Generally, the shape of photopeaks is gaussian (when spectrum have sufficient counting statistics). That shape you can observe in the case of gamma/X spectrometry. The reason that peaks in alpha spectrometry is changed from the left (low-energy) site is that, the alpha particles emitted from the source at the large angles must travel longer distance until they reach an active volume of a detector, therefore they loose more energy, so the peak is not gaussian anymore.

The reason you can not observe peaks  at the spectrum of beta-decay nuclied source is that electrons from beta decay are not monoenergetic, so they do not produce peaks around a certain energy at the spectrum.

@Kinga: The question I raised to discuss reason for the peak shapes. I will appreciate if reasons can be discussed.

I have observed the photo peak as gaussian and the reason being statistical fluctuations in the phenomenon taking place while interaction in detector medium.

In case of alpha particle the shape is having a tail on lower side where as higher energy side it falls steeply.

In case of monoenergetic electrons such as internal conversion ones, I don't know whether there is a peak shape?

I though I have gave you the reason regarding peak shape from alpha particles. The high-energy side is remained gaussian-type, whereas the low-energy side has longer tail, because the distance of alpha particles which do not travel perpendicular to the detector surface but at different angles (than 90deg) is larger therefore they loose energy. That is the reason of this tail on the spectrum.

As for the electrons, in case of monoenergetic electrons you could be able to observe also gaussian peak. I guess the lack of peak can be connected with the kind of detector you are using. For example, in case of plastic scintilator you can not observe peaks.

Kinga's explanation correctly accounts for the differences in peak shapes.

An x-ray or gamma photoelectron peak is nearly Gaussian. A full photon energy photoelectron is produced by a photon that has had no prior scattering losses. The photoelectric effect occurs in the detector so all the energy of the photoelectron is deposited in the detector. The Gaussian shape results from stochastic effects of the collection process in the detector and the conversion process to an electronic pulse.

Full energy peaks of alpha and beta particles have the same stochastic collection and conversion processes, so a full energy alpha or beta particle will produce a nearly Gaussian shape. Alpha and beta particles originate outside the detector, unlike the photoelectron produced in the detector. Alpha and beta particles lose energy in the process of reaching the detector and the detector active volume. Alpha and mono-energetic beta particles will show a low energy tailing because of the losses. The larger the angle of incidence on the detector, the larger the losses.

A good, introductory spectrometry text should include the explanation of peak shapes.

Kinga´s and Joseph´s arguments and explanations are perfect. If you want to change the alpha peaks you could experimentally use very small aperture in front of your detector. The energy losses especially of alphas will diminuish, in case of electrons you will get sharp gaussian peaks. One condition for the alpha spectrometry is a very small distance of alpha source and detector because alphas are scattered and loose a significant part of energy already in some cm of air. And your detector must use a very thin entrance window. My recommendation is, try to study theoretically the stopping power of the used materials for alpha radiation.

A literature tip is ICRU report  49.

Kinga´s , Joseph´s and Hnno's  arguments and explanations are perfect.My recommendation is, try to study theoretically the stopping power of the used materials for alpha and beta particles.