Bragg peak spectroscopy of low-energy heavy ions

244

Nuclear Instruments and Methods in Physics Research A257 (1987) 244-252 North-Holland, Amsterdam

BRAGG PEAK SPECTROSCOPY OF LOW-ENERGY HEAVY IONS R. KOTTE, H.-J. KELLER, H.-G. ORTLEPP and F. STARY Zentralinstitut für Kernforschung, Rossendorf DDR-8051 Dresden, GDR

Received 24 June 1986 and in revised form 2 January 1987 A small ionization chamber with the electric field lines parallel to the particle trajectory (Bragg ionization chamber) allowing the determination of the nuclear charge and the energy of low energetic heavy ions (E/A - 0.8-2 .0 MeV) is presented together with an adapted shaping amplifier. Using the customary technique of pulse shaping with a long and a short time constant the energy and the Bragg peak resolutions for projectiles between 24He and 16 32 S were determined . With purified n-pentane an energy resolution of d E/E =1 .0% and a nuclear charge resolving power of Z/d Z = 47 were measured for 1.5 MeV/amu 28 Si ions elastically scattered on 197Au . No significant pulse height defect is observable for the energy determination of particles with Z _< 17 . The Z resolution obtained via Bragg peak measurements was found superior to the Z determination using energy loss measurements which at low energies, in particular, suffer from large uncertainties due to straggling.

1. Introduction

2. Detector construction and electronics

Bragg curve spectroscopy (BCS), first proposed by Gruhn et al . [1], proved to be an efficient and widely used method [2-7] for nuclear charge identification of heavy ion reaction products. It is based on the generation of an ionization current signal the time dependence of which contains information on the specific ionization along the particle track. If the particle is stopped inside the active volume of the detector, the specific ionization distribution has a characteristic shape known as the Bragg curve. The height of the Bragg curve (Bragg peak, BP) is directly related to the nuclear charge number Z, the area of the Bragg curve is proportional to the energy E, and its length corresponds to the particle range in the gas volume . In this paper we report on a small Bragg ionization chamber (BIC) with homogeneous electric field for defining optimum working conditions and attainable parameters together with an optimization of the electronical processing of the information. The results are useful for designing the detector modules of the 41r spectrometer PHOBOS [8,9] which is intended for the study of medium energy (E/A -- 10-30 MeV) heavy ion reaction products at the JINR Dubna tandem cyclotron facility U-400/U-400M.

The detector (fig. 1) is mounted on a plexiglass flange and inserted into a stainless steel cylinder of 15 cm inner diameter which carries the gas inlet/outlet and the signal output connectors . The particles enter the active volume through an entrance window of 2 .5 cm diameter . It was made of stretched polypropylene foil of thickness 8=100 ;kg cm- z supported by an etched brass grid (180 pin broad strips, 2 mm mesh width, 83% transmission) which serves as cathode together with the brass entrance window holder and an aluminium flange supporting the whole detector construction . The design of the grid holder ensures that the entrance window is placed in the correct cathode position and allows a rapid and simple window exchange . The distance between the cathode and the Frisch grid (FG) amounts to L = 5.3 cm. Equidistant guard rings with an inner diameter of 4 cm maintain a homogeneous electric field. A divider chain of 8 X 20 M62 supplies the voltages for these electrodes . The cathode is held at negative potential with the high voltage plug on the high vacuum side preventing electric breakdown in the gas volume . The FG-anode distance amounts to G = 0.65 cm. The grid consists of parallel copper-beryllium wires of 70 Am diameter and 0.5 mm spacing. Its screening inefficiency was calculated to be 1.0% according to Buneman et al . [10] . The anode consists of a brass disc of 5 cm diameter embedded in a teflon plate ensuring low noise. It is maintained on positive potential through the input of an Ortec-120 charge sensitive preamplifier. N-pentane was used as stopping gas, its high electronic stopping power permits operation at

* Eq . (1) is a corrected version [19] of the faulty eq . (15) in ref. [l8], which has also been used in refs . [20-22], fitting the experimental data quite well, too. 0168-9002/87/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)

R. Kotte et al. / Braggpeak spectroscopy SUPPORTING FLANGE

ENTRANCE WINDOWCATHODE

ANODE

Fig . 1 . Cross section of the Bragg ionization chamber.

relatively low gas pressures and, therefore, the use of thin entrance windows. This is of importance if low energetic ions are to be measured because of the significant contribution of the entrance window straggling to the energy resolution . The commercially available liquid n-pentane (96`x) was purified from water and oxygen admixtures by a threefold destillation [11] . During the short test measurements the Bragg chamber was operated only with static gas fillings of the 4 1 volume. Longer runs, however, resulted in a full energy peak shift of 1-2% per hour due to an increasing contamination with electronegative admixtures, showing

of low-energy heavy ions

245

the necessity of a gas flowing through regime [12] . The pressure was chosen such that the particles with the longest range provided a 5 cm track length . The cathode-FG voltage was adjusted to get a reduced field strength E/p = 3 .8 V/cm Ton corresponding to a drift velocity vd - 3 cm/ps [13] delivering a maximum pulse length of 1 .8 As . In order to guarantee an effective collection of all electrons drifting to the anode a ratio of the FG-anode field strengths I E+/E- I = 3 .5-4.0 was used. The preamplifier signals were fed into two different shaping amplifiers - a standard spectroscopic amplifier with 2 As shaping time for the energy determination and a second one with a shaping time comparable with the electron drift time between the Frisch grid and the anode, usually in the order of 0.1 As. However, most of the available spectroscopic amplifiers have shortest shaping times of 0 .25 As or 0 .5 its. Therefore, a new shaping (BCS) amplifier/peak stretcher unit was developed meeting the requirements of Bragg curve spectroscopy. The simplified circuit diagram of the amplifier part is shown in fig . 2 . The shaping is performed by a pole-zero compensated differentiation just at the input and a fourfold decoupled RC integration between the second and the third amplification stage. Time constants of 22, 47 and 105 ns are selectable resulting in shaping times T (with the commonly used normalization) of 47, 100 and 220 ns, respectively . The amplification factor, defined as the step response amplitude divided by the step height, may be adjusted from 50 to 150 . In difference to usual amplifiers the change of the differentiation time constant is performed by changing the resistance before the virtual ground of the first stage . In this way the step response amplitude becomes proportional to the inverse of the shaping time. The BP amplitude, however, remains nearly independent of the shaping time . The shaping times were chosen shorter than usually employed in order to extend the Bragg curve spectroscopy to lower heavy ion en-

Fig . 2 . Simplified circuit diagram of the shaping (BCS) amplifier.

246

R. Kotte et al. / Braggpeak spectroscopy of low-energy heavy ions

ergies . The output signal of the shaping amplifier when viewed on an oscilloscope should be representative of the Bragg curve .

1500

3. Experimental results

1000

First test measurements were undertaken with «-particles from 210 Po and 241Am sources . Further measurements with heavy ions were carried out at the Rossendorf 5 MV tandem accelerator.

Z 0 U

500

3.1 . Tests with a -particles Tests were performed to study the gas properties. The chamber was operated at pressures providing 5 cm track lengths . Three types of stopping gas were used at the same reduced field strength E/p = 3 .8 V/cm Torr (table 1). The energy resolution of the Bragg ionization chamber is denoted by 4 E. Additional charge recombination along the particle track [12] causes a pulse height loss SE which was determined by comparing the BIC pulse height with the pulse height of a transverse-field ionization chamber [14] measured with indentical electronics . Obviously, purified n-pentane gives the lowest charge loss . Fig . 3 shows a typical a-particle energy spectrum with purified n-pentane, which was used as gas filling in all further measurements . In order to study the gas density dependence of the energy peak the pressure was increased by 20 Torr. The energy signal decreased by only 0 .9% corresponding to a constant pressure coefficient of 4 .5 x 10 -4 Torr -1 whereas the relative BP shift is equal to the relative pressure (density) variation. A typical a-particle BP spectrum is given in fig. 4. The BP resolution was determined with 100 and 220 ns shaping times resulting in resolving power values of Z/d Z = 31 and 45, respectively. In order to compare the BP resolution with the energy loss resolution of a gas-solid state d E-E telescope the chamber was operated with 115 Torr n-pentane corresponding to a relative energy loss of 4 E/E = 70% . The

0

Fig. 3 . a-particle energy spectra measured with the Bragg ionization chamber .

BRAGG PEAK

tn

Z

400L I

PULSER

p - 150 torr n-pen tone lpuritied1 Ua=-30kV Ua = 1 < kV shaping time -01Ns

Fig . 4. a-particle Bragg peak spectrum measured with the Bragg ionization chamber.

Table 1 Energy resolution and pulse height loss of alpha particles in different gases Gas

p [Torr]

d E(fwhm) [kev]

8E/E [%]

n-pentane (purified)

150

32

4 .2

n-pentane (nonpurified)

150

35

10.6

2-methylpentane (purified)

135

37

14 .0

Fig. 5 . Bragg peak height vs energy distributions of 21opo/241Am a-particles for three different shaping times T.

R . Kotte et al. / Bragg peak spectroscopy of low-energy heavy ions

247

Fig. 6. Measunng electronics. measured relative resolution of d(d E)/a E = 4.0% is inferior to the measured BP resolution but sufficient for the Z identification of a number of light particles. If the range of the lightest particles to be measured exceeds the active depth of the Bragg ionization chamber a thin metallized anode foil and a surface barrier E-detector of

Fig. 7. Bragg peak height vs energy distributions of

corresponding diameter behind the anode offers the possibility to operate such an arrangement both as Bragg ionization chamber for the heavy and AE-E telescope for the light particles. The typical tendencies caused by the variation of the shaping time z are obvious from fig. 5. The BP resolu-

210Po/241 Am a-particles and (cellulose absorber used).

160, 12C

ions elastically scattered from

19'Au

R. Kotte et al. / Bragg peak spectroscopy of low-energy heavy ions

248

Table 2 Energy resolution d E and the atomic number resolving power Z/dZ measured with different ions and shaping times. E denotes the energy deposited. Z/dZ

'r

p [Torr]

5 .3

31 45

100 220

150

350

21 .7

37

100

150

8

400

24.2

42

100

150

14 Si

400

37 .2

27 35 47

47 100 220

60

32s

460

26 .6

26 35 47

47 100 220

60

ZX

dE

4He

34

[keV]

12c 6 160

16

E

[MeV]

[ns]

tion improves with increasing shaping time, whereas the dynamic range over which the BP height becomes nearly independent of the energy shortens . A shaping time of T =100 ns corresponding to half the drift time of the electrons through the FG-anode gap seems to be a compromise between an acceptable BP resolution and a sufficient dynamic range similar to that obtained with T = 47 ns . This result was confirmed by measurements with heavy ions. 3.2. Tests with heavy ions The following ions were used for the measurements of the energy and BP resolutions and response func5 + (26 .0 MeV), 35017+ tions: 1204+ (22 .75 MeV), 16 0 28Sí6+ MeV), (32.2 MeV), 28 Si 7+ (36.8 MeV), (36.0 28Si8+ (41 .4 MeV), 32 S6 + (31 .5 MeV), 79 Br7+ (36.0

Z 8 6 4

MeV) . These particles were elastically scattered from a 10 jug cm-2 197Au target on a 10 Fig cm -2 carbon backing and in some cases from the aluminium target holder . The scattered products were observed at a lab. angle 5 = 30° with an opening angle c = ± 1° . At different times a cellulose absorber behind the target served for producing a continuous energy distribution of the scattered particles. The energy and BP signals were digitized and fed into a microcomputer-controlled two-dimensional analyzer with colour display [15] (fig. 6) . For off-line analysis the data were stored on magnetic tape cassettes . First measurements were undertaken with carbon and oxygen ions and a shaping time of T =100 ns. In order to get the a-particle BP as third point of the BP vs channel number response the chamber was operated with a pressure of 150 Torr ensuring that also the a-particles were fully stopped. The measured three BP-E distributions are given in fig. 7. The deduced energy and BP resolutions are listed in table 2. The BP response function given in fig. 8 shows the expected linear increase of the specific energy loss at the Bragg maximum with the atomic number Z (fig . 9) [16,17]. For the heavier ions the chamber was operated with p = 60 Torr. In fig. 10 typical BP-E distributions for 100 and 220 ns shaping times are presented. Fig. 11 shows the corresponding BP singles spectra for a fixed energy window, and in fig . 12 the deduced BP response functions are given . The decrease of the BP width with increasing shaping time is obvious from fig. 13 showing typical BP and energy spectra of 28 Si 7+ ions elastically scattered from "'Au . The deterioration of the energy

150

x E

P 100

5_ s dN W a

50

2

CHANNEL NUMBER Fig. '8 . Atomic number Z vs Bragg peak height response for the distributions given in fig. 7 .

Fig . 9. Maximum of the specific ionization d E/d x vs atomic number Z deduced from ref. [17] .

249

R. Kotte et al. / Bragg peak spectroscopy of low-energy heavy ions

BP 200 vi

192

28s)

p=60 torr n=pentane T =100ns E= ( 26 .0±0 .7)MeV

Z

tr W

O U

m Z

27

13A1

d

-34

50

W Z Z Q 2 U

50

100

150

BP

CHANNEL NUMBER

64

150

0

20

30

EIMeV

VI H Z

p=60torr n - pentene T =220 ns E= (26 0±0 .7) M .V

28 S) 14

b

2

O U

27 13 At

T 50

dZ =47

100

150

BP

Fig. 11 . Bragg peak singles spectra for a given energy window and T =100 ns (a) or T = 220 ns (b) shaping times.

Fig. 10. Bragg peak height vs energy distribution of 28Sis+ ions elastically scattered from target (Au), backing (C) and target holder (Al, Mg, Na, O) for T =100 ns (a) and T = 220 ns (b) (cellulose absorber used).

resolution (compared to table 2) is due to a rather old (7 h) gas filling which barely affected to BP resolution . In order to check the linearity of the energy vs channel number response most of the energy points measured with the Bragg ionization chamber are compared in fig. 14 . E corresponds to the energy value deposited in the BIC with the energy losses in the entrance window subtracted, which were determined by the stopping

power tables of ref. [161 for (CH 2 ) . Not only the energies of the scattered projectiles and the recoils were used, but also the energy losses of particles not fully absorbed in the gas volume . The energy losses within the cathode-FG distance plus half of that within the FG-anode gap were determined with the help of the range tables of ref. [171 for C5H12 . A linear energy response may be defined up to charge numbers Z =17. Slight deviations for 35C17+ and 28Si7+ were ascribed to the above mentioned long-term instability of the gas composition . For the heavier particles, such as "Br and 197Au, a mass dependent pulse height loss occurs resulting from charge carrier recombinations within the high charge density tracks of those particles and/or the ionization deficit due to an increasing contribution of nuclear collisions for energies below 0.1 MeV/amu. 4. Energy straggling considerations Precise energy loss measurements with a transversefield ionization chamber filled with isobutane or argon methane were analysed by Schmidt-Böcking and

R. Kotte et al. / Bragg peak spectroscopy of !ow-energy heavy tons

250 z 16 14

i

a Z-0 .1176 * N " 0.69

12

b

z

14

"

,IL - 47

Z=0 .1183*N+1 .30

/

12

Z -34 . / YZ

dZ

10

10

8

8

6

6

4

p-60 torr n -pentane shaping time T-100ns

4 2

CHANNEL NUMBER

p=60 torr n- pentane shaping time : t,=220n9

2 N

0

i 100 50 CHANNEL NUMBER

Fig. 12. Atomic number Z vs Bragg peak height response deduced from fig. 10 for T =100 ns (a) and

Eq . (1) is a corrected version [19] of the faulty eq . (15) in ref. [18], which has also been used in refs . [20-22], fitting the experimental data quite well, too.

28 Si 7+ ELASTICALLY SCATTERED

V) IZ

= 220 ns (b) shaping times.

where A E is the mean energy loss, K a target dependent constant and S(E) the stopping power of the projectile (E, ZP) in the target material (Zt). To compare the experimental Z resolutions deduced from the BP measurements of sect . 3.2. we put eq. (1) in the form fwhm/A E = A Z/Z, where A E is the energy loss of the projectile around the Bragg maximum. It corresponds to the energy loss (taken from ref. [17]) within the effective FG-anode gap, Gefr =

Hornung [18]. They suggested that the fwhm of the energy loss spectra can be represented by the semiempirical formula Zl/2 S(E-~E) fwhm = K A E0 .53 (1) E) X Zt13 + Zt/3 ' S( P t

47ns

T

N

shaping time 100 ns

220ns

ON 197 Au 2 Ns E =32 .6 MeV

BP

BP P

CHANNEL NUMBER Fig. 13 . Energy resolution and dependence of the Bragg peak resolution (given in charge units) on the shaping time 28Si7+ ions elastically scattered from 197Au (P : pulser lines) .

T

for 36 .8 MeV

R. Kotte et aL / Bragg peak spectroscopy of low-energy heavy tons

E MeV

28S i 7+(197ÁU)

E =0 .1655*N-1.30 MeV

30

35CI 7-1(197ÁU)

251

.

28S i 6+(197ÁU)~ ,"' 27A[ (28S,8+)

79Br7+(197Áu),

,r 32S6+(197 Áu ) 1605+(19 7Mut

~12C4+(197ÁU

20

197Á u

(79Br7 +)

)

12C4+(12C ) o -12C4+( JE) 16 0 3+(197Áu )

10

à 150 torr 60 torr 0 70 torr n-pentane

0

100 CHANNEL NUMBER

200

N

Fig. 14. Energy vs channel number response of the Bragg ionization chamber filled with purified n-pentane . 28Si8+(197Áu) denotes 28Si8+ ions elastically scattered from 197 An . 27AI( 28 Si 8 +) denotes z 7AI ions recoiled by z8 Si 8+ ions. 12 C a+ (QE) denotes energy loss measured for 12 C 4+ ions recoiled by z8 Si 8+ ions, etc .

2 VG + (2TVd ) 2 , determined from the effective drift time teff, i .e . by folding the drift time G/Vd with the amplifier integration time 2T. Furthermore, S(E - A E) = S(E) can be assumed around the ionization maximum. In fig . 15 the reduced Z resolution (,A Z/Z )( Zp/3 + ZI/3)/ZP/2 VS the energy loss AE is drawn. The points give the experimental values of table 2 corrected for electronic noise contributions . The solid line has been computed from eq. (1) with the value K=0 .126 MeV, which is concluded from the data of ref . [18], replacing the value Z1 = 6 (refs . [18,21]) by the more realistic average value (also used for the experimental points) Z 1 = (5 x 6 + 12)/17 = 2 .47 for pentane. For relatively light ions and the energy region considered the Bragg peak measurement delivers a 40% better Z resolution than the traditional energy loss measurement . This difference may be attributed to the range straggling which influences only the energy loss determination .

5 N

0

.} I

N

a

N

x 3

N IN 4

2 _AE

MeV Fig. 15 . Reduced Z resolution vs the energy loss around the Bragg peak (points : measured values, solid line : semiempirical formula (1) of ref. [18] for pentane) .

Acknowledgements The authors wish to thank Mrs . 7 . Fiedler for her careful preparation of the thin entrance windows and the Rossendorf tandem accelerator operating crew for

R . Kotte et al. / Bragg peak spectroscopy of low-energy heavy ions

252

the rapid and skilful sputter source handling providing excellent beams. References

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[9] H. Sodan, Yu .E . Penionzhkevich, LV. Kolesov, R. Kupczak, Yu.Ts . Oganessian, H.-G. Ortlepp, W . Seidel and D . Walzog, ZfK Rossendorf annual report 1984, ZfK-559 (1985) p. 144 . [10] O . Buneman, T . Cranshaw and J . Harvey, Can . J. Res . 27A (1949) 191 . [11] D . Walzog, W . Seidel and H. Sodan, ZfK Rossendorf annual report 1984, ZfK-559 (1985) p. 155 [12] H. Braun, W. Cohausz, H . Gemmeke, L . Lassen and A . Richter, MPI Heidelberg annual report (1980) p . 36. [13] W. Seidel, JINR Dubna report D7-82-891 (1982) p . 66 . [14] W. Seidel, ZfK Rossendorf annual report 1984, ZfK-559 (1985) p . 154 . [15] W.D . Fromm and M. Kahlenbach, ZfK Rossendorf annual report 1983, ZfK-530 (1984) p . 131 . [lb] L .C. Northcliffe and R .F . Schilling, Nucl. Data Tables A7 (1979) . [17] J . Henniger and B. Horlbeck, JINR Dubna report D1083-366 (1983) . [18] H . Schmidt -B6cking and H. Hornung, Z. Phys . A286 (1978) 253 . [19] H . Geissel, GSI-report GSI-82-12 (1982) . [20] N .J . Shenhav and H. Stelzer, Nucl . Instr. and Meth. 228 (1985) 359 [21] A.N . James, P .A . Butler, T.P. Morrison, J . Simpson and K .A . Connell, Nucl . Instr. and Meth . 212 (1983) 545 . [22] R. Kotte, H .-J . Keller, H.-G . Ortlepp and F . Stary, ZfK Rossendorf report ZfK-591 (1986) .