HAL Id: jpa-00215769
https://hal.archives-ouvertes.fr/jpa-00215769
Submitted on 1 Jan 1974
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
POSSIBLE IMPROVEMENTS OF MÖSSBAUER SPECTROMETERS WITH THE USE OF MULTIWIRE PROPORTIONAL COUNTERS
G. Bonazzola, T. Bressani, E. Chiavassa, G. Dellacasa, A. Musso, B. Minetti
To cite this version:
G. Bonazzola, T. Bressani, E. Chiavassa, G. Dellacasa, A. Musso, et al.. POSSIBLE IM- PROVEMENTS OF MÖSSBAUER SPECTROMETERS WITH THE USE OF MULTIWIRE PRO- PORTIONAL COUNTERS. Journal de Physique Colloques, 1974, 35 (C6), pp.C6-687-C6-690.
�10.1051/jphyscol:19746153�. �jpa-00215769�
JOURNAL DE PHYSIQUE Colloque C6, suppltment au no 12, Tome 35, Dtcembre 1974, page C6-687
POSSIBLE IMPROVEMENTS OF M~SSBAUER SPECTROMETERS WITH THE USE OF MULTIWIRE PROPORTIONAL COUNTERS
G. C. BONAZZOLA, T. BRESSANI, E. CHIAVASSA, G. DELLACASA, A. MUSS0 Istituto di Fisica Superiore dell'universita, I 10125 Torino, Italy
Istituto Nazionale di Fisica Nucleare, Sezione di Torino and
B. MINETTI
Istituto di Fisica Sperimentale del Politecnico, I 10129 Torino, Italy
RBsumB. - Nous discutons les caracteristiques d'un spectromt?tre Mossbauer construit en utili- sant une chambre proportionnelle multifils. La resolution en vitesse et l'efficacite de detection pour rayons y sont analys6es. Details de construction et resultats experimentaux sont report&.
Abstract. - We describe Mossbauer spectrometer utilizing a multiwire proportional chamber.
Velocity resolution and detection efficiency for y-rays are discussed. Details of construction and experimental results are given.
1. Principle of the method. - Recently we demons- trated [l] the possibility of constructing Mossbauer spectrometers which are able to measure an entire velocity spectrum at the same time. The apparatus consists mainly (see Fig. 1) of a multiwire proportional
TO THE ELECTRONICS
DRIFT SPACE ' MESHES
\
d
FIG. 1. - Sketch of the MWPC Mossbauer spectrometer.
chamber (MWPC) shaped like the sector of a cylinder.
The y-ray source is located at its center of symmetry.
It moves forwards and backwards with constant speed v,, along a fixed straight line on the plane normal to the axis of the cylinder, covering only a small spatial sweep. The absorber also has the shape of a sector of a cylinder and is coaxial with the MWPC. In this geome- trical configuration counting pulses arising at each wire are caused by y-rays emitted at an angle 8 withrespect to the direction of the motion of the source. These y-rays have therefore been transmitted through the absorber with a Doppler shift corresponding to the relative velocity between source and absorber of
We note that when the motion of the source reverses than the angle 8 has to be replaced by (71-8) and the relative velocity associated with each wire simply changes sign.
In the apparatus sketched in figure 1 the angle 8 ranges from 7116 to 7112 a n d therefore the relative velocity varies between 4312 vo and - J3/2 v,. The advantage of the proposed Mossbauer spectrometer is that this device contains no y-rays collimator ; the solid angle subtended by the detector on the source can be as great as desired ; for example it is possible to attain a value of 2 7c sr with a cylindrical chamber and a point source.
We showed that the proposed method works success- fully by using a flat MWPC of 10 x 10 cm2 area with
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19746153
C6-688 G . C . BONAZZOLA, T. BRESSANI, E. CHIAVASSA, G. DELLACASA, A. MUSS0 AND B. MINETTI
a wire spacing of 2 mm [I]. By placing this chamber tangentially onto a cylinder of 30 cm radius we obtained the spectra depicted in figure 2. The usefulness of the MWPC spectrometer becomes evident when the spectra are compared with those obtained with the help of a conventional constant velocity Mossbauer spectro- meter.
(900:1111 I l ~ l l l l i t I t l l f l { l f
t I
t i t
4 2 0 0
-
=I I ' i 1 I t'000- , , , , t
r i m m l r )
, , I , , . I
- 6 -5 - - 3 -2 -1 0 I 2 3 1. 5
FIG. 2. - Mossbauer transmission spectra of an enriched iron absorber, obtained by a simple line 57C0 14.4 keV y-ray source : a) with MWPC Mossbauer spectrometer having uo = 0.5 cm/s and 0 between 30° and 90° ; b) same arrangement but with v o = 1 cm/s and 9 between 450 and 900 ; c) with a standard constant-velocity spectrometer ; all spectra have identical velocity
scales.
In the following we examine, in some detail, geo- metrical problems concerning the velocity smear related to each wire and technical problems encounter- ed in the construction of MWPCs with high efficiencies for y-rays, in the energy range useful for Mossbauer spectrometry.
2. Geometry of the apparatus. - In a MWPC we may assume that each wire detects particles producing ionization in a volume centered around the wire and having the following dimensions : height b (lenght of the wire), thickness s (that of the chamber) and width A (the wire spacing).
We consider now a thin disc-shaped source of radius R,, with its center at a distance R from the wire, travelling back and forth the length d ( f d/2 around its center position). We assume that the plane of the source forms an angle 8 with the direction of motion.
The problem to solve is : how wide is the angular spread A8 of the y-rays detected by each wire ? We solved this problem by means of a Monte Carlo cal- culation ; we evaluated a large number of distribu- tion in angle of events (1 000 for each step) detected by each wire. The desired value of A9 is deduced by evaluating the total width at half maximum of the distribution found. The calculations were performed in the following way : first we disregarded the chamber thickness and we fixed experimental parameters like :
a) the height of the wire b : 5 cm ;
b) the spatial sweep of the source D : 0.5 cm ; c) the angle a between the source plane and the motion direction : 450.
The reasons for this choices are :
a') the value of 5 cm for b leads to a good solid angle and imposes no serious problem in chamber construc- tion ;
b') the value of the spatial sweep must be small with respect to the chamber radius R ;
c') the value of 450 for the angle a seems to be best for the exposure to a planar source of a chamber shaped like a sector of cylinder.
With this set of parameters we chose different values for the wire spacing A and the radii R and R, of the chamber and the source. Event distributions for wires placed at angles ranging from O0 to 1800 in steps of 100 were then generated. In figure 3 we show the results for six different combination of A, R and R,. To evidentiate the meaning of this calculation we do not report A8 in figure 3, but the related velocity smear Av obtained by utilizing the simple formula :
A = & m m R 300 mrn Re2 5 m m
A=G rnm R = 2 5 0 m m
A=G rnm R.200 m m qz2.5 m m
A=3 mrn R.250 rnm Rc2.5 m m
Oo 30' 60' 90' 120" 150' 180' 6 I degrees)
FIG. 3. - Velocity resolution Au for a MWPC Mossbauer spec- trometer, with a MWPC shaped like a cylindrical sector of radius R, wire spacing A and source radius R,. The length of the wires is 5 cm, the travel length of the source is 0.5 cm and the angle between the source plane and direction of motion is 450.
The constant velocity v o is assumed to be equal to 10 mm/s.
POSSIBLE IMPROVEMENTS TO M~~SSBAUER SPECTROMETERS c6-689
EfJiency of a detector, Jilled with A or Xe for dzyerent values of the eflective thickness, of the counting gas pressure and o f the y-rays energy
The value of Av is directly proportional to v, which we assumed to be equal to 10 mm/s. In this case it appears that a maximum smear of 0.2 mm/s can be easily obtained with reasonable dimensions of the chamber, i. e. a radius R of about 25 cm and a wire spacing larger than the one commonly used. (We recall that in high energy physics experiments MWPCs work with A = 2 mm ; in some cases even with A = 1 mm.)
Referring to figure 3 it is interesting to remark that if symmetrical values for the velocity resolution around 8 = 900 (that is v = 0) are desired, the source dimen- sion must be small in respect to the wire spacing.
Under these conditions the source looks in fact like a point source and the solid angle subtended by the sensitive region of each wire depends only on the distance R and not on any other source parameter, like a. We remark finally that in the limit of negligible source travel length and a point source the velocity resolution curves are simple sinusoids.
Energy (keV)
nent with high 2. Chambers filled with Xenon have been successfully operated [2] ; from table I it appears that the efficiency improves considerably ;
11) to increase the pressure in the chamber. The efficiency grows exponentially ;
111) to increase the thickness of the useful space. The efficiency grows exponentially.
We designed our MWPC for Mossbauer spectro- metry considering the above arguments.
The chamber was enclosed in an aluminium contai- ner which had a mylar window for the y-rays. The container could be evacuated, then filled and sealed off at pressures up t o 3 atrn ; this is useful in thecase of Xenon which, because of its cost, cannot be used in a flow chamber.
Secondly we increased the useful thickness of the chamber by adding two drift spaces [3] of 2.1 cm, one before the first and after the second cathode. In these 3. MWPC properties and construction. - MWPC
usually work with a mixture of gases (argon, isobutane, freon are typical ones) ; the mixture flows continuously through the chamber at atmospheric pressure. The chamber is composed of a plane of anode wires placed 5000 between two cathode planes separated typically by 1.4 cm. We can evaluate the efficiency E for such a detector using the simple formula :
6 = 1.4cm
where p is the attenuation coefficient of the y-rays and 6 the chamber thickness. In table I we give some values of E for different y-rays energies, for a chamber with a useful thickness of 1.4 and 5.6 cm, filled with Argon and Xenon at 1 and 3 atm.
It is evident that a conventional chamber with Argon filling has a rather unsatisfactory efficiency for the y-rays in question. There are some possibilities to overcome this difficulty :
6 = 5.6 cm p = 1 atrn
I) t o change the gas mixture introducing a compo-
p = 1 atrn p = 3 atrn
Background
I l
p = 3 atm I
- - - - p = I atrn.
-
p =2.2atrn.14.L KeV
57 Co I
FIG. 4. - Energy-spectra of a 57Co source as measured by a MWPC, shaped like a cylindrical sector, at two different values of
counter gas pressure.
C6-690 G. C. BONAZZOLA, T. BRESSANI, E. CHIAVASSA, G. DELLACASA, A. MUSS0 AND B. MINETTI two spaces there is no gas multiplication phenomena The wires (gold-plated tungsten) used as anodes had a but electrons liberated in these volumes can reach the diameter of 20 ym.
amplifying region, thus increasing the useful thickness With this chamber we performed some experimental to a value of 5.6 cm. The gain in efficiency can be seen tests ; mainly for observing the behaviour as a function in table I. In conclusion, the detector consists, as of gas pressure. In figure 4 the resulting pulse height sketched in figure 1, of eight frames each 7 mm thick : spectra of a 57Co source are depicted ; an Argon- I) frame supporting the mesh which polarizes the Methane mixture was used. It is evident that, at-one first drift space ; atmosphere, the efficiency of the chamber for detection of 14.4 keV y-rays is unsatisfactory, while at 2.2 atm 11) frame defining the first drift space ; the 14.4 keV peak appears clearly. From these data we 111) frame supporting the cathode wires or mesh ; conclude that an A&&-~ethank mixture can be used IV) frame supporting the anode wires ; up t o 20 keV, whereas for greater y-rays energies one V) frame supporting the cathode wires of mesh ; needs a Xenon filling.
As a final remark we recall that each MWPC wire VI and VII) frames defining the second drift space ; can count up t o lo6 particlesls, thus no problems seem VIII) frame supporting the mesh which polarizes to arise from the of view of the counting rate.
the second drift space. We are grateful to Dr. A. Alessio for her help.
References
[l] BARBERO, C., BONAZZOLA, G. C., BRESSANI, T., CHIA- [3] BRESSANI, T., CHARPAK, G., RAHM, D., ZUPANEIE, C., VASSA, E., MUSSO, A., Lett. Nuovo Cimento 4 (1972) 19. Proc. Int. Seminar on Filmless Spark and Streamer [2] KAUFMANN, L., PEREZ-MENDEZ, V., STOKER, G., ZEEE Trans. Chambers, Dubna, 1969 (JINR, Dubna, 1969) 275.
on Nucl. Science N S 20 (1973) 426.
