Characterization of R-134a Superheated Droplet Detector for Neutron Detection

Transcription

1 Characterizati f R-34a Superheated Drplet Detectr fr Neutr Detecti Prasaa Kumar Mdal *, Rupa Sarkar ad Baru Kumar Chatterjee Departmet f Physics, Bse Istitute, 93/ A. P. C. Rad, Klkata , Idia. prasaa_id_82@yah.cm (P.K. Mdal) sarkar_rupa2003@yah.cm (R. Sarkar) baru_k_chatterjee@yah.cm (B.K. Chatterjee) * Crrespdig authr: Tel.: ; fax: prasaa_id_82@yah.cm (P.K. Mdal) Abstract R-34a (C 2 H 2 F 4 ) is a lw cst, easily available ad chlrie free refrigerat, which i its superheated state ca be used as a efficiet eutr detectr. Due t its high slubility i water the R-34a based superheated drplet detectrs (SDD) are usually very ustable uless the detectr is fabricated usig a suitable additive, which stabilizes the detectr. The SDD is kw t have superheated drplets distributed i a shrt-lived ad i a relatively lger-lived metastable states. We have studied the detectr respse t eutrs usig a 24 AmBe eutr surce ad btaied the temperature variati f the ucleati parameters ad the iterstate kietics f these drplets usig a tw-state mdel. Keywrds: Superheated drplet detectr, R-34a, eutr detectr, ucleati, ucleati efficiecy, tw-state mdel.. Itrducti The radiati detectrs based the emulsi f superheated liquid drplets i viscelastic gel r i sft plymer matrix are beig used fr eutr detecti, eutr dsimetry ad eutr spectrmetry fr ver three decades (Apfel et al., 982; d Erric et al., 200, 2002; Mukherjee et al., 2007). Cmpared t the ther cvetial eutr detectrs the superirity f these detectrs has already bee well established. I superheated drplet detectr (SDD), the active liquid is dispersed i the frm f micrsize drplets i a viscelastic gel medium. It is well kw that the superheated state is a metastable state f the liquid, where a small perturbati like, the thermal fluctuatis, eergy depsiti by eergetic radiati etc., culd trigger the frmati f a stable vapur phase. The SDD is used i almst all braches f radiati physics, icludig health physics, medical physics, space physics, uclear physics ad high eergy physics (Apfel et al., 989; Harper et al., 995; Ig et al., 999; Gu et al., 999). Superheated liquid based detectrs are als used i dark matter search experimets (Behke et al., 20; Archambault et al., 202; Felizard et al., 202), sice by chsig the peratig temperature ad pressure it ca be made cmpletely isesitive t majrity f the backgruds assciated with these experimets. COUPP (Behke et al., 20), PICASSO (Archambault et al., 202) ad SIMPLE (Felizard et al., 202) are the three Preset address: Departmet f Chemical, Bilgical & Macrmlecular Scieces, S.N. Bse Natial Cetre fr Basic Scieces, Blck JD, Sectr III, Salt Lake, Klkata , Idia. prasaa@bse.res.i.

2 grups wrkig the dark matter search experimets usig superheated liquid based detectrs. Fr the preparati f SDD differet lw bilig pit liquids are used, such that at the peratig temperature it ca be used fr the detecti f iizig radiatis. The R-2 (CCl 2 F 2 ; b.p C) based SDDs are well studied ad are widely used i eutr detecti (d Erric et al., 2002; Mdal et al., 203). Hwever due t its chlrie ctet it has a ze depletig ptetial (Slm, 999) ad hece it is baed i may cutries. R-34a (C 2 H 2 F 4 ) has a bilig pit f C ad i SDD it ca be used as a pssible alterative f R-2 (Harper et al., 995; Das et al., 200). We have studied the temperature variati f the ucleati parameters f R-34a SDD prepared by a mdified SDD fabricati techique (Mdal ad Chatterjee, 203) that prduces a highly stable detectr which ca be used eve years after its fabricati. Recet study has idicated that i SDD there exist tw grups f drplets e with a much shrter lifetime tha the ther (Sarkar et al., 2008). The decay f these drplets is mdeled usig a tw-state decay scheme (Mdal ad Chatterjee, 2009). We have used this mdel fr fittig the eutr irradiati data f R-34a SDD. The experimets are perfrmed i the temperature rage f 20 t 40 C, where fr eutr irradiati a 3 Ci 24 AmBe eutr surce is used. The ucleati rate data is fitted with the tw-state mdel, which gives the ucleati parameters, ucleati efficiecy ad iterstate trasiti kietics f the R-34a drplets. 2. Thery The superheated state is a metastable state f the liquid, where the liquid phase is maitaied either at a temperature higher tha its bilig pit at a give pressure r at a pressure lwer tha its saturati vapur pressure at a give temperature (Avedisia, 985). I superheated liquid there exists a dyamic ppulati f micrbubbles, which grw t a maximum size ad the cllapse back. If a micrbubble reaches a size larger tha a certai critical size (r c ), bubble ucleati ccurs, ad the the vapur bubble grws sptaeusly vaprisig the superheated liquid. Here a eergy barrier (W) (Ry et al., 987), due t the iterplay betwee the surface ad vlume frces, gvers the stability f the micrbubbles. Whe a micrbubble has sufficiet eergy t vercme this barrier it causes sptaeus hmgeeus bubble ucleati. The bubble ucleati may als be triggered by the eergetic radiati which depsits eergy i the liquid ad causes radiati iduced ucleati. Fr bubble ucleati t ccur, the eergy depsiti has t be greater tha the threshld eergy (W) eeded t frm a critical size (r c ) micrbubble. Fr eutrs the recil i, prduced due t the eutr-ucleus elastic scatterig, depsits eergy alg its path ad triggers the bubble ucleati. The frequecy f sptaeus ucleati is usually quite lw cmpared t the radiati iduced ucleati, which eables e t use SDD as a radiati detectr. Whe a SDD is expsed t eergetic radiati the superheated drplets vaprise idepedet f each ther. The vaprisati f superheated drplet is accmpaied by the emissi f a acustic pulse ad a chage i vlume, bth f which ca be detected electrically (Apfel ad Ry, 983; Mdal ad Chatterjee, 2008). I a radiati field the superheated drplets expected t decay mtically. Hwever, whe a SDD is irradiated multiple times with a radiati-ff perid i betwee tw irradiatis, the bserved ucleati rate at the begiig f the irradiati fud t be much higher tha the ucleati rate at the ed f the previus irradiati (Sarkar et al., 2008). This discrepacy i the ucleati rate data idicates that i SDD the drplets are i tw metastable states, the rmal metastable state ad the secd metastable state. The

3 drplets ctiuusly mve frm e state t the ther ad at lw ucleati rate the drplet ppulati reaches equilibrium. A drplet i the rmal metastable state has a lifetime which is much lger tha its lifetime i the secd metastable state. Whe irradiated with eutrs the drplets i the secd metastable state decay much faster tha the thers i the rmal metastable state, givig a sharp fall i the ucleati rate, after which the shrt-lived drplet ppulati decreases csiderably ad the lg-lived drplets maily ctribute i the decay data (Mdal ad Chatterjee, 2009). Durig the radiati-ff perid the shrt lived drplets reppulate frm the rmal metastable state resultig i a icrease i the ucleati rate at later irradiatis. It was bserved that d, the trasiti rate frm secd metastable state t rmal metastable state, is larger tha the trasiti rate frm rmal metastable state t secd metastable state c (Mdal ad Chatterjee, 203). Durig eutr irradiati the superheated drplets i SDD decay due t sptaeus ad iduced ucleatis. The ucleati frequecy f the drplets i rmal ad secd metastable states ca be expressed as (Mdal ad Chatterjee, 2009), spt iduced b = b + b = kv + kvψ () ad spt iduced = a + a = q v + q vψ. (2) a Here v is the drplet vlume, ψ is the eutr flux, k, q are the sptaeus ucleati rate per uit vlume fr the rmal ad secd metastable states respectively ad k, q are the eutr iduced ucleati frequecy per uit vlume per uit flux fr the rmal ad secd metastable states respectively. The first ad secd terms the right had side f Eqs. () ad (2) accut fr the sptaeus ad eutr iduced ucleatis respectively. The ucleati parameters k, q, k ad q, ad the iterstate trasiti rates c ad d ca be btaied by fittig the multi-expsure ucleati rate data with the twstate decay mdel (Mdal ad Chatterjee, 2009, 203). 2.. Nucleati efficiecy The ucleati efficiecy η f a superheated liquid is defied as the prbability f bubble ucleati fr each scatterig evet f the eutr (Sarkar et al., 2006). Sice differet is have differet LETs (liear eergy trasfer), the prbability f triggerig a ucleati is als differet fr differet is. Here, due t the difficulty i idetifyig which i has triggered the ucleati it is t pssible t calculate the ucleati efficiecies fr all the is separately. Thus we have take the average ucleati efficiecy fr all the is η, which ca be expressed as (Sarkar et al., 2006), k η = (3) ρliqn A iσ i M i where ρ liq is the liquid desity, M is the mlecular weight, N A is the Avgadr umber, i is the atmicity f the i th uclear species f the mlecule havig the eutruclei elastic scatterig crss-secti σ i. Similarly, fr the secd metastable state the ucleati efficiecy η ca be writte as *

4 * η = ρn M A q By fidig k ad q the ucleati efficiecies f the liquid btaied usig Eqs. (3) ad (4). i. (4) σ i i η ad * η ca be 3. Detectr fabricati The R-34a SDD ca be prepared by a simple emulsificati prcess (Ry et al., 998) usig R-34a as the active liquid. Durig the emulsificati the liquid breaks it small drplets which remai suspeded i a viscelastic gel medium. The viscelastic gel is prepared by mixig glycerl ad cmmercial ultrasud gel i a prprti such that it ca hld the drplets i suspesi. We have bserved that the R-34a based SDD is usually very ustable due t the high water slubility f R-34a (0.5 wt% at bar ad 25 C), which results i a diffusi f the liquid it the gel gradually vaishig the drplets. I rder t vercme this prblem we have prepared the SDD by a mdified SDD fabricati techique (Mdal ad Chatterjee, 203). I this techique befre the emulsificati we have added Twee 80 surfactat i the gel, which ehaces the stability f the R-34a drplets i the viscelastic gel. A plydisperse emulsi f superheated drplets (abut 8500 drps/ml) is btaied by this methd. Here the drplet size distributi (Fig. ) is measured usig a techique reprted by Mdal et. al., 200. I Fig. (b) f (v) represets the rmalized drplet vlume distributi f the emulsi, which is used i fittig the ucleati rate data f R-34a SDD. It is bserved that i R- 34a SDD the drplet size varies i the rage f abut 5 t 90 µm. Fig. The drplet size distributi i R-34a SDD. We have als de a quatitative cmparis f the stability f surfactat free ad surfactat added R-34a SDDs by measurig the chage i drplet ppulati with the detectr ageig fr the tw cases. Fr this study we have prepared tw batches f SDDs, e is surfactat free ad ther e is surfactat added. I bth the cases same amut f viscelastic gel ad R-34a liquid were used fr emulsificati ad 0 vials f SDDs were prepared i each cases. The vials were stred at abut 4 C ad were used at differet ageig time fr cutig the umber f drplets preset i the vial. The ttal umber f drplets preset i each vial were measured experimetally by vaprisig all

5 the drplets preset i the vials. I a typical experimet the vial was wrapped with a heatig cil, usig which the detectr temperature was icreased i small steps up t a temperature such that all the drplets are vaprised. The acustic pulse geerated durig vaprisati f a superheated drplet was cverted it a electric pulse by a BaTiO 3 piezelectric trasducer. These electric pulses were cverted it TTL pulses with the help f a pulse shapig device (Sarkar et al., 2006) ad were cuted usig a data acquisiti card (Advatech USB 47) peratig i a LabView platfrm. The ttal umber f cuts recrded i such a experimet gives the ttal umber f drplets iitially preset i the vial. The variati i drplet ppulati as a fucti f ageig f the surfactat free ad surfactat added SDDs is shw i Fig. 2. Sice the vlume f the detectr was differet i differet vials the drplet ppulati is rmalized with the detectr vlume. Fig. 2 Variati f the rmalized drplet ppulati i surfactat free ad surfactat added R-34a SDD. 4. Experimetal methd Fr the characterizati f R-34a SDD the experimets were perfrmed at differet temperatures usig a 3 Ci 24 AmBe eutr surce. The schematic diagram f the experimetal setup is shw i Fig. 3. I these experimets abut 8 ml SDD was take i a glass vial ad was placed the tp f a BaTiO 3 piezelectric trasducer. The vial was wrapped with a heatig cil, which was cected t a variac usig which the detectr temperature was ctrlled. The detectr temperature was icreased i small steps up t a desired temperature at which the SDD was irradiated with eutrs. The eutr prduces recil i which, while passig thrugh the active liquid, depsits eergy alg its path ad iduces the bubble ucleati whe sufficiet eergy is depsited withi a certai critical legth (Ry et al., 987). As discussed earlier the vaprisati f the superheated drplet is assciated with the geerati f a acustic pulse, which ca be detected by the piezelectric trasducer. With the help f a pulse shapig device (Sarkar et al., 2006) the electric pulse frm the piezelectric trasducer is cverted it TTL pulse. Usig this device we have btaied the ucleati rate data, i.e. the umber f drplets vaprised ( N ( t)) durig a preset dwell-time at time t. The ucleati rate data was acquired as a fucti f time usig a MCS (multichael scaler) prgrammed i a LabView platfrm.

6 Fig. 3 Schematic diagram f the experimetal setup used fr btaiig the ucleati rate data f R-34a SDD usig a 24 AmBe eutr surce. I rder t btai the ucleati parameters f tw metastable states a multiexpsure ucleati rate data is required (Mdal ad Chatterjee, 2009). The tw-state fittig f multi-expsure ucleati rate data remves the degeeracy i the btaied ucleati parameters. I ur experimets, at a cstat temperature ad at ambiet pressure, iitially the R-34a SDD was irradiated with eutrs fr a few miutes. The radiati was the tured ff fr a perid f time (by remvig the eutr surce) ad the tured back agai by placig the surce. Such switchig ad ff the irradiati was repeated agai where the radiati-ff perids were varied. A typical multi-expsure experimetal data at temperature 35 C is shw i Fig. 4. T uderstad hw the sptaeus ad iduced ucleati rates chage with temperature ad als fr btaiig the temperature variatis f the iterstate trasiti rates the experimets were perfrmed i the temperature rage f 20 t 40 C. I all these experimets the R- 34a SDD was irradiated 3 times with eutrs. Fig. 4 A typical experimetal ad fitted ucleati rate data btaied with R-34a SDD at 35 C.

7 5. Results ad discussi The ucleati rate data f R-34a SDD were fitted usig the tw-state mdel. The detail f the data fittig methd has bee reprted earlier (Mdal ad Chatterjee, 2009). Sice the ucleati frequecies b ad a (Eqs. -2) is depedet the drplet vlume, the drplet vlume distributi f (v) plays a imprtat rle i the data fittig (Sarkar et al., 2004). Here we have used the measured drplet vlume distributi f (v), shw i Fig.. By fittig the data e ca btai the parameters k, q, k, q, c ad d. Usig these parameters e ca als btai ther quatities, like the ucleati efficiecies, equilibrati time ( /( c + d) ) ad drplet ppulati i differet metastable states. A typical experimetal ad fitted data fr R-34a SDD is shw i Fig. 4. The temperature variati f the parameters btaied by fittig the ucleati rate data are shw i Figs. 5-0, which shw sme characteristic features as discussed bellw. 8.E- 6.E- k (cm -3 s - ) 4.E- 2.E- 0.E Temperature ( C) Fig. 5 The temperature variati f the sptaeus ucleati frequecy ( k ) fr rmal metastable state q (cm -3 s - ) Temperature ( C) Fig. 6 The temperature variati f the sptaeus ucleati frequecy ( q ) fr secd metastable state.

8 4.E-05 3.E-05 k (cm - ) 2.E-05.E-05 0.E Temperature ( C) Fig. 7 The temperature variati f the eutr iduced ucleati frequecy ( k ) fr rmal metastable state. q (cm - ) Temperature ( C) Fig. 8 The temperature variati f the eutr iduced ucleati frequecy ( q ) fr secd metastable state.

9 3.0E E E-04 c (s - ).5E-04.0E E E Temperature ( C) Fig. 9 The temperature variati f the trasiti rate (c) frm rmal t secd metastable state..0e E-04 d (s - ) 6.0E E E E Temperature ( C) Fig. 0 The temperature variati f the trasiti rate (d) frm secd t rmal metastable state. The temperature variatis f k ad q, the sptaeus ucleati frequecy per uit vlume f the active liquid fr rmal ad secd metastable states respectively, are shw i Fig. 5 ad Fig. 6. It is bserved that k ad q icrease with icrease i temperature. This happes because with icrease i temperature the threshld eergy fr ucleati (W) decreases while the umber f micrbubbles per uit vlume f the active liquid icreases makig the superheated liquid mre ad mre ustable. Fr this reas with icrease i detectr temperature the prbability f bubble ucleati icreases, resultig i a icrease i k ad q. The eutr iduced ucleati frequecies f the tw metastable states, k ad q, als icrease with icrease i temperature, as shw i Figs Here k ad q are the eutr iduced ucleati frequecies per uit vlume f the active liquid fr

10 rmal ad secd metastable states respectively. I case f eutr iduced evets, i additi t the threshld eergy ad micrbubble desity, the eutr-ucleus iteracti crss-secti ad the LET affect the ucleati rate. As the threshld eergy fr ucleati decreases with a rise i detectr temperature, mre ad mre is ctribute t the ucleati, resultig i a icrease i k ad q. The temperature variati f the trasiti rates c ad d are shw i Figs It is bserved that with a icrease i temperature, c decreases, while d icreases. This idicates that, as the temperature icreases the prbability f trasiti frm rmal t secd metastable state decreases while the prbability f trasiti frm secd t rmal metastable state icreases. Fr this reas the drplet ppulati i secd metastable state decreases with a icrease i temperature, as shw i Fig.. At the equilibrium the fracti f drplets preset i secd metastable state is = c /( c + d) (Mdal ad Chatterjee et al., 2009, 203). Usig c ad d, the P S temperature depedece f P S is btaied (Fig. ). Fig. Temperature variati f the shrt-lived drplet ppulati i R-34a detectr. The equilibrati time f the system ca be estimated usig the trasiti rates c ad d. I absece f ay ucleati evets after a time τ = /( c + d), kw as the equilibrati time, the system will reach equilibrium ad the there will be substatial chage i the drplet ppulatis f differet metastable states. The temperature variati f τ is shw i Fig. 2, which shws that with icrease i temperature the drplet ppulatis equilibrate amg themselves i shrter times.

11 Fig. 2 Temperature variati f the equilibrati time τ. As discussed earlier, usig k ad q values i Eqs. 3-4 e ca btai the * ucleati efficiecies η ad η f the liquid i rmal ad secd metastable state * respectively. The temperature variatis f η ad η fr R-34a are shw i Figs. 3-4, which idicate that the prbability f bubble ucleati fr a drplet i secd metastable state is always higher tha that i rmal metastable state. Fig. 3 The temperature variati f the ucleati efficiecy ( η ) fr rmal metastable state.

12 Fig. 4 The temperature variati f the ucleati efficiecy ( η * ) fr secd metastable state. 5. Cclusi We have fabricated a stable R-34a SDD which is eutr sesitive at rm temperature. The respse f this SDD is studied usig a 24 AmBe eutr surce. Experimetal result shws a strg presece f secd metastable state i R-34a SDD. The temperature variati f the ucleati parameters ad iterstate trasiti kietics f the superheated drplets are studied i the temperature rage f 20 t 40 C. The large * ucleati efficiecies ( η ad η ) per uit vlume f R-34a make it a gd eutr detectr. Refereces Apfel, R.E., Chu, B.T., Megel, J., 982. Appl. Sci. Res. 38, 7. Apfel, R.E., L, Y.C., 989. Health Phys. 56, 79. Apfel, R.E., Ry, S.C., 983. Rev. Sci. Istrum. 54, 397. Archambault, S., et al., 202. Phys. Lett. B 7, 53. Avedisia, C.T., 985. J. Phys. Chem. Ref. Data. 4, 695. Behke, E., et al., 20. Phys. Rev. Lett. 06, Das, M., Sarkar, R., Mdal, P.K., Saha, S., Chatterjee, B.K., Ry, S.C., 200. Pramaa - J. Phys. 75 (4), 675. Felizard, M., et al., 202. Phys. Rev. Lett. 08, Gu, S.L., et al., 999. Radiat. Meas. 3, 67. Harper, M.J., Baker, B.W., Nels, M.E., 995. Health Phys. 68, 670. Ig, H., Nulty, R.A., Cliffrd, E.T.H., Gamer, J.S., Camer, D.R., 999. Radiat. Prt. Dsim. 85, 0. Mdal, P.K., Chatterjee, B.K., Meas. Sci. Techl. 9, Mdal, P.K., Chatterjee, B.K., Nucl. Istr. Meth. A 604, 662. Mdal, P.K., Chatterjee, B.K., 203. Appl. Radiat. Ist. 77, 6. Mdal, P.K., Sarkar, R., Chatterjee, B.K., 200. Meas. Sci. Techl. 2, Mdal, P.K., Seth, S., Das, M., Bhattacharjee, P., 203. Nucl. Istr. Meth. A 729, 82.

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