The Active Personnel Dosimeter (APD) provides a digital readout of events caused by neutrons interacting with superheated liquid droplets. The droplets are suspended in a gel held in a replaceable cartridge. Upon neutron interaction, the superheated droplet vaporizes, forming a bubble. The sound produced in this process is recorded by transducers that sense the accompanying pressure pulse. The APD electronically discriminates against spurious noise and vibration. Studies with the production prototype APDs indicate that the detector response is linear up to about 0.40 mSv, with large variations sometimes from predicted values and between cartridges at higher dose equivalents. The response to standard neutron sources (bare {sup 252}Cf, PuBe, PuB, PuF, PuLi) is reported and compared with the expected response. Unirradiated cartridges self-nucleate when heated to temperatures of 46{degree}C. The APD is insensitive to low-energy photons but responds to high-energy photons and electrons. 9 refs., 2 figs., 3 tabs

Busick, D. D.

Donahue, R. J.

Ipe, N. E.

UNT Digital Library

< 5 # . f " * ^""^ SLAC-PUB—5122 DE90 004430 THE ACTIVE PERSONNEL DOSIMETER — APFEL ENTERPRISES SUPERHEATED DROP DETECTOR* N. E, Ipe, R. J. Donahue, and D. D. Busick Stanford Linear Accelerator Center Stanford University. Stanford, CA 94309, USA Abstract — The Active Personnel Dosimeter {AFD) provides a digital readout of events caused by neutrons interacting with superheated liquid droplets. The droplets are sus­pended in a gel held in a replaceable cartridge. Upon neutron interaction, the superheated druplct vaporises, forming a bubble- The sound produced in this process is recorded by transducers that sense the accompanying pressure pulse. The APD electronically dis­criminate against spurious noise and vibration. SLudies with the production prototype APDs indicate that the detector response is linear up to about 0.40 mSv, with large variations sometimes from predicted values and between cartridges at higher dose equiv­alents. The response to standard neutron sources (bare 2 5 2Cf, PuBe, PuB, PnF. PuLi) is reported and compared with theexpected response. Unirradiated cartridges self-nucleate whet) heated to temperatures of 4fi*0. The APD is insensitive to low-energy photons but responds to high-energy photons and electrons. Pfrsrnttd «t tht 9th lutirnatianal Conferrnn an Solid Stnli f)<wimrjrj,. \'un»a. Austria, Xovrmber fi 10. 19t>9. * VW = hupiK.ri.nl h> ntpariiix'tii of Kiternv outran l>E-AC03-76SK005t& INTRODUCTION The APD available for neution dosimetry from Apful Enterprises is an electronic instrument that provides a digital readout of events caused by neutrons interacting with droplets of supr heated liquid.'1' The droplets art' suspended in an immiscible, inert, impurity-free gel which is bold in a replaceable cartridge that fits in the instrument. Upon neutron interaction, the superheated droplet vaporises, forming a bubble which slowly rises out of the gel. The sound produced by the vaporisation of the droplet is recorded by transducers that sense the accompanying pressure pulse. Each event results in an increment in the digital display and an audible "beep." The APD is battery powered and electronically discriminates against spurious noise and vibration. According to the manufacturer, the energy response of the APD follows the K'RP 21-dose equivalent response within 40% for neutron energies above 100 KeV and within a factor of 10 for energies below 100 KeV. The detector under-rcsponds in the thermal region. The APD has a high sensitivity, and a minimum delectable limit of 1 /iSv, It is omnidirectional and insensitive to photons. The sensitivity of the detector, hnwvcr, increases as the temperature of the detector increases. In this paper, the results of studies performed with the APL)s purchased as part of a production preview program (i.e., a program in which the experi mentor reports the results U> the manufacturer so that the manufacturer can improve the technology) are reported. Characteristics investigated are: effect of temperature; linearity and re­producibility; response to standard nrulron sources (bare M aCf, PuBc, PuB, I'uLi and Put*"); effect of storage; and effect of photon and electron irradiation. o Apfi'l Knlrrpriws, Inc. '2b Scicnci- I'nrk. N<-«v Havrii, CT 06011, USA 2 MASTER • " -_ T • — - . -' • " ' J I ' I f t i ; ! , _. ' . " " • " ; - > i •• • • ^ - • * — t j _ . • # PRINCIPLE OF OPERATION The basic principle of operation of the superheated drop detector (SDD) is that boiling in a superheated liquid can be initiated by the presence of charged particles formed by radiation interacting with the detector.' 2 , 3 1 A liquid can be raised to a temperature above il» boiling point without boiling actually occurring. This stale of the liquid, known as the superheated state, is unstable; a slight disturbance or introduction of any impurities will cause the liquid to hoi). The normal boiling point of a liquid is the temperature at which its saturation vapor pressure is equal to the external pressure. An increase in the external pressure results in an elevation of the boiling point and vice versa. When the SIM) is sensitised {i.e., when the external pressure has been reduced), the droplets are in a superheated state. Neutron irradiation results in the formation of secondary charged particles which deposit energy along their path, thus initiating vapor bubble formation. The. vapor nucleation will depend on the energy deposited which, in turn, is dependent on the liquid and the degree of superheat. The mure superheated a liquid is, the less energy it will require to trigger bttbble formation. The nucleation step requires that a vapor bubble of a minimum size is funned. Bubbles smaller than this critical size are unstable and tend to collapse. Bubbles greater than this critical size will grow to a visible size by the evaporation of the superheated liquid until the whole droplet is consumed. The neutron energy threshold of detection at a given temperature will thus depend on the critical radius of tbe bubble and the stopping power of the secondary charged particle. EXPERIMENTAL METHODS Two APUs were purchased from Apfel Enterprises AN part of a product inn preview program. 3 Two hatches, each consisting of thirty cartridges, were also purchased. The cartridges in Batch I were used witliin a month of their arrival, whereas the Batch 2 cartridges were stored in the refrigerator for about seven months prior to usage. According to the manufacturer, the lifetime of these cartridge* exceeds four months. Each cartridge has its own storage cap. When the cap is removed, the pressure is released, and the cartridge is "sensitised" or ready for use. The cartridges in Batch 1 could be used for as long as one week after sensitisation, while those in Hatch 2 could be used for as long as three days. The cartridges were stored in the refrigerator when not in use. Shortly before use, they were taken out of the refrigerator and allowed to equilibrate with the ambient thermal environment. Each 4 ml glass cartridge contains about 0.03 ml of Freon 12 (OCljFY boiling poim = -29 .6T) droplets suspended under pressure in a host mixture, The host mixture, either a polymer or a mixture of water, glycerine and gel, holds the drops such that when one drop evaporates it will not trigger other drops. All neutron irradiations were performed outdoors in a low-scatter environment with the APDa mounted on a water phantom. Corrections for anisotropy of the neutron sources were made in calculating the neutron fluences. The ffuences were then converted to dose equivalents using the methods outlined in NORP 79.'6' Corrections for neutron scatter also were applied. The calculated values of the dose equivalent are reported. A thermometer probe mounted on tin- phantom adjacent to the APD monitored the ambient temperal uri'. Tin* temperature and detector readings were taken at intervals of 5 to 10 minutes. Temperature corrections were applied to all data using the temperature dependence curve (for Am Be neutrons) supplied by the manufacturer. According to this, the sensitivity increases by about 5% per °C-•l The photon exposures were made at a dose rate of 4.8 Gy/min with a dinar 1800 medicJ accelerator. An APD was placed on the treatment couch inside the primary beam in the patient plane. A field size of 35 cm x 35 cm was used. Another API) was placed outside the beam al a distance of 50 cm from the isoccnter in the patient plane. The electron exposures were also made in a similar manner but at a dose rate of 3.8 Gy/min with the Clinac 1300. A field size of 25 cm x 25 cm was used. The APD outside the beam was at a distance of about 30 cm from the isocenter, in the patient plane. RESULTS A N D DISCUSSION Additional results and detailed discussion can be found in an extended version of this paper.*61 BACKGROUND The average background on sensitised unirradiated detectors (Batch 2) varied from 0 to 4 counts in one day. This is in agreement with the manufacturer's claim of 1 to 5 counts per day. Because the APD has such a low background, no background subtraction was made in the data reported. LINEARITY A N D REPRODUCIBILITY Figure 1 shows the response of 5 APD cartridges when exposed to 0.83 mSv/h of PuBe neutrons. The solid line provided by the manufacturer shows Unexpected response to 0.29 mSv/h of Ami}', at 23l>(.\ The response is gem-rally linear up to about 0.40 mSv with large variations between some cartridges and from the expected response at higher dosr equivalents. 5 RESPONSE TO N E U T R O N SOURCES The bubhle detectors were exposed to neutrons from standard sources, bare M I Cf, PuBe, PuB, PuF, and PuLi, with average energies of 2.15, 4.5, 2.1, 1.5, and 0.5 MeV, respectively. Figure 2 shows the response of the APDs to 2i2C( and PuBe at dose equivalent rates of L.l and 1.23 n>Sv/h, respectively. The response to PuBe and 2 5 2 C f closely follows the expected response to AmBe (0.29 mSv/h), Detectors also were exposed to about 0.08 mSv of PuBe, PuB, PuF, :md PuLi neutrons at a dose equivalent rate of about 0.04 mSv/h. The results are shown in Table 1. The relative sensitivity is the ratio of the sensitivity in the linear region to the expected sensitivity (to AmBe neutrons) claimed by the manufacturer. The response to these different neutron sources is within 40% of the expected response. EFFECT OF TEMPERATURE Unirradiated detectors self-nucleate when heated tu temperatures of 46°C or greatnr. EFFECT OF STORAGE After storage for seven months in the refrigcrat r, some cartridges retained their expected response to neutrons, while others had a lowered response. EFFECT OF PHOTON IRRADIATION The APD is insensitive to low-energy photons. This was confirmed by exposing the detector to 12 CJy of 6 0 Oo gamma rays. No efFcc! was observed. At low onergiw. the photon* interail mainly with I he electrons and are therefore not wry ef*'<ient nl depositing their energy locally/ 7' (i ! hi* eHei*: *•'. higher e!«*!»> ph->*.-.'*is was >iudii'd In i-\p>»mp the AI'Ds both inside and outMile I In* piitn.tr>- hreiiisslraliluiifi beam produced by <i Mi*V and I") Me\' electrons. The result*: are shouu m laltle 2. lor the (• MeV exposure, the response inside I be l«\am ran be attributed to protons gnu-raled by the photo lisiiitegration of deuterium in hydrogen. Hydrogen is a constituent of the detector medium. The isotopic abundance of deuterium is 0.01.1%. This fraction has an energy threshold of 2.22 MeV. Accelerators operating above 10 MeV pradun.- significant quantities of photoneutrons. At these energies, other photonuclear react ions also arc possible. Since the response inside and outside the beam is practically the same, most of the response at 15 MeV can be attributed to the photoneutrons produced in the accelerator. The same cartridge was used for both the 0.5 and 1.5 Gy exposures. The lower readings for the 5 Gy exposure reflects the mmlinearity of the AIM) response. Cartridges from Batch 2 were usi'd for these exposures. E F F E C T OF E L E C T R O N IRRADIATION Table 3 shows the results of exposure to electrons of energies 12, 16, and 20 MeV from a d i n a r 18011 medical accelerator. The response inside the beam <an be attributed to the scattering of nuclei in the detector medium by these relativist ic electrons. Only one cartridge (from Batch 2) was used at each energy. No significant response above background was obserwd for the detectors outside the primary beam. Response to electrons ill the energy range of 9 to 20 MeV also has been observed in the Chalk River Nuclear Laboratories neutron hubltle detector.'** C O N C L U S I O N S The AIM) has some desirable* • harartcristics. such as its quick and easy readout, low background and insensitivity lo low-energy photons. \ he <U'(v>ttir if- i-eiiaitive t<t liij>.li energy plini<ins and elcetrons. H<:s|ioiisr to '*'(! iiixl I'ulle neutrons < l<isely follows the expected ifsjn>ii>a* (to Am Or neutrons) claimed by the manufacturer. Al low dose equivalents, the response to Pulle, PuH, PuF, and PuLi neutron sources is within 40% of the exported response. The detector self-nucleates when heated to temperatures of 4t>°C or greater. The dependence of sensitivity upon temperature is a limit'ng factor in the practical aspects of dosimetry. Other disadvantages include the large variations sometimes observed between cartridges and the limited range of linearity of response. These results were obtained with the 1988 production prototype detectors. Since then, according to the manufacturer,'*' there has been considerable improvement in the dynamic and linear range of the detector. The manufacturer also claims that the new cartridges can be used up to four weeks aiter sensitisation. ACKNOWLEDGMENTS The authors thank C. Donahue and F. Vandeithaak of Stanford University for their help with the accelerator exposures. Acknowledgments also are due to the SLAC Publi­cations Department, for help with the preparation of this manuscript. REFERENCES I. Apfcl, R. K. Active PersonndDetector/Dosimeter APD(Manwt). ApfeiEnterprises, Inc. CT. USA. 2 Apfel. R. K. and Roy, S. ('. Superheated Drop Detector: A Possible Alternative for Srutron Dosimetry. Hadiat. Prot. llosim. 10 (1 4), 327-330 (1985). ;». Apfel, It. K.. Hoy. S. C . and l.o. Y. C. Prediction of thr Minimum Neutron Energy te> AW/ra/r Vopnr [htbblf* ni Suptrhratrd Liquids. Phys. Rev. A31, 31*1-3198 (1985). 8 4. Apfr), II. K. and Lo, Y. C. Practical Srutnm Do&tmftry with Suptrhcatrd Praps. Hoalth Phys. 56:1, 79 83 (1989). 5. NCRI*. Srvtrtm Contamination from Medical Flrclron Accelerator*. Report No. 7!) (Bothesda, MD: NCMP Publications) pp. 42-46 (11)84). 6. Ipe, N. E.. Donahue. R. J. and Busick, D. D. The Active Personnel Dosimeter — Apfel Evtrrpns'n Superheated Drop Detector (extruded version). SLAC PUB-5122. Stanford. CA. USA (198!>). 7. Apfel, II. E. Thr Suprrhroied Dn>p Detector. Nucl. InHtrum. ant) Mcth. 162, 603-008 (1979). 8. lpe, N. £. and Itusick, D. D. BD-100: The Chalk River Nuclear laboratories' Neutron Bubble Detector. SLAC-PUB-4398, Stanford, CA, USA (1987). 9. Apfel, R. E. Private communication. 9 Table 1 Response to Neutron Sources Source ill Relative Sensitivity PuBe 4.2 1.28 1.41 PuB 2.1 1.13 1.17 PuF 1.5 1.10 1.05 PuLI 0.5 1.30 1.29 Table 2 Effect of Photon Irradiation Energy Dose Inside Beam Counts/Gy Inside Beam Outside Beam e 5 5 41 27 1.6 0.8 15 0.05 0.5 5 206 197 111 214 208 85 Table 3 Effect of Electron Irradiation Energy (MeV) Dose Inside Beam (Gy) Counts/Gy Inside Beam Outside Beam 12 2 4 23 21 0.5 0.25 16 1 121 4 20 0.05 0.5 180 206 0 0 10 FIGURE CAPTIONS 1. Linearity and reproducibility. 2, Exp«ur« to PuBe and '"Cf. II 4000 3000 -CO i -3 2000 o o 1000 -I I • • — • J ^ ^ — • X • it • • * > * * o 0 < mSv/h " i / Data I - PuBe AmBe 0.83 0.29 (Apfel) I I 1©-» flHUAT 0.5 1.0 1.5 2.0 DOSE EQUIVALENT (mSv) Fig. 1 4000 r 3000 -to o o 2000 -1000 1 1 mSv/h • PuBe 1.23 * 252Cf 1.10 —Am-Be 0.29 (Apfel) 10 89 65D2AB 0 1 DOSE EQUIVALENT (mSv) 

The Active Personnel Dosimeter: Apfel Enterprises superheated drop detector

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The Active Personnel Dosimeter: Apfel Enterprises superheated drop detector

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