Response to 350 MeV 90Zr ions of polycarbonate detectors

168

Nuclear Instruments and Methods in Physics Research B51(1990) 168-172 North-Holland

RESPONSE

TO 350 MeV 9oZr IONS OF POLYCA~ONA~

Jolly

Swarnali

RAJU,

GHOSH,

Atul SAXENA

* and K.K.

DETEXTORS DWIVEDI

**

Department of Chemistry, North-Eastern Hill University, Shiliong 793 003, India Received 4 January 1990 and in revised form 2 April 1990

Makrofol-KG and Polycarbonate (Bayer) plastic detectors were exposed to 350 MeV WZr ions for studying the heavy ion tracks. Successive etching of these detectors in 6N NaOH at 55 * C has been performed in order to develop latent tracks and to evaluate bulk and track etch parameters. The detectors have been character&d by correlating the measured track-etch rate (VT) with residual range ( R) and total energy-loss rate (d E/dx). A linear dependence of VT on both R and d E/dx has been observed in these detectors for %Zr ions above 15 MeV. The significance and importance of the experimental results have been discussed.

1. Introduction Development of newer detector materials and the availability of energetic heavy ion beams have made Solid State Nuclear Track Detectors (SSNTDs) versatile in their use [l-10]. Amongst plastic track detectors, Makrofol-KG and Polycarbonate (Bayer) are quite suitable for their application in production of ion track filters [ll,lZ], which are capable of a mechanical separation of small particles suspended in a liquid or gaseous medium. Ion track filters are advantageous over conventional filters as they are defined by a few independent parameters such as the track length, track diameter aud area1 density of ion tracks. Moreover ion track technology offer definite pore size and shape as required. Studies on the superfluid analog of the superconductive Josephson effect is in progress [13]. This development needs the manufacture of sufficiently small apertures, for the “weak links”-potential barrier through which tunneling occurs between two neighbouring superfluids. Due to the smaller coherence lengths of superfluids in wmpa~son with superconductors these structures have to be smaller. Single pore cell may act as a weak link for that purpose. A thin film microbridge formed by etching down a superconducting film can act as a weak link for observing the Josephson effect, In order to observe the superfluid analog of the Josephson effect by the use of ion track technique, a number of aperture (weak link) geometries are possible: 1) single cone obtained by one sided etching, 2) overlapping cones due to one sided etching with more than one ion track present, 3) stepped cones obtained by one sided

etching, sputter deposition of the top layer and further enlargement of the bottom cone by a second etch step, 4) double layer material with different etch rates and elliptical pore due to nonisotropic lateral etch rate. Polycarbonates have been found very useful in production of ion track filters and single pore membranes [14]. In addition they may be used to prepare microapertures useful in biomedical research [6]. In order to prepare microapertures of known shape and size, knowledge of the various track parameters (track-etch rate (I’,), bulk-etch rate (Vo), track length) are needed. Thus the optimal use of any SSNTD largely depends on standardization of various etching parameters such as bulk-etch rate and track-etch rate with respect to the energy-loss rate (dE/dx) of the track forming heavy ion. ~~retic~ly the range-velocity equation of Bethe [15] and Bohr 1161 show a dependence of energy-loss rate (dE/dx) on the penetration depths of the heavy ion. A relationship between Vr and (dE/dx) for any ion in any media provides one with an understanding of track formation mechanism and a calibration curve to produce microholes of the desired geometry in the media. The aim of the present study is to evaluate the response of two different polycarbonate plastic detectors to 350 MeV ?Zr ions in terms of the track parameters (I& and I’,) and to find the correlation of Vr with d E/dx and residual range (R).

2. Experimental 2.1. Detector preparation

* Present address: Department of Physics, Paehbunga University College, NEHU, A&w&796 001, India. ** Author to whom all correspondence should be addressed.

and irradiation

M~ofol-KG and Polycarbonate (Bayer) detectors were prepared from commercially available foils manu-

0168-583X/90/$03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland)

169

J. Raju et al. / Response to 350 MeV 9oZr ions of Polycarbonate detectors Table 1 Properties of detectors Properties Composition Chemical name Molecular weight Density (g/cm3) Thickness (urn) Uniformity Surface view Chemical etchant Clarity Colour

2.3. Measurement

Makrofol-KG

Polycarbonate (Bayer)

G5%4

G&03

Bisphenol A polycarbonate 254.0 1.18 42.0 good optical grade

Bisphenol A polycarbonate 254.0 1.20 315.0 good optical grade

NaOH average greenishyellow

NaOH average colourless

of tracks parameters

Track lengths and face diameters were measured by a Leitz optical microscope. The detector surface was scanned in the x- and y-directions in order to identify lo-15 partially etched tracks. The track diameters were recorded as the minor axis of the elliptical face of the track, and in addition the projected track lengths were measured. These measurements were carried out for all identified tracks after every etching. The accuracy was 0.45 urn at 50 x and 0.20 urn at 100 x objective magnification. After complete etching about 200 tracks were measured for the detector resolution.

3. Results and discussion 3.1. Detector resolution

factured by Bayer AG, Leverkusen, FRG by cutting into size of 2.0 x 2.0 cm2. Properties of these detectors are listed in table 1. After removal of surface protecting layers the detectors were washed in soap solution and dried inside a vacuum desiccator. Detectors were mounted on a 5 x 5 cm2 slide glass backing and fitted into sample holders. These holders were then placed into a special perspex magazine for irradiations. All the samples were exposed to a collimated beam of 350 MeV 90Zr ions at the X0 channel of UNILAC, GSI, Darmstadt. The incident angle was 45 o and the area1 dose was lo4 cmm2. 2.2. Chemical etching for track development Latent damage tracks were developed by suitable chemical etching. The etchings were carried out in 6N NaOH at 55 + 0.5O C. Prior to etching the detectors were washed in lukewarm soap solution in order to remove all greasy substances from the surfaces. Maximum etchable track lengths were obtained by etching the detectors successively during intervals of lo-15 min. Successive etching allows a determination of the rate of increase of track-length versus etch time and in addition to find the time needed to completely etch the tracks. Etching was terminated when rounded track tips were formed. A few test etchings were performed to determine the complete etch time (t,) for tracks of different lengths in the two detectors. tc depends on the etch conditions, the track forming ion and its energy. After every etching the detectors were washed in running water for lo-15 min followed by two or three washings in distilled water. All etched track detectors were dried by pressing them gently between layers of soft filter paper and subsequently placing them in a vacuum desiccator.

The track detectors are usually not affected by a high flux of less ionizing radiations and thereby offer excellent mass, charge and energy resolution for energetic heavy ions. The relative energy resolution of a detector is defined as the full width at half maximum (FWHM) divided by the mean ion energy (E,) obtained from a peak centroid of the track length distribution curve (fig. 1). The relative energy resolution, the FWHM and standard deviation in each of the cases are given in table 2. 3.2. Error analysis The accuracy in track length measurement is generally found to be +0.9 urn leading to an error of *1.3 urn in the true length measurement. This leads to a cummulative error of +4.0 urn h-’ in the track etch rate. The accuracy in the measurement of track diameter is + 0.5 urn which leads to an error in Vo of + 0.04

Table 2 Energy resolution of 350 MeV 90Zr in two different detectors Parameters

Makrofol-KG

Polycarbonate (Bayer)

Energy (MeV)

350 5.00 2.1 40.5 14.0 53.5 11.57

350 6.00 2.5 47.7 15.0 55.5 13.63

LFWHM 0~

64

EFWHM uE

(pm)

(MeV)

(Mew

-L (pm) Resolution (a)

L,,,,: track length corresponding to FWHM. EFwH,: energy corresponding to FWHM. or: standard deviation in measurement of track length. on: standard deviation in energy. L,: most probable track length.

J. Raju et al. / Response to 350 MeV 9oZr ions of Polycarbonate

170

Table 5 Values of the coefficients and the validity region of the calibration curve between VT and residual range (R) for 350 MeV 90Zr ion in the two detectors

1

0

aD

[a)

MKG

SSNTD

BPC

lb)

detectors

Coefficients

t

a

Makrofol-KG Polycarbonate (Bayer)

Validity

2.31* 0.36

8.46

1.77 kO.20

hh’ for Polycarbonate

pm

region

b > 10 MeV

-3.48

Makrofol-KG (Bayer).

and

> 34 MeV

f0.08

pm

hh’

for

3.3. Etch-rates

OLO

I, t

I

I

50

60

I

TRACK

&O

60

50

LENGTH

x

Bulk-etch rates (Vs) for Makrofol-KG and Polycarbonate (Bayer) have been determined using track diameter technique [17]. The results are given in table 3, which also shows the time (t,) corresponding to complete etching of damaged tracks in each case. The track-etch rate (Vr) at any point on the tracks of %Zr ions in Makrofol-KG and Polycarbonate (Bayer) were obtained from the slopes of the curves showing the true track length versus etch time.

I 7t

(pm)

Fig. 1. Track length distribution of 350 MeV WZr in (a) Makrofol-KG and (b) Polycarbonate (Bayer). The most probable track lengths are indicated by the arrows. Table 3 Values of complete maximum etchable detectors

etching time t,, the bulk-etch rate V, and track length of 350 MeV %Zr ions in two

Table 6 Values of coefficients and validity region of the calibration curve between VT and energy-loss rate (dE/dx) for 350 MeV 90Zr ion in two detectors

Polycarbonate

Makrofol-KG

(Bayer) Etching

conditions

6N NaOH/55 75 +3 1.1 kO.04

t, (mm) Vo (pm h-r) Track length (urn) experimental ‘) theoretical b,

oC

6N NaOH/55 140 f6 1.56 * 0.08

oC

C

55.5 k22.5 54.45

53.5 f 2.1 55.23

Makrofol-KG Polycarbonate (Bayer)

a) Values representing most probable track lengths. b, From computer code RANGE (ref. [19]). Table 4 Values of measured

VT, residual

range and the calculated

energy-loss

VT

bmh-')

range (pm) 12.00 7.00 4.00 1.50 0.25

a) dE/dx

35.29 22.22 20.00 12.00 6.00

calculated

from computer

dE/dx a> (MeV mg-’

Residual cm2)

38.50

code RANGE

(ref. [19]).

0.56 + 0.05

Validity

region

d - 27.60 6.00

> 10 MeV > 16.20 MeV

for 350 MeV seZr in both the detectors

(Bayer) VT(P~~-')

range (pm) 27.5 25.5 20.5 17.5 13.5 9.5

28.50 24.50 20.50

1.60+0.03

rate (dE/dx)

Polycarbonate

Makrofol-KG Residual

Coefficients

SSNTD

42.8 41.38 40.00 33.33 30.00 20.00 11.41

dE/dx a) (MeV mg-’ 66.49 63.26 56.25 48.40 43.31 25.00

cm’)

r...

J.

-r_ mev *_ I790.Q ions of Polycarbonate detectors KaJu er al. / Kesponse 10 su

I

.I,..

I

171

I

15

0

ReZidual Rangb4j_Jm) Fig. 2. A calibration curve for Makrofol-KG detector with wZr in terms of the measured VT as a function of the residual

dE/dx (MeV.mg-’ .cm2) Fig. 4. A calibration curve for Makrofol-KG detector with wZr in terms of the measured VT as a function of the calculated total energy-loss rate (dE/dx).

3.4. Comparison Iength

of experimental

with theoretical

track

The maximum etchable track lengths of 350 MeV 90Zr ion in these detectors have been obtained by applying suitable etching corrections [23] to the observed track lengths. Fig. 1 shows the distribution of track lengths of 90Zr in Makrofol-KG and Polycarbonate (Bayer) detectors. The most probable track lengths are found to be 53.5 k 2.1 pm and 55.5 f 2.5 pm respectively. Table 3 lists the experimental values along with the corresponding theoretical values obtained from the computer code RANGE [19] based on the stopping power equations of Mukheji and coworkers [20-231. A reasonably good agreement has been observed between the measured and calculated values. This has further supported the earlier views [24-261.

3.5. Characterization

of track detectors

We have calibrated the two detectors (Makrofol-KG and Polycarbonate (Bayer)) with 350 MeV 90Zr ions by two methods. These methods are based on the dependence of the track-etch rate (Vr) on the residual range (R) and the energy-loss rate (d E/dx). Table 4 shows the measured values of Vr at different points on the track, and the corresponding theoretical total energy-loss rate (dE/dx) and the residual range (R). 3.5.1. Dependence of VT on R Plots of track-etch rate (VT) versus residual range (R) for %Zr ions in the mentioned detectors are given in figs. 2 and 3, respectively. A linear dependence of VT on R has been observed within the mentioned energy region: VT = a(R)

+ b,

(1)

80

20

dE/dx (f%l.mg-1.c~2) Fig. 3. A calibration curve for Polycarbonate (Bayer) detector with “Zr in terms of the measured VT as a function of the residual range.

Fig. 5. A calibration curve for Polycarbonate (Bayer) detector with 90Zr in terms of the measured VT as a function of the calculated total energy-loss rate (dE/dx).

172

J. Raju et al. / Response to 350 MeV “Zr ions of Potjxarbonate detectors

where VT is in units of nmh-’ and R in pm. Coefficients a and b have been evaluated from the slope and intercept respectively of the Vr-R plots (figs. 2 and 3) and are listed in table 5. The VT-R calibration is found to be valid between 10 and 350 MeV for Makrofol-KG and 34-350 MeV for Polyc~bonate (Bayer). 3.5.2. Dependence of VT on total energv-loss rate Figs. 4 and 5 show plots of VT as function of total energy-loss rate (dE/dx) for Makrofol-KG and Polycarbonate (Bayer) respectively. A linear dependence of VT on dE/dx has been observed for both the detectors: VT= c(dE/dx)

+ d,

(2)

where VT has the same units as expressed in the previous section and dE/dx in MeV mg-l cm2. Coefficients c and d for both the detectors have been determined from the slope and intercept respectively of the corresponding Vr-dE/dx plots and are listed in table 6. The linear relation has been found to be valid between 10 and 350 MeV for Makrofol-KG and 16.20-350 MeV for Polycarbonate (Bayer).

4. Conch&on The registration and development of latent tracks created by 350 MeV 90Zr in Makrofol-KG and Polycarbonate (Bayer) have been studied in terms of bulk and track-etch rates, complete etching time and maximum etchable track length. Our results show that in 6N NaOH at 55 o C etching times of 75 f 3 min and 140 + 6 min are required to etch 350 MeV %Zr tracks completely in Makrofol-KG and Polycarbonate (Bayer), respectively. Using appropriate etching corrections, the true maximum etchable track lengths of %Zr in Makrofol-KG and Polycarbonate (Bayer) were found to be 53.5 & 2.1 pm and 55.5 + 2.5 Pm respectively. A reasonably good agreement between measured track lengths and the corresponding theoretical data from computer code RANGE has further established the usefulness of the stopping-power equations 120-231 to obtain reliable heavy ion ranges in complex media. A linear dependence of VT on (dE/dx) supports a “ionexplosion spike” model of track formation in these polycarbonates.

Ackuowkdgements We wish to thank Dr. R. Spobr, Dr. J. Vetter and other staff at UNILAC, GSI, Darmstadt for providing

irradiation facilities. We also thank the German Agency for Technical Cooperation (DGTZ) and DAAD, Bonn for an equipment grant.

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