma-rays. Fg.. Peak-to-total rato o varous cylndrcal NaI(l) detectors. 6