Gamma spectroscopy pdf

In Figure 3. The energy of the incoming photon is divided between the scattered photon and the recoil nucleus by a relationship that is dependent on the scattering angle. This case is the case with the maximum energy transfer between the incoming gamma-ray and the electron. Because of this, a continuum of energies can be transferred to the electron. This continuum can be seen in Figure 3. This energy has a range from 0 all the way to the maximum predicted by Equation 1.

The Compton continuum will be observed at the lower energy end of the spectrum. There is a minimum amount of gamma-ray energy that is required for this process to take place.. This minim place minimum um energy is the mass of the elec electrontron-positron positron pair, 22m m0 c2. Th Thee posi positi tion on of this energy is called the double escape peak in an actual gamma-ray pulse height spectra. This Th is is a ver ery y comp compli lica cate ted d proce process ss due to the the fa fact ct that a pos posit itro ron n is no nott stab stable le..

It will will annihi ann ihilat latee whe when n it com comes es into into con contac tactt wit with h an ele electr ctron, on, whi which ch are ve very ry abu abunda ndant nt.. Upon annihilation, two 0. This process will happen quite quickly and will appear to be in coincidence with the original pair production. In som somee cas cases, es, only one of the anni annihil hilati ation on pho photon tonss is abs absorbe orbed d ins inside ide the det detect ector.

Thi Thiss produces a peak that is known as the single escape peak on the pulse-height spectrum. It can be see seen n tha thatt all of the gamm gamma-r a-ray ay int intereractions are absorbed by the detector. The large peak that is labeled full-energy peak, is often known as the photopeak in detectors that do not absorb all of the gamma-ray energy. Figuree 3. Figur Figuree 3. It can b bee seen that the ener energy gy of the photon pho ton is no lon longer ger a dis distin tinct ct peak, but has a who whole le spectr spectrum um of energ energies ies..

Thi Thiss spec spectru trum m 10 Rittersdorf Gamma Ray Spectroscopy of energi energies es depend dependss on man many y fac factor tors. The size size,, sha shape, pe, geo geomet metry ry,, and compos compositi ition on of the detector dete ctor are all fact factors ors that attribute to the shape of the pulse-h pulse-heigh eightt sp spectr ectrum.

A coupl couplee of spectrum properties that are often quoted are the photofraction , the ratio of the area under the photopeak to the area under the entire spectrum, and the ratio of the area under a single or double escape peak to the area under the photopeak. These high energy particles have a high probabilit probab ility y of penetr penetrating ating the detect detector or and leaki leaking ng out of the system.

This elect electron ron leakage leak age will distort the response function function.. The Compton contin continuum uum along with other low amplitude amplit ude energie energiess will be shift shifted ed to fav favor or lower amplit amplitudes. Since eve events nts are escaping the system, the photopeak will be reduced as well. This loss of events from the photopeak will also reduce the photofraction. This causes a probl problem em when the brems bremsstrah strahlung lung photons are emitted emitted and not reabsor reabsorbed bed by the detect detector.

Product Production ion of brems bremsstrah strahlung lung photons is proportional to Z to Z 2 of the absorber. This alters the response function in the same manner that the escape of secondary electrons alters the response function. If this process takes place near the surface of a detector, the characteristic X-ray may escape the detector and alter the response function, as Figure 3. By escaping, the characteristic X-ray creates a new peak in the response function.

This peak appears a distance of the characteristic X-ray energy away from the photopeak and is known as the X-ray escape peak. This phenomena is prevalent in low energy incident photons and detectors with a large surface-to-volume ratio. Both of these types of secondary radiations will increase the overall energy that is detected in the syste system.

These secon secondary dary radiati radiation on energies will be superim superimposed posed onto the response function.

The result in this is a shift of the spectrum to favor higher amplitudes. It is not uncommon for the results of these interactions to then be absorbed by the detector. In Fig Figure ure 3. This additi additional onal peak in the response func function tion is a resul resultt of the detec detector tor absorbing the characteristic X-rays that are emitted from the surrounding materials. Event 2 corresponds to the backscattering.

This is a wider peak because of the broad range of energies a backscattered photon can have. However, due to the energetics of the backscattering process,, the bac process backscat kscatterin teringg p peak eak alwa always ys occurs at energ energies ies of 0. In addition to the expected spectrum detecto show as a dashed line , the representative histories shown at the top lead to the indicated corresponding features in the response function.

This seem seemingly ingly instant instantaneous aneous emission of separ separate ate gamma-ray gammarayss is kno known wn as coinc coincidenc idence. In this situation, the detect detector or will see both of those energies ener gies as one large largerr energy deposite deposited d in the detec detector. A tell-tale sign of a summation peak is a prominent peak that has the same energy as two characteristic 14 Rittersdorf Gamma Ray Spectroscopy gamma-rays of the source.

All that will be discussed here is why semiconductors are used over scintillators or other types of detectors and the basics of how they work.

The periodic lattice of crystalline materials establishes allowed energy bands for electrons that exist within within a solid. Cer Certai tain n ene energi rgies es are for forbid bidden den,, and thes thesee are visu visuali alized zed as gap gaps. There The re are tw twoo ty types pes of ban bands ds tha thatt are of inter interest est:: the va valen lence ce band and the con conduc ductio tion n band. The next higherhigher-lying lying band is know known n as the conduction band and represents electrons that are free to migrate through the crystal.

If an electron lies within the conduction band then it contributes to the overall electrical conductivi condu ctivity ty of the materia material. The gap betw between een these bands is known as the bandgap. The bandga ban dgap p for a sem semico icondu nducto ctorr is les lesss tha than n the diel dielect ectric ric for tha thatt mat materi erial. The size of thi thiss gap is quoted as an energy, Eg. If an electron gains an energy of Eg or greater, than it will gain the ability to jump up into the conduction band and then lend itself to electrical conductivity.

In a semiconductor, the value for Eg is less than the dielectric for that material. The excitation of an electron process not only creates an electron in the conduction band, but it also leaves a hole in the valence band. The combination of these two is known as an electron-hole pair.

Of Often ten being at room tem tempera peratur turee wil willl sup supply ply ele electr ctrons ons with enough energy energy to jump into the conduct conduction ion band in many mate materials. Becau Because se of this, semiconductors with small bandgaps need to be severely cooled in order to be of use in detection. In other words, a small bandgap leads to a high resolution detector.

Creating the electron-hole pairs is the mechanism by which the semiconductor detects radiation. When a cha charged rged particl particlee passes through a semic semiconduc onductor, tor, electro electron-hole n-hole pairs are created crea ted along the path of the charg charged ed particle particle..

These elec electrons trons allow for the conduction of electricity. In short, this conduction of electricity allows a pulse to be formed. The larger the energy of the incident particle, the more electron-hole pairs are formed, and thus a higher pulse is the result. The reason that high level of purity in the material mater ial is desir desired ed has to do with the depleti depletion on region.

The deplet depletion ion region is desir desired ed to be as large as possible. At thi thiss lev level el of impurity, a depletion depth of 10 mm can be obtained with a reverse bias voltage of less than V. In order to do this, our detec detector tor has as coaxial geomet geometry ry as shows in Figure 3. There are electrodes connected to contact on the inside of the coaxial and the outside of the coaxial coaxial.. A pote potent ntial ial is app applie lied d acr across oss the coa coaxia xiall and a pote potent ntial ial is app applie lied d acr across oss the detector.

Bec Becaus ausee thi thiss collection time is not constant, the pulse shape is also not constant as well. This lack of funct functionali ionality ty at room-te room-temperat mperature ure stems from the large thermallyther mally-induc induced ed leaka leakage ge curr current ent that results at this temperatur temperature. In order to get around 18 Rittersdorf Gamma Ray Spectroscopy this, the germanium detector is cooled to the point where this thermal leakage no longer spoils the excellent excellent energy detect detections.

This temperat temperature ure happens to be 77 K and is ach achiev ieved ed through the use of liquid nitrogen to cool the detector. The detector requires constant cooling from the liquid nitrogen in order to maintain the great gre at energ energy y res resolu olutio tion n tha thatt it has has..

A spec special ial appar apparatu atuss has been con constr struct ucted ed to aid in thatt task. Appe tha Appendi ndix x H has a sk sketc etch h of suc such h a dev device ice.. The det detect ector or mus mustt be hou housed sed in a vacuum-tight cryostat to inhibit thermal conductivity between the detector crystal and the surrou sur roundi nding ng air air..

The cry cryost ostat at is jus justt cap capsul sulee tha thatt hou houses ses the ger german manium ium crys crystal tal.. A thi thin n window is usually located near the crystal to minimize attenuation of gamma rays before they the y enter enter the germ germani anium. As it is show shown n in Append Appendix ix H, the cry cryost ostat at is mou mount nted ed on a liquid nitrogen dewer.

The dewer houses all of the liquid nitrogen and allows the germanium crystal to maintain its low temperature. A high energy resolu resolution tion means that the detector can discriminate between gamma-rays with similar energies.

Some of these factors will dominate over the other factors, but this is dependent on the energy of the radiation and the size and quality of the detector in use. The natural radioact radioactivit ivity y of the const constituen ituentt mater materials ials of the detect detector or itse itself. The natural radioactivity of the ancillary equipment, supports, and shielding placed in the immediate vicinity of the detector.

Radio Radioactiv activity ity in the air surround surrounding ing the dete detector. The primary and secondary components of cosmic radiation. Cosmicc radiat Cosmi radiation ion also con contribu tributes tes to bac backgroun kground d radia radiation. The planet is const constant antly ly b bomombarded with high energy particles such as muons, pions, neutrons, electrons, positrons, etc.

The high energies can produce large pulse heigh heights ts in a spectr spectrum um and give false readings for a source that is being measured.

Fig Figure ure 5. Afte Afterr doing this, a sourc sourcee was placed on the detect detector. The purpose of this 22 Rittersdorf Gamma Ray Spectroscopy next reading is to calibrate the MCA so that each channel would correspond to an energy. The source sourcess were long long,, cle clear ar rods wit with h a bla black ck tip at the end end.. The blac black k tip hous housed ed the radioactiv radioa ctivee source.

This part was placed in the middl middlee of the detec detector tor to minim minimize ize error due to the solid angle of the measu measureme rements nts..

The measurement was taken until there were at least counts. Afte Afterr doing this for the followin followingg isoto isotopes, pes, 57 Co, 60 Co, Cs, 22 Na, Ba, Cd, 54 Mn the data was ready to be analyzed. The centroid channel number for all prominent peaks as well as the channel number num ber location of all other int interes eresting ting feature featuress we were re recorded recorded..

The FWHM in channels channels , , area under each photopeak, and the area under the spectrum was also recorded, as was instructed by part 4 of the lab instructions. A majority of the spectra that were gathered in lab were of very poor quality.

It is possible thatt the coun tha countt tim times es were too lo low w to build up a dec decen entt spec spectru trum. Als Also, o, the bac backgr kgroun ound d 23 Rittersdorf Gamma Ray Spectroscopy spectrum consists of about 16 total counts. The low quality background count made subtracting this spectrum from the other spectra meaningless. This will also provide later complications in analyzing the background spectrum.

This curve is the energy calibration curve, and can be seen in Figure 6. There The re are a cou couple ple of thin things gs that are of not notee in Fig Figure ure 6. This is an excellent result. This means that the MCA is linear; each each channe channell has the same width of energ energy y.

The bricks that contain the germanium germanium dete detector ctor are made of lead. It sho should uld be not noted ed tha thatt the erro errors rs bet betwe ween en the measu measure remen mentt and the actuall value of the X-rays are very small. This error coul could d be reduced even further with more MCA channels. The photopeak was detected right where we would expect it be, at The error between the measured and the actual here is almost nonexistent1.

It should be noted that there are no single escape, double escape, or annihilation annihi lation peaks on this spectrum. This is exactly what is expected, expected, as the gamma-r gamma-rays ays emitted by this source are not energetic enough 1. For 60 Co, the spectrum in Figure 6.

This spectrum is also distorted, although the features are more distinguishable than the those on the 57 Co spectrum. The next two noteworthy features on the spectrum Figure 6.

It is here that an interesting phenomenon occurs. Each of the two gammas will create their own ow n Com Compto pton n con contin tinuum uum on the spectr spectrum. The These se con contin tinuum uum overl overlap, ap, but the Compt Compton on edges are still distinctly distinctly visible for each region. These Comp Compton ton edges are labeled on Figure 6.

Using Eq. Professor Kai Siegbahn —, Nobel Laureate The constant k was determined by measur- ing a photoelectron line using two known excitation energies [4]. This issue was carefully discussed by ES [3], where it was pointed out that this meant that one had to measure the work func- tion of the spectrometer, and that the sample had to be in metallic Fig.

Carl Nordling and Evelyn Sokolowski working in the laboratory with the contact to the spectrometer. The instrument was two orders vant binding energies for a solid are related to the Fermi level.

In of magnitude more sensitive than earlier constructions due to the double focussing Ref. This is not far from the present value of The basis for the energy determi- energy. However, ference in retardation voltage between the Fermi edge of a metal calibration sample and the studied core photoelectron line.

The chemical shift The remarkable growth of research based on electron spec- troscopy is to a large extent connected to the presence of core level binding energy shifts. Small chemical shifts had been observed since in X- ray emission spectroscopy.

Chemical shifts in X-ray absorption spectra were observed much later. Today, modern, very precise syn- chrotron radiation monochromators have enabled very accurate measurements of near edge X-ray absorption spectra NEXAFS. In fact, his method is a very important tool for chemical understand- ing of matter. Interestingly enough, one researcher in Uppsala had actually observed a very clear chemical shift in X-ray absorption at a very early stage.

Magnusson [7]. This spectrum is shown in Fig. However, in Magnusson, and his colleagues and supervi- Fig. It was obtained in but was in press in January The sample that the shift between the two chemically inequivalent N1s lev- was metallic Cu evaporated on a backing. It was instead explained as due to experimental errors. ES describes the process: By we had our sensitive equipment quite well under control.

But then we got into more serious trouble. The beautiful K electron lines of copper which we had used for some time, suddenly became irreproducible, jumping in a random way between two energy positions.

Had somebody started another cyclotron?? Sometimes in science you are just lucky, and this time I happened to be the lucky one. Carl Nordling had a family commitment that made him take time out for a while, and I decided to use those weeks to prepare myself for the last remaining theoretical Ph D examination.

The photo-line of the cupric oxide, were plotted with crosses. However, as discussed in the text, most of shift is due the chemical form of the sample. To our knowledge our copper samples had always been prepared in the same way — by evaporation in vacuum — but neither the vac- in , one of her reasons to do so was directly related to this uum, the temperature nor the thickness of the copper layer had been problem: well controlled.

It seemed quite possible that what we had thought was pure copper had undergone different stages of oxidation.

Four years earlier course that this might open a new way of analyzing surfaces. I had used a small scholarship The evaporation technique had been chosen in order to give thin from Uppsala to attend a summer course in Italy on reactor physics, samples where scattered electrons would not distort the undis- with the most stimulating faculty I had ever met: Nobel Laureates turbed line.

But here nature, together with the high resolution of Aage Bohr and Isidor Rabi Enrico Fermi should have been there but the spectrometer had solved the problem: The energy loss in the had died of cancer and the director of Oak Ridge National Labora- scattering process was mostly distinct, and large enough to sepa- tory Alvin Weinberg.

It seems I may have electrons, and still got a usable undisturbed line, which brought the been wrong. The observation of shifted Cu1s core electron lines see Fig. The decisive observation of the chemical shift was published in Physical Review [9]. The reported effect of core level chemical shifts was still a of chemical shifts a much clearer result had to be obtained. When ES left the unstable. The careful discussion Fig.

Together with Carl Nordling he continued the nuclear physics of this result was well motivated, and turned out to be correct. And to be only about 1 eV [10]. The observed shift of 4 eV must there- they spent considerable time investigating if the apparatus could fore mainly be due to sample charging.

None of these results attracted much international attention, Fig. The S2p photoelectron spectrum of pure sulphur and sodium thiosulphate. The colour coding and the reports were rarely cited. The project seemed to be run- for the two sulphur atoms and the corresponding electron lines are the same in ning into a dead end. Siegbahn even started to indicate to the group Figs.

The picture is worked out based on the originals in Refs. When arriving to Uppsala in he created a multitude of new projects and the persons involved often research and it eventually lead to a Nobel Prize.

His main interest was shown in Fig. In he was e. Serendipity entered into the project! There were no further discussions about terminating the One Sunday evening [13,14] 50 years ago Nordling and project. The of new samples. They made a sample Chemical Analysis for the newborn technique. During the trum of sodium thiosulphate see Fig. It is enough to look at the reference list of this book to understand how intense the work must have been. Plans to study liquid samples were also discussed.

The electronics, the vacuum equipment and the detectors were modernized. Results from electrostatic instruments Fig. The chemical composition of the thiosulphate ion. The pioneering years were over. This spectrum very pedagogically demonstrates the core electron binding energy shift. All the chemically inequivalent carbon atoms give rise to a C1s core photoelectron line, and the binding energy of the lines follows the electronegativity of the ligands.

This had to await a third generation of instruments. After the pioneering years molecule shown on the cover of the two ESCA monographies. In Fig. It is hard to overestimate the impact of the two ESCA volumes.

In this spectrum the instrumental resolution spectrometer and Electron spectroscopy was internationally recognized not only in X-ray monochromator was about 20 meV to be compared with physics but also in chemistry, biology, surface science and other about 1 eV in Refs.

When pushing the resolution to this modern disciplines. We have not discussed the partly parallel but level, completely new information can be extracted. Not only the very impressive activity of Ultraviolet Photoelectron Spectroscopy chemical shifts of the different carbon atoms in the molecule can launched by the group of Turner in the UK in the beginning of be observed. The spectral shape shows that the studied molecule the s. However, this falls out of the scope of the present arti- is a mixture of the two conformers anti-gauche and anti-anti, that cle since Turner and coworkers did not deal with core levels and originate from rotation of bonds.

Moreover, the vibrational struc- chemical shifts and only used UV excitation. New generations of electron analyzers have tation of rotations and to dissociation when ionizing on the carbon been developed, see e. The development of Syn- in the -CF3 group. This shows the importance of resolution, since chrotron Radiation X-ray sources is perhaps the most important now also the dynamics of the core ionized stated can be assessed.

The amazing increase of brilliance of the X-ray radiation by a factor well above for storage rings implies that core 7. Concluding remarks level spectroscopy can today be performed at a resolution of about 10 meV, or even better.

This rapid understanding was due to the fact that the possibility of chemi- cal shifts had been carefully discussed already several years earlier, when different Cu1s binding energies could be correlated to differ- ent chemical states of Cu. The authors behind the report on Cu1s realized that the results were somewhat ambiguous, and today we know that the largest fraction of the observed shifts was due to sample charging. In contrast, Magnusson observed very clear chemically shifted peaks in the absorption spectrum of NNO already in Also in this case the peaks originated from chemically inequivalent atoms in a molecule.

The N1s related peaks were much better resolved than in the core photoelectron spectra from S2p but Magnusson could not make a correct assignment. The lines do not have equal intensity. See further in the text. From Ref. Sokolowski, Ark. Nordling, Ark. Lebugle, U. Axelsson, R. Nyholm, N. Martensson, Phys.