We present new calibrations of different production batches (from 1989 to
1999) of the INTERCAST CR-39, using the BNL-AGS 1 A GeV iron beam. The
comparison with previous results, obtained with the 158 A GeV lead beam from
the CERN-SPS shows that, while each production batch has a different
calibration curve (mainly due to minor differences in the production
conditions), the aging effect is negligible. We also tested the dependence of
the CR-39 response from the time elapsed between exposure and analysis (fading
effect). The fading effect, if present, is less than 10%. It may be compatible
with the experimental uncertainties on the bulk etching rate vB.Comment: 9 pages, 4 EPS figures, .pdf file. Talk presented by M. Giorgini at
  the 20 Int. Conf. on Nuclear Tracks in Solids, Portoroz (Slovenia), Aug
  28-Sep 1, 200

Cecchini

Fleischer

G. Giacomelli

Giacomelli

L. Patrizii

M. Giorgini

Noll

P. Serra

Particle Data Group

S. Cecchini

English

arXiv

Crossref

Radiation Measurements

New calibrations and time stability of the response of the INTERCAST CR-39

We present new calibrations of different production batches (from 1989 to 1999) of the INTERCAST CR-39, using the BNL-AGS 1 A GeV iron beam. The comparison with previous results, obtained with the 158 A GeV lead beam from the CERN-SPS shows that, while each production batch has a different calibration curve (mainly due to minor differences in the production conditions), the aging effect is negligible. We also tested the dependence of the CR-39 response from the time elapsed between exposure and analysis (fading effect). The fading effect, if present, is less than 10%. It may be compatible with the experimental uncertainties on the bulk etching rate vB

Cecchini, S

Giacomelli, G

Giorgini, M

Patrizii, L

Serra, P

CERN Document Server

120/11/2000New Calibrations and Time Stabilityof the Response of the INTERCAST CR-39S. Cecchinia,b, G. Giacomellia, M. Giorginia *, L. Patriziia, P. SerraaaDip. di Fisica dell’Universita’ di Bologna e Sezione INFN di Bologna,Viale C. Berti Pichat 6/2, I-40127 Bologna, ItalybCNR, Istituto TESRE, Via P. Gobetti 101, I-40129 Bologna, ItalyAbstractWe present new calibrations of different production batches (from 1989 to 1999) of theINTERCAST CR-39, using the BNL-AGS 1 A GeV iron beam. The comparison withprevious results, obtained with the 158 A GeV lead beam from the CERN-SPS shows that,while each production batch has a different calibration curve (mainly due to minordifferences in the production conditions), the aging effect is negligible. We also tested thedependence of the CR-39 response from the time elapsed between exposure and analysis(fading effect). The fading effect, if present, is less then 10%. It may be compatible withthe experimental uncertainities on the bulk etching rate vB.KeywordsCR-39; calibration; aging effect; fading effect.* Corresponding author:Dr. Miriam GiorginiDipartimento di Fisica dell’Universita’ e Sezione INFN di Bologna,Viale C. Berti Pichat 6/2, I-40127 Bologna, Italy.Tel : +39-51-2095235  ;  Fax : +39-51-2095269  ;  E-mail : miriam.giorgini@bo.infn.it21. IntroductionThe INTERCAST CR-39 is used as a nuclear track detector in the MACRO (MACROColl., 1993) and SLIM (SLIM Coll., 2000) experiments. MACRO is a large areadetector, located in the Hall B of the underground Gran Sasso Laboratory. One of itsprimary purpose is the search for magnetic monopoles with various subdetectors, one ofwhich exploits the 1200 m2 nuclear track detector CR-39. SLIM is a 400 m2  passivedetector devoted primarely to a “light” magnetic monopole search; it is located at theChacaltaya high altitude Laboratory at 5230 m above sea level; it is assumed that itsexposure will last 4 years.     In both cases the CR-39 was produced in different batches with slightly differentsensitivities. The long exposure times require the knowledge of aging and fadingeffects. The aging effect is the loss of capability of detection in the period between theproduction and the exposure. The fading effect is the partial or total loss of the damagedue to a highly ionizing particle in the time elapsed from exposure to analysis. For theaging effect, we compared the response of CR-39 exposed to ion beams at differenttimes after production and analyzed immediately after exposure. For the fading effectwe compared the CR-39 response in sheets exposed at the same time to the same beamand analyzed after different periods.     To establish the response of the nuclear track detector CR-39 (calibration curves),stacks of CR-39 sheets were exposed to a 158 A GeV (A GeV means GeV/a.m.u.)Pb82+ ion beam at the CERN-SPS in Dec. 94 (Lead94) and Dec. 96 (Lead96) and to a 1A GeV Fe26+ ion beam at BNL-AGS in Dec. 99 (Iron99). The typical thickness of aCR-39 sheet was ~ 1.4 mm. The range of the Fe26+ and Pb82+ ions in CR-39 is ~ 14 cmand some meters, respectively.The CR-39 was manufactured by the INTERCAST Europe Co. of Parma (Italy) from1989 to 1999 in different production batches, using the same chemical composition andthe same curing cycle (MACRO Coll., 1991, 1993).3    In each stack, the ion beam crossed a target (where the incident nuclei fragmented)and CR-39 sheets before and after the target. The thickness of the targets (~ 1 cm) waschosen in order to have about 50% of survived beam. Both the beam and the fragmentions with electric charges above the detector threshold were recorded. The exposureswere carried out at normal incidence; the beam density was ~ 400 ions/cm2. Afterexposure, the CR-39 foils were etched in a 6N NaOH water solution at 70 °C for 30hours in the Pb cases and for 45 hours in the Fe case. During the etching, the CR-39 isremoved at a bulk velocity vB and at a track velocity vT, greater than vB, along an ion path.Therefore, for ions with Z/β greater than the detector threshold (5e for the INTERCASTCR-39), the etching process leads to the formation of a conical etch-pit on each side of adetector foil along the ion path (Fleischer et al., 1975; Cecchini et al., 1996; Giacomelliet al., 1997, 1998). In the used etching conditions, the bulk etching rate vB for theINTERCAST CR-39 ranges between 1.092 and 1.231 µm/h.The reduced etch rate p = vT /vB is used to measure the detector response. It wasdetermined using the relation (Fleischer et al., 1975)p = [1+(D/2vB t)2] /[1-(D/2vB t)2]           (1)For normally incident particles, D is the diameter of the etch-pit cone base. Previouscalibrations had shown that the reduced etch rate p is a function of the Restricted EnergyLoss (REL) only (Fleischer et al., 1975; Cecchini et al., 1996). For each detected charge,the REL was computed using the Bethe-Block formula (Particle Data Group, 2000) withenergy cut-off at 200 eV and is a function of (Z/β)2. This means that once an ion hasbeen identified by its base cone area (or diameter), we know its p-value and REL. Wechecked that the REL values on a CR-39 sheet were constant for Pb and Fe ions; inthese conditions the tracks have conical shapes.4The track diameters were obtained from the base cone area measurements made with theElbek automatic image analyzer system (Noll et al., 1988). About 50000 tracks weremeasured on each foil. Fiducial marks and patterns of tracks were used to track the pathof each detected ion along the stack. The vB was determined via thickness measurementsbefore and after the etching procedure.     Each stack was composed of CR39 foils from different batches; it was then possibleto determine the response of each batch to the same incident ion.For each batch we measured three consecutive foils (6 faces) for the Lead96calibrations, two consecutive foils (4 faces) for the Lead94 calibrations, one foil (2faces) for the Lead94 calibrations used for studying fading effects and for the Iron99calibrations. A “signal” was defined as the spatial coincidence of similar base coneareas in the required number of faces; the average of the measured base areas wascomputed.     A typical average base area distribution is shown in Fig. 1a for the Lead96 beam. Itrefers to CR-39 made in Nov. 95 and etched for 30 h in 6N NaOH at 70 °C shortlyafter exposure. A large number of peaks is evident. Each peak corresponds to a nuclearfragment with different charge. Due to the worsening of the charge resolution withincreasing charge and the relatively small number of measured faces, only fragmentswith charge up to Z ~ 35e are clearly separated.At the first peak the charge Z = 5e was assigned by comparison with our previouscalibrations (Cecchini et al., 1993, 1996) and assuming that the threshold of the CR-39was the same as for samples of the same age at the time of exposure and etched underthe same conditions.Fig. 1b refers to CR-39 exposed to the iron beam, etched for 45 h in 6N NaOH at 70 °C(2 measured faces). The CR-39 used was made in Dec. 96. Here all the nuclearfragments with charges from 6e to 26e are present. Due to some background not5completely removed using only two faces, we had to cut low values of the averaged arealoosing the 5e ion signal.2. Calibrations with the Iron BeamIn Fig. 2 are shown the calibration curves (p vs REL) for various CR-39 productionbatches since Apr. 89 to Apr. 99 using the BNL-AGS Fe26+  beam of 1 A GeV. TheApr. 99 CR-39 was exposed with an aging time of about 8 months, so its calibrationcurve is particularly high (Cecchini et al., 1996). As far as the other curves we can saythat each production batch has its own calibration curve with no evident connection withthe production time (see for istance the calibration curves of Nov. 89 and Nov. 97production batches that are very similar in spite of their really different productionages). The variation is mainly due to not perfectly identical conditions during theproduction. Further studies of these effects are in progress.3. Comparison of the Calibrations with Iron and Lead BeamsWe have compared the calibration curves of CR-39 from the same production batches,exposed to the CERN-SPS lead beam (Lead96) and the BNL-AGS iron beam (Iron99)to test the reproducibility of  the calibrations and the aging effect. Fig. 3 shows theresults for the Nov. 89 and Nov. 95 production batches. Within errors no effect isvisible.4. Fading Effects in CR-39In order to test the dependence of the CR-39 response from the time elapsed betweenexposure and etching (fading effect), several sets of samples belonging to the sameproduction batch were exposed in Dec. 94 to the 158 A GeV lead ion beam and etchedafter different times. In Fig. 4 the results for two sets etched 0.5 y and 4.5 y after theexposure, respectively, are shown. The response of the samples etched 4.5 y after6exposure is less than 10% lower than that for the samples etched shortly after exposure.The difference is within our experimental uncertainties on bulk velocity, shown as bandsin Fig. 4. That means that our detectors after 4.5 y are still well active.5. ConclusionsThe experimental results quoted above indicate that the response of the INTERCASTCR-39 used in the MACRO and SLIM experiments shows a fair time stability. Thereare no indications of aging effects for material older than 10-12 months. From the studyof fading effects we can be confident that, up to now, the detectors are at full sensitivity.AcknowledgmentsWe gratefully acknowledge the staff of the CERN-SPS and BNL-AGS for the helpduring the exposures. We acknowledge the collaboration of E. Bottazzi, L. DegliEsposti, D. Di Ferdinando, V. Togo, C. Valieri and the general services of the BolognaSection of INFN.References,Cecchini S., Dekhissi, H., Giacomelli, G., Katsavounidis, E., Margiotta, A.R., Patrizii, L.,Pedrieri, F., Serra, P. and Spurio, M., 1993. Calibration of the INTERCAST CR-39,Nucl. Tracks Radiat. Meas. 22, 555-558.Cecchini, S., Dekhissi, H., Garutti, V., Giacomelli, G., Katsavounidis, E., Mandrioli, G.,Margiotta, A.R., Patrizii, L., Popa, V., Serra, P., 1996. Calibration with relativistic andlow velocity ions of a CR-39 nuclear track detector, Il Nuovo Cimento 109 A, 1119-1128.Fleischer, R., Price, P.B. and Walker, R.M., 1975. Nuclear Tracks in Solids, Universityof California Press.7Giacomelli, G., Patrizii, L., Popa, V., Serra, P. and Togo, V., 1997. New results fromexposures of CR-39 Nuclear Track Detectors, Nucl. Tracks Radiat. Meas. 28, 217-222.Giacomelli, G., Giorgini, M., Mandrioli, G., Manzoor, S., Patrizii, L., Popa, V., Serra, P.,Togo, V. and Vilela, E.C., 1998. Extended calibration of a CR-39 nuclear track detectorwith 158 A GeV Pb ions, Nucl. Instrum. Meth. A 411, 41-45.MACRO Coll., 1991. Improvements of the CR-39 polymer for the MACROexperiment at the Gran Sasso Laboratory, Nucl. Tracks Radiat. Meas. 19, 641-646.MACRO Coll., 1993. First supermodule of the MACRO detector at Gran Sasso, Nucl.Instrum. Meth. A 324, 337-362.Noll, A., Rusch, G., Rocher, H., Dreute, J. and Heinrich, W., 1988. The Siegenautomatic measuring system for nuclear track detectors: new developments, Nucl.Tracks Radiat. Meas. 15, 265.Particle Data Group, 2000. Review of Particle Physics, Eur. Phys. J. C 15, 165.SLIM Coll., 2000. Search for “light” Magnetic Monopoles, (SLIM proposal) hep-ex/0003028.Figure CaptionsFig. 1. (a) Base cone area distribution for the CR-39 exposed to the CERN Pb82+ beam of158 A GeV; the CR-39 was etched in 6N NaOH at 70 °C for 30 hours. The base areaswere averaged over 6 faces. (b) Base cone area distribution for the CR-39 exposed to theFe26+ beam of 1 A GeV; the CR-39 was etched in 6N NaOH at 70 °C for 45 hours. Thebase areas were averaged over 2 faces.Fig. 2. Calibration (p vs Restricted Energy Loss) curves for various CR-39 productionbatches exposed to the 1 A GeV iron beam (BNL-AGS, 1999).Fig. 3. Calibrations of CR-39 from the same production batch exposed to the CERN-SPSlead beam and to the BNL-AGS iron beam with different aging times.Fig. 4. Calibrations of CR-39 from the same production batch exposed to the CERN-SPSlead beam and analysed 0.5 y and 4.5 y after exposure.89

New calibrations and time stability of the response of the INTERCAST
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New calibrations and time stability of the response of the INTERCAST CR-39

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