276 Nuclear Instruments and Methods in physics Research B36 (1989) 276-281 North-Holland, Amsterdam

ENERGY-LOSS MEASUREMENT OF 132Xe IN LEAD BY A NUCLEAR TRACK ~C~IQUE

Atul SAXENA and K.K. DWIVEDI *

Deparfment of Chemisfv, _Vorth-Eastern Hill University, Shiliosg 793 003, India

E. REICHWEIN and G. FIEDLER

II. Physikalisches Insfitut, Justus-Liebig Uniuersitiit, D-6300 Giessen, FRG

Received 1 July 1987 and in revised from 2 November 1988

A simple nuclear track technique has been described to measure energy-loss rate of any heavy ion in any medium. Here we present the results obtained from our measurements of energy-loss rate of 17.0 hileV/u r3’Xe ions in lead using special zinc phosphate (ZnP) glass detector. The errors in measurement range from 5-10%. Experimental energy-loss data has been compared with calculated values. The significance of the results and scope of the track technique is discussed.

1. Introduction

Solid state nuclear track detectors offer several ap- plications not only in the field of nuclear physics but also in many diverse fields of study [1,2]. During the last couple of years some new detectors such as CR-39 and ZnP-glass were identified and developed 13-5) with enhanced sensitivity and resolution. The shape and size of heavy ion tracks in such detectors are dependent on the mass, charge and energy of the track forming ions as well as on the stopping-power of the media. Thus after appropriate calibration, it is possible to use these track detectors for measuring energy-loss rate of any heavy ion in any medium. Recently, track technique has been employed to measure ranges and energy-loss of heavy ions in certain track forming solids [6,7]. Saxena et al. [6] have measured ranges and energy-loss of 16.34 MeV/u =‘U in Makrofol-N on the other hand Swarnali Ghosh et al. [7] have presented data for 18.56 MeV/u 4oAr in Lexan poiycarbonate.

In this paper we describe a simple method for mea- suring energy-loss rate of 17.0 MeV/u t3’ Xe in metallic lead using ZnP-glass track detector. Our experimental results are compared with the theoretical values ob- tained from computer code DEDXT [S] based on stop- ping-power equations of Mukhetji and coworkers [9-111.

2. The track technique

The basic principle involved in this track technique is very simple. First of all a sensitive track detector is

* To whom all correspondence should be addressed.

0168-583X/89/$03.50 Q Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

calibrated for a desired heavy ion in terms of maximum etchable track length as a function of ion energy. Then several targets of any material with precisely known thickness are placed before the detectors and exposed to a collimated beam of the same heavy ions. The energy- loss of the transmitted ions may be directly obtained from the values of the measured maximum etchable track lengths and the calibration curve.

The basic principle of the track technique is il- lustrated in a schematic diagram (fig. 1). It represents the depth of penetration of ‘32Xe (17.0 MeV/u) ions in a few ZnP-glass detectors after passing through lead targets. From these penetration depths (track lengths, L,), the transmitted energy (E,) of the ion is de- termined and an energy-loss curve is obtained by plot- ting E, as a function of target thickness. For a given target thickness X, the energy-loss may be obtained by the relation

AE=(E;-E;‘), (1)

where Ei and EC are energies of the ion before and after its passage through an effective target thickness AX respectively. The value of AX is equal to the thickness difference of any two nearby targets X, and X, and is given by

AX= (Xz - Xi). (2)

Hence, the energy-loss rate (ELR) can be determined from the following equation.

(ELR) E,,X, = E, 2 1

where E, and X, are the mean energy of the ion and

A. Saxenn et al. / Energy loss of ““Xe in lead 211

2LO 160 80 7 L, , , , , 7 ,“,O ,16,0 ,2rjOp

Collipated ions -----_

Ei = 17.0 MVIU I / ---__

P b-targets ZnP-glass detectors

Fig. 1. A schematic diagram showing the basic principle of the measurement of energy-loss of 17.0 MeV/u i3’Xe ions in lead

using ZnP-glass detector.

the mean target thickness respectively. These corre- spond to

E,=(E;+Er)/2

and

X,=(X,+X,)/2. (5)

In the present work, the energy-loss rate of 132Xe in lead is determined from the experimental energy-loss curve.

3. Experimental

3. I. Preparation of detectors and targets

Thin plates of ZnP-glass detectors were obtained from bulk material (chemical composition: B,.,,O,,.,,-

*I,.,Si,,,,P,,.,,Zn,., and specific gravity 2.686 g/ml) by cutting in the size of (25 X 15 X 1.5) mm. These detector plates were polished in order to produce back- ground free and optically plain surfaces. Each detector plate was examined under microscope for surface smoothness and then weighed over semi-microbalance.

High purity (4nines) lead was used to prepare targets by vacuum evaporation-deposition technique. Fifteen detector plates were fixed in a mount and placed inside

the vacuum chamber at a height of nearly 15 cm above the boat containing lead metal. After deposition, all the

target-detector plates were weighed again. It was found

that fairly uniform (within 3%) targets of Pb were deposited irrespective of their positions. For the present

experiment, targets of varied thickness (1.6-124.8 mg/cm2) were prepared by successive evaporations and their thickness were determined by weighing technique. In order to protect these targets from air and moisture, which attack the metal and form thin surface layer of oxycarbonate, vacuum desiccators were used to preserve the targets. This has prevented oxidation and thereby mirror like surface lusture of the targets was retained.

3.2. Irradiations

All irradiations were done at X0 channel of UN- ILAC, GSI Darmstadt. Samples were fitted in special holders and arranged in sample magazines. An auto- matic sample carrier-cum-changer was used to align

samples in any desired orientation with respect to ion beam. A well collimated beam of 132Xe ions with an initial energy of 17.0 MeV/u was used to expose targets and detectors at incident angle of 45O with respect to the surface. An optimum ion dose of 2 X lo4 crnm2 has been used. Several ZnP-glass detectors without any target material were also exposed to 13’Xe ions of varied energies (1.4-17.0 MeV/u) in order to obtain a calibration curve between 132Xe ion energies and maxi- mum etchable track lengths in ZnP-glass detector.

3.3. Chemical etching

After irradiation, layers of Pb targets were removed by dissolving in aqua regia at room temperature. ZnP- glass detectors were then thoroughly washed in distilled water and dried in air. All the glass detectors were then etched in 6N NaOH at 55 o C. The etching process was carried out in steps and terminated when rounded track-tips were observed. The etching time ranges be- tween 40-100 min. After appropriate etching and wash- ing the detectors were dried under vacuum.

3.4. Meusurement of truck length

Well defined, narrow conical tracks of ‘32Xe ions in ZnP-glass were observed at ordinary magnifications. Lengths of the etched tracks have been measured at random all over the detector surface in order to mini- mize the effect due to variation in target thickness. Track diameters and projected track lengths were mea- sured with the help of an optical microscope at a magnification of 1000 X . A measuring accuracy of + 0.5 pm has been achieved at this magnification. From mea- sured data, the maximum etchable true track lengths

278 A. Saxena et al. / Energy loss of ‘32Xe in lead

were obtained from the equation given by Dwivedi and Mukheji [12].

It is well known [13] that chemical etching reveals only that portion of tracks for which energy-deposition rate is above a critical value (dE/dX),. This critical energy-deposition rate is also called track registration threshold and the sensitivity of SSNTD is characterized by this parameter. If the total length of damaged trail (range) is R and the maximum etchable track length is L, then the portion of range remaining unetched or the range deficit (AR) is obtained by

AR=(R-L). (6)

For ZnP-glass detector the track registration threshold (dE/dX). was found to be 13.0 MeV mgg’ cm2 [14]. This corresponds to a range deficit of less than 2 pm and to a lower energy cut-off value of about 7 MeV for ‘32Xe ions. Since the detector calibration is done in terms of maximum etchable track lengths, therefore, the value of lower energy cut-off has no influence on the experimental results.

3.5. Detector calibration

ZnP-glass detectors were calibrated for energy mea- surements of ‘32Xe in terms of maximum etchable track lengths. Several ZnP-glass detectors were exposed to ‘32Xe ions of varied energies (1.4-17.0 MeV/u) at 30 o and 45O with respect to detector surface. Ion energies were precisely measured with the help of a TOF system at UNILAC. Nuclear tracks were fully developed by chemical etching in 6N NaOH at 55 o C for a period of 40-100 min. Maximum etchable true track lengths were measured as mentioned in the previous section. Mea- sured track lengths and energies of ‘32Xe ions are listed in table 1. A few low energy data are taken from other references [15,16]. Fig. 2 shows a calibration curve for i3* Xe ions in ZnP-glass detector. It is drawn by fitting a one dimensional third order polynomial of the type

n=3

E,= c GG, (7)

n=O

where E, is the transmitted energy (in MeV) of the ion after traversing through a target of thickness X. Lx (in pm) is the track length of the ion of energy E, in the track detector and (Y, is the best set of coefficients of one dimensional polynomial fit. As obtained in the present experiment, the values of coefficients (LY,) are listed in table 2. Using these coefficients and the meas- ured track length in ZnP-glass detector, the transmitted energies of 13’Xe have been obtained with fair accuracy.

3.6. Energy-loss measurement

An energy-loss curve may be constructed by plotting energy of the transmitted ions as a function of target

Table 1 Maximum etchable track lengths of 13’Xe ions in ZnP-glass at different energies. Tracks are etched in 6N NaOH at 55OC. Only statistical uncertainties are shown here.

Energy Maximum etchable (MeV/u) track length (pm)

1.40 16.2kO.5 =) 4.70 42.4 + 0.6 =) 7.35 60.3 f 0.6 7.50 62.0 i 1.1 b, 8.38 69.0 f 1.2 b, 8.82 71.0*1.4 9.83 s2.2*1.3

10.87 87.0 * 1.3 12.00 93.4+ 1.1 13.18 104.5 * 1.1 14.13 112.0 * 1.2 15.05 122.3 + 0.8 15.60 127.8 + 0.6 16.08 133.0i0.6 16.54 137.1* 0.9 17.00 145.4i 0.6

a) Data from ref. [15]. b, Data from ref. [16].

132 - zoo- Xe in ZnP Glass E

t 160-

Experimental data from

. Present work

S o Cmmbach

G

IZO- . Routenberg

:: 80-

2 co-

o ,,,,,,,,,,,,,I III, I 0 2 4 6 8 10 12 14 16 18

ION ENERGY (MeVlu)

Fig. 2. A calibration curve obtained by a one dimensional third order polynomial fit to the measured track length-energy data

of 13*Xe ion in ZnP-glass detector.

Table 2 Values of best set of coefficients of one dimensional third order polynomial fit (a) (Y,: for calibration curve between energy and track length and (b) &: energy-loss curve between energy and target thickness.

” Coefficients

%I Pm 0 0.1699 E+02 0.2254 E+04 1 0.1025 E+ 02 -0.1417 E+02 2 0.1236 E+ 00 -0.4414 E+OO 3 -0.6110 E-03 0.3613 E-02

A. Saxena et al. / Energy loss of ‘32Xe in lead 279

ION ENERGY (MeVlu 1

0 2L 6 8 10 12 11 16 18 20 1 II!III 1 1

g- 132 Xe in ZnP Glass (11

I21

3.7 Experimental errors

TRACK LENGTH (pm1

Fig. 3. Typical energy and track length spectra are shown for r3*Xe ions in ZnP-glass detector after passing through Pb- targets of varied thickness. The initial beam energy was 17.0

MeV/u.

The energy of heavy ions impinged on the targets is measured accurately (within 0.1%) with the help of a TOF system at UNILAC, Darmstadt. It was found that Pb-targets were uniform within 3% except the thinnest one for which it was nearly 7%. Track lengths are measured within an accuracy of +0.5 ym. The broad- ening of energy distribution of the degraded ions is clearly reflected in track length distribution curves. Fig. 3 shows such distributions for initial energy and 5 degraded energies of ‘32Xe in lead targets. Nearly 200-300 tracks were measured for each distribution. It has been observed that full-width at half maximum (FWHM) gradually increases with target thickness. Ta- ble 3 contains several parameters such as energy-loss and track length distribution of 132Xe ions along with target thickness and standard deviations. The experi- mental errors in the energy-loss measurements have been estimated to be about 5-10%.

thickness. For ‘32Xe ions in lead, the experimental data are fitted by a one dimensional third order polynomial which is represented as

4. Results and discussion

n=3

Ex= c ,4,X”, (8) II=0

where E, is the transmitted energy (in MeV) of the ion after penetrating through a target thickness X (in mg/cm2) and p, is the best set of coefficients of the polynomial fit. These coefficients are also given in table 2. Using eq. (S), the energies Ei and EC may be obtained for any two target thickness Xi and X, re- spectively. From these the energy-losses (AE) are de- termined for any corresponding target thickness (AX).

In table 4 we present our experimental results ob- tained for energy lost by 132Xe ions in passing through lead targets of varied thickness. The energy (E,) of the transmitted ion is derived from the calibration curve or from eq. (7) for a measured track length (L,). An energy-loss curve is generated by plotting E, against target thickness (X) and is shown in fig. 4. From this curve, the values of energy-loss rate of 132Xe in lead as a function of mean target thickness (X,) and mean ion energy (Em) have been obtained using the method described in section 3.6. Table 5 lists the values of experimental energy-loss rate (ELR) for every 5 mg/cm2

Table 3 Energy-loss and track length distribution of 13*Xe ions in a few lead targets of varied thickness

Parameters a) Reference number of the peak in fig. 3

(1) (2) (3) (4) (5) (6)

Xi @g/cm* ) 0.0 8.51 19.5 34.2 53.0 72.2 AE (MeV/u) 0.0 1.0 3.0 6.1 11.1 14.8 -% (MeV/u) 17.0 15.9 13.9 10.6 5.9 2.2 Lx (pm) 144.0 132.1 111.8 86.6 51.0 21.2 eE (MeV/u) 0.1 0.1 0.15 0.2 0.3 0.5 eL (pm) 1.2 1.5 1.8 2.0 3.0 5.5 N 305 240 225 215 260 220

‘) Xi = Pb-target thickness in mg/cm*. A E = energy lost by ‘32Xe ion of 17.0 MeV/u. E, = energy of the ion as obtained from the calibration curve. L, = length of the maximum etchable tracks for ion energy E,. oE = FWHM with respect to ion energy distribution. oL = FWHM with respect to track length distribution. N = number of tracks measured.

280 A. Saxena et al. / Energy loss of ‘32Xe in lead

Table 4

Values of maximum etchable track length (L,) and corresponding transmitted energy (E,) of 13’Xe ions after traversing through lead targets of thickness (X).

Target thickness X

(pm)

No target

(mg/cm’ )

No target

Maximum etchable track length L,

(pm)

144.0 + 1.2

Energy of the transmitted ion

E,

(MeV) (MeV/u)

2244.0 17.0 1.4kO.l 1.6 +O.l 141.6 + 1.3 4.2+0.1 4.8 + 0.1 138.8 + 1.6 7.5 + 0.2 8.5 f 0.2 132.1+ 1.0

12.0 + 0.3 13.6+0.3 119.6kl.l 17.2 + 0.4 19.5 +0.4 111.8+1.5 22.2 * 0.5 25.2*0.6 101.1* 1.0 24.5 + 0.6 27.8 + 0.7 96.6 + 1 .O 30.1+ 0.8 34.2 + 0.9 86.6 + 1.5

38.2 & 0.8 43.4 + 0.9 65.4kl.S

46.7 + 0.9 53.0+1.0 51.0 f 2.2

60.0+1.1 68.1+ 1.2 28.5 k 3.5 63.6rt1.4 72.2 + 1.4 21.2k5.7

97.0 + 1.6 110.1 f 1.7 no tracks 110.0 + 2.0 124.8 + 2.2 no tracks

2229.5 2178.0 2104.1 1987.9 1836.1 1675.1 1597.2 1399.2 1104.8

801.2 386.7 293.0

-

16.89 16.50 15.94 15.06 13.91 12.69 12.10 10.60 8.37

6.07 2.93 2.22

of target thickness, the corresponding mean ion energy and the calculated energy-loss rate of 13’Xe in lead using computer code DEDXT [8] based on stopping- power equations of Mukherji and coworkers [9-111. Plots of (ELR) versus Em and X, are shown in fig. S(a) and fig. 5(b), respectively. Discrepancies between ex- perimental and calculated energy-loss rate range from 2 to 15%. From this comparison we cannot make any meaningful assessment about the validity of the stop- ping-power equation of Mukherji and coworkers [9-111 unless the measurements are done for several other ions in different elements and complex media.

5. Conclusions

Energy measurement of heavy ions require highly sophisticated instruments such as recoil proton spec-

- 201 I

13’Xe in Lead

(Initial energy 17.0 MeVlu)

01 , , , , , , 0 IO 20 30 40 50 60 70 60 90 100

TARGET THICKNESS (urn)

Fig. 4. A plot showing energy-loss data for 13’Xe in Pb. Experimental data points are fitted with a one dimensional

polynomial of third order.

trometer [17-191, magnetic spectrometer [20], Time-of- flight (TOF) [21,22] and double time of flight (DTOF) [23] systems. Although these systems are capable of more precise measurements as compared to the pro- posed nuclear track technique, but due to its simplicity and low cost the track techniques may be considered as

Table 5 Values of experimental and theoretical energy-loss rate of ‘32Xe in lead at various mean target thickness and ion energies

Mean target Energy-loss rate (ELR) Mean ion

thickness X, (MeV mg-’ cm’) energy E,

(w/cm* ) Exp. ‘) Theoret. b, (MeV)

5.0 20.2 23.6 10.0 22.5 24.0

15.0 24.9 24.5 20.0 27.2 25.1 25.0 28.4 25.7 30.0 29.6 26.4 35.0 30.8 27.1 40.0 32.0 27.7

45.0 32.0 28.1 50.0 32.0 28.5 55.0 30.8 28.4 60.0 28.4 27.9 65.0 26.0 26.7 70.0 21.3 24.3 75.0 17.8 19.7 80.0 14.2 15.1

85.0 9.5 11.2 90.0 7.2 8.7

a> Experimental - present work. b, Theoretical - refs. [S-11].

2172.5 2071.8 1954.3 1822.9 1680.3 1531.2 1372.2 1212.2 1051.8

893.6 740.5 595.2 460.3 338.5 232.6 145.3 79.3 37.2

A. Saxena et al. / Energy loss of '32Xe in lead 281

::r-.-_!i

100 800 1200 1600 2000

E, IMeV 1

0 20 LO 60 80 100

x, tmglcm2)

Fig. 5. Plots of experimental energy-loss rate of i3*Xe in lead along with theoretical values [S-11] as a function of (a) mean

target thickness E, and (b) mean ion energy X,,,

quite handy and useful. Though in this technique, the track detector needs to be calibrated for each ion, but once it is done, the detector can be used to measure energy-loss of any ion in any material. Present investi- gation provides a simple experimental technique for measuring energy-loss rate of any heavy ion in any media using a sensitive solid state nuclear track detec- tor.

We wish to thank Dr. R. Spohr, Dr. J. Vetter and other staff at UNILAC, GSI, Darmstadt for providing irradiation facilities. One of us (K.K.D.) thanks DAAD (Bonn, West Germany) and UGC (New Delhi, India) for the award of academic exchange fellowship. We also thank the German Agency for Technical Cooperation (DGTZ) FRG for an equipment grant.

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